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1981 CB750 "DOHC" ISSUE

 
Old 09-11-2010, 12:41 AM
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Default 1981 CB750 "DOHC" ISSUE

Recently I purchased a 81 CB 750 custom, aside from the tank it looked to be in excellent condition except a few minor dent in the tank but the owner said he never tried to get it running and had no idea what was wrong with it... I thought for 300 dollars I want it! Got it into the garage and got into tinker with it... cleaned and lubricated the carbs up real nice... gotta battery onto it and tried to turn it over with nothing but some clicking... Pulled the plugs to release the compression and see if it would turn over... Still nothing... Thus I have a seized engine and need a little guidance as to what my best options are gonna be... I'm willing to rebuild it... I will be pulling it apart soon to see whats actually seized whether its in the top end or bottom...
So: A couple questions
1: Is it common for these engines to seize? could there possibly be something that i'm missing?

2: If i get new pistons and rings how exactly do I know what to get the cylinders bored to??? (of all the ****ing around i've done on quads and bikes i've never had to replace a piston) Lucky me i suppose...

3: I'd love to take in any pointers involving this engine I currently don't know a pile about it... Which as of tonight I realized i'm gonna get to know this engine better than I thought I would lol
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Old 09-12-2010, 04:07 AM
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Default Re: 1981 CB750 "DOHC" ISSUE

Originally Posted by Dwrecktum View Post
So: A couple questions
1: Is it common for these engines to seize? could there possibly be something that i'm missing?
(
no it is not... try turning over the engine via the drive sprocket/countershaft using a wrench. if the engine is frozen from age, try putting some wd40/oil in to the chambers to break loose the the age

Originally Posted by Dwrecktum View Post

2: If i get new pistons and rings how exactly do I know what to get the cylinders bored to??? (of all the ****ing around i've done on quads and bikes i've never had to replace a piston) Lucky me i suppose...

(

bore size is usually determined in steps over the original bore... another words, after a seizure, you need to go to the next step that will allow you to remove the damaged areas. they normally go up in bore size by .25(one step) .50(two steps) etc

Originally Posted by Dwrecktum View Post
.

3: I'd love to take in any pointers involving this engine I currently don't know a pile about it... Which as of tonight I realized i'm gonna get to know this engine better than I thought I would lol
try turning it over by hand and go from there
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Old 09-12-2010, 12:35 PM
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Default Re: 1981 CB750 "DOHC" ISSUE

Thanks a lot for your reply MSchu
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Old 09-12-2010, 12:35 PM
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Default Re: 1981 CB750 "DOHC" ISSUE

Here's a little more info on what I've done... Checked voltage @ battery its good. Checked starter resistance to ground. GOOD Performed voltage drop checks between all connections including the solenoid... Checked voltage output on starter side of solenoid is good...
Removed spark plugs and starter would still not turn engine.
Removed timing cover... Here's my big mistake... The bolt i chose to turn on said 10 on it... But what i didn't realize that it was actually machined down to more of a 7 or 8mm shank... W/ a screwdriver in the #4 spark plug hole my partner felt the piston move upwards only slightly before i broke the bolt... used a pin style punch to remove portion of bolt from the end of the crank... (got incredible lucky)
Got a new bolt from a bike wreckers today...

Little info about me: I'm a H/D technician i work on on road trucks in alberta canada... I'm incredibly familiar with starting systems and issues because as you can imagine at -25 to -40 Deg Celcius during the winter trucks tend to not start... So I traced everything to my knowledge...
In my free time I fix old quads and the occasional dirt bike... Always only one cylinder though... This will be the first engine of this type i've played with... So i'm more than willing to take all advice that anyone here has as i'm sure you've all dealt with a lot of similar issues...
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Old 09-12-2010, 12:37 PM
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Default Re: 1981 CB750 "DOHC" ISSUE

And story continues! reading back I realize my first post sounded like i'm an idiot... It was 3 am.. I was tired and frustrated... But that night i did spray a pile of fluid film into each cylinder... I like how you call it breaking loose the age

This evening... First... Pulled the starter and confirmed that it spins like a dream... Tried turning the crank on the genny side... No movement at all what so ever... Pushed it down the road at all our might in 5th dropped the clutch and turns out the clutch slips... lol... likely adjustment... seems like its cranked up pretty tight... Rolled it back in the garage, and put it up on the stand...

Tore the engine out... when removing it was easy to see in the exhaust pipes that #3 had been running lean or hot... and #1 rings had been passing a bit of oil...

Engine on the bench:
Tore the valve cover off and unbolted the cams... "E" is the only damaged one showing excessive wear but cam appears fine with no real pitting and surface is in good condition... All other components are good ish... cept 1 cap screw that broke when trying to remove it... i haven't figured out how i'm getting it out yet... Removed the cylinder head... Took a bit of mild tapping but it eventually came... Now the issue... RUST #3 and #4 being the worst... used a wood dowel to hammer on the pistons that were near TDC and eventually got them freed up... Some gradual cranking back and forth and eventually full rotations were possible with no real restriction... Removed the cylinder block... Which was a nightmare to get off!! and Voila...

#3 and #4 wrist pins don't move freely... #1 and #2 look like new ish... So ultimately I need pistons rings pins... Everything below looks good except some surface rust on the #4 connecting rod which seems to come off no problem... I've decided to refuse to split the case... There's no reason to do so... I'll need to either hone or bore the head... I'm thinking its gonna get bored a mill or two over...

**!!Now i really need the suggestions... What should i get... and from where and what else should i be looking for just in case... !!**

Sadly this bike needs a little more tlc for an oil change but i'll love it all that much more when its done...
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Old 03-28-2011, 07:42 AM
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Icon6 Re: 1981 CB750 "DOHC" ISSUE

I'm reading your saga with interest. I tried to buy a dohc wreck several years ago but the owner wouldn't deal at all, then surprise, called me and said I can have it. It has sat out in the weather for years so it's gotta come completely apart. I have visions of a slick cafe racer. One thing I saw years ago was a marine guy stripping a Merc outboard that had gone swimming in the chuck, then sat in a garage til it was white with that alum ozide. He got the pistons out in a jif, just put Coke classic in the bores and left them for about 30 mins. I couldn't believe it. Anyway, I would like to learn all I can from your project.
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Old 04-05-2015, 11:17 PM
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Default Re: 1981 CB750 "DOHC" ISSUE

That's a B(ad)-(H)itch that you broke a cam-cap bolt, those are the hardest tool-steel hardware in the whole bike. Really a hassle to fix. Only worth doing on a much better head than the CB750C item, for instance a CB900F head or CB1100F head etc.


I realize this is an old thread, but hey - anybody who's trying to rebuild an old CB750C engine should realize that what they WANT is another engine anyhow, even if the 'C engine IS in good shape! Ha-ha. There are a lot of shaft-drive CB900C's out there, would make great donors for engine parts - ME I wanna figure out a way to adapt a belt-drive directly off the CB900C's output shaft - For clarification, that's the regular ENGINE output shaft, exposed when the sub-transmission is removed. See the CB900C engine is just like the CB900F engine only it's got this odd-ball sub-transmission stuck on the output, which drives a 2-speed tranny to a shaft-drive on the OTHER side of the rear wheel. A complicated and heavy mess, IMHO. They're still great bikes, don't get me wrong. But the CB750C chassis has the lightest frame, less dead weight to haul around - and a belt-drive is even better than a chain, which is better than a shaft in terms of transmitting horsepower to the rear wheel. A belt-drive would be equivalent to putting a hot-rod head job into the engine, or a hot pipe etc. Because more power is transmitted to the rear wheel it's basically equivalent to having more power in the first place. Right?


So yeah - what you WANT from all of the DOHC family:


- the 39mm fork from the CB900F or at least the 37mm version from the '82 CB750F. Any which way you slice it, you don't want the "leading axle" type forks because the forces on them don't act straight up and down in the telescopic fork I guess they supposedly introduce angular vector forces which induce stiction and impede proper suspension dampening action, etc etc. The TRAC (torque reactive anti-dive control) forks seem like a heck of an improvement but require a rebuild to work properly so it's an added complication that not everybody wants. But ME heck I want one - I want the 41mm with 20mm front axle, found on the GL1500 six-cylinder Goldwing. It's a lot like Freddie Spencer's Daytona winning superbike's fork..... Anyway yeah, if you can get your hands on one locally score one of these thicker OEM forks. Plenty of CB900F folks have replaced theirs with 43mm forks from CBR etc, so the 39mm version is an available "hand-me-down" for the smaller bikes I'd grab 'em while you can!


- the caliper hangers from CBX pro-link or GL1100A Aspencade (Non TRAC anti-dive version) which will work with 296mm rotors OR even 300-310mm rotors like Brembo discs from a Ducati or any of the CBR rotors - the mid'90s version of CBR900RR rotors should bolt straight up to the GL1000 wire-spoke front hub, while plenty of similar five-bolt rotors can be adapted to the stock Comstar hubs. The point being, with the OEM caliper hangers from these few models you don't need to make complicated custom-made caliper hangers from alloy plate/billet etc.


-the dual-disc systems are superior for braking power but ALSO for the fact they input that braking force into both fork legs, which means the fork doesn't twist under the input. This is the real reason why old race-bikes used double-sided DRUMS up front, which had a weight penalty for the extra side-plates etc.


-what you want on ANY of these forks is a good FORK BRACE. At the bare minimum, the one from inside the oem fender. DON'T ditch the fender, your front end will become really sloppy as a result. However if you can't stand the fender and you WANT mud all over the front of your motor, use a Telefix or "Super-Brace" style fork brace that clamps to the tops of the fork's lower half the "sliders" or "stanchions" etc. A good fork brace can make a 35m fork just as stable as the 37mm or 39mm without one. Oddly enough, the fatter tube versions don't seem to weigh all that much more than the skinny ones.


- the 900cc engines or at least the 'F type engine which is tuned hotter than the 'C or 'K versions


- barring a motor replacement you want either the top end from the 900 (the 750 cases are the best for building the strongest motor, so don't throw 'em away!) or a head, or cams, all sorts of bits you should look for from the other bikes BEFORE you consider replacing the 'C cams or 'K cams etc which would cost about the same anyhow....


-OIL PUMPS and SUMPS and OIL COOLERs - the 750's all had a single stage pump, whereas the 900 1000 & 1100's all had two-stage parallel pumps which ran a loop to an oil cooler, through the sump. It's true, the 750 motors are better designed for cooling, however I've already burned one out from overheating. So henceforth I've vowed to only run 'em with a huge oil cooler. What you need is the pump, the "oil path plate" from under the sump, you need the sump itself and the cooler and lines.


-HOWEVER I've also used the 750 sump drilled & tapped for little hose-barbs with clamps really "old's cool" style. Though the OEM version is much better than that, they're a bit of a pain to adapt to non-OEM oil coolers, like bigger universal coolers with AN- fittings an stainless braided lines in the iconic hot-rod style. But yeah, the pump and the path-plate would be enough, if you're happy using hose-barbs and clamps.


-I've ALSO built a DEEP SUMP using a sandwich layer from a 750 sump, with a 900 sump underneath it. Got inspiration from the "Motoren Israel" stuff for BMW airheads. It's well proven tech, it's been done on other bikes. There WERE aftermarket deep sumps offered, I think some of 'em might have even come from the factory, just not certain. But yeah, deep sumps were used on racers in the '80s though probably not as common as the DRY-SUMP parts used on Freddie Spencer's 1982 winning Daytona Superbike CB750F 1032cc monster, or on the "RCB" or "RS1000" endurance racers over in Europe.


-Essential to ANY of these bikes but even more essential to a deep sump, there are people on the 'F-orums who modify the oil sump pick-up screens for you, using a fatter tubing. Then you seal up the whole mess with RTV silicone aka "Honda-Bond" all over the pick-up screen tube and the "path-plate" etc. This keeps the pump from cavitating and causing the oil to foam up.


-IF the oil foams up it will damage the crank-shaft. this is a Honda Tech site for auto enthusiasts so you're probably quite familiar with this idea. Most other '70s-'80s Japanese Superbikes, as well as Harleys and Brit-bikes, all had ROLLER BEARING CRANKS which were more tolerant of low oil pressure or air infiltration etc. Paradoxically, the automotive "plain-bearing" type crank on the Honda DOHC-four has less than 10% of the friction which those roller-bearing cranks (GS1000, KZ1000 etc) it's just gotta keep up a constant oil pressure. In truth, it's too bad these Hondas can't use an oil PRE-pressurization system when starting up, the way many autos do. Would be nice to have a back-up battery-powered oil pump inside an oil tank or something like that. Used only during start-up. But yeah, I have never heard of it being done. And it's probably a huge weight penalty!


-the point being, DON'T leave your oiling system as stock! Get over to the www.CB750C.com and/or www.cb1100f.net sites and do everything you can afford to upgrade your lubrication system. And consider the cost-saving methods I suggested about the drill-&-tap hose-barbs & simple clamped lines for a universal oil cooler, because the SOHC bikes used stuff like that on the likes of LOCKHART oil coolers back in the day!


-as for spin-on oil filters, they're helpful in the sense of being convenient to change. However the stock OEM paper oil filter actually WORKS much much better than what's inside most spin-on filters. There's an "approved list" of filters on the 'F-orums so you CAN use that type of stuff.


-Now if you're considering using the spin-on filter adapter type oil cooler adapters, consider that you're adding a bunch of back-pressure to a pump that's already a bit stressed. It would be a great way to do it only IF you could figure out how to run the twin-stage CB900F/CB900C/CB1000C/CB1100F/CB1100R oil pump into just ONE loop, with opened up passages for better flow and the entire volume flowing through the cooler - I really like the idea of it - But yeah if you're gonna run an oil cooler loop you're smarter to use the twin-stage pump. It's cleaner to run the lines off the sump anyhow, even if the loop is just dumping pointlessly without lubricating the engine. I always wanted to figure out how to dump that cooled oil into the head or something like that!


-Speaking of the head - far better to close off a leaking tachometer drive than repair it! Heck of a lot of work blocking it, that's true. But with all the hassle of mechanical drive cables, it's much smarter to replace the guts of your tachometer with an electronic tacho drive, where the pick-up is near your coils or spark-plug cables etc.


-Same deal with a wheel swap: don't bother trying to fudge together a mechanical speedo that works. I've seen some crazy stuff on the 'F-orums over the years, like people adapting speedo drives that run off the engine output sprocket on other bikes, meaning a whole custom sprocket cover needs to be built with an adapter etc. INSANE. Modern speedometer drives (from 2000 onward pretty much) run with a small magnetic "hall effect sensor" pickup zip-tied to the fork leg, and a small rare-earth magnet tucked into a bolt-head or some such. The magnet trips the sensor with each rotation of the wheel, and you could even add more magnets for even quicker response. There are programmable adaptable universal electronic speedo tacho units out there, but it's just as easy to use clocks from a crashed crotch-rocket which used the same tech as stock. One way to match it up is to use a unit from a bike with the same size front wheel, but an even more interesting way would be to use your computer to draw up a new clock face, which changes the interval of the numbers on the clock dial in proportion to the change in wheel size. A bit of math screwing around with it, but there are situations where it would work really well and for cheap too. Plenty of folks out there selling reproduction clock face stickers I'd bet they could make one for you if you calculated the intervals for yourself....


-Speaking of wheel-swaps: Some of the coolest most "period correct" wheels for the DOHC-four Hondas are the "Boomerang Comstar" rims in 2.50x18" & 3.00x18" rear, OR a GL1100A front Comstar wheel in 2.50x18" with a stock rear wheel, or ideally 2.75"x18" from the CBX pro-link models. OR - the wire-spoke rims from the CB750K models. IF you're gonna stick with the rear drum brake, but you want the boomerang rims - it would be interesting to use a rear rim from the XBR500 with it's smaller lighter rear drum - the neat thing about Comstars these days is people are rebuilding 'em with parts from a broad selection of disparate rims! Ah, but probably far more reliable to rebuild some WIRE SPOKE wheels using the hubs from certain '79-'82 CB750K versions. Incidentally, you're probably better off to go with a front hub from GL1000 and a rear drum from just about any of the SOHC 750's provided you use the '78 sprocket carrier or the carrier from the 'C model or 'K model DOHC 750's - if you can change the off-set with a specialty rear sprocket then the CB500 rear hub is smaller and lighter and commonly used on CB750 SOHC racers etc.


-speaking of chain-line off-set, the pro-link ('81, '82) CBX six used a sprocket which fits the DOHC-four motor but kicks the chain something like 5mm to the left, so if you're doing a wheel swap of any type this is helpful stuff! The RC30 & certain other V-four Honda sprockets also fit the same spline, as did some Yamaha FZR750 sprockets so you could play mix-&-match to make a wheel swap work.


- as for rim sizes, the DOHC-four will take some decent fat rear wheels, people put 170/55-17 rear sport-bike wheels on 'em all the time, they'll also take a 160/60-18 sport touring radial provided you've got a wide enough rear rim ie 4.25x18"-4.5x18"-5.00x18" for this size.


- the stock CB750K wire-spoke rims are 2.50x18" front and 2.50x17" rear (same as the GL1000 rear and CB750K '78 or CB750A Automatic's rear rim, only THOSE bikes have the rim in ALUMINUM which would be a great swap if you're looking for a replacement in stock sizes!) If you're gonna re-lace or rebuild these wheels AT ALL they deserve to get some nice alloy rims there are 40-hole Supermoto rims to play around with (I scored some Harley spec 4.5x17" & 5x17" rear rims for $40ea on eBay) the front 2.50x18" is pretty much the same as a rear rim from early '70s Harley and they're Aluminum un-marked BORRANI Italian rims, so you could use one I'm using one right now - there are also 3.00x16" chopper rear rims, in matching Borrani patterns are the best to go with but there are also Akront and the best of all "Super-Akront" both in 3.50x16" which is a great rim if you're really digging the "CUSTOM" vibe of your CB750C


-stock wheels on the CB750C are what? 2.15x19" & 2.50x16"? TERRIBLE - the rear wheel pinches the tire, causes it to wear prematurely. I think some of the last year CB750C's might have the 3.00x16" from the CB900C or CB1000C spec, but even so, there's better stuff out there so if you're replacing a wheel don't just replace it with the same stock wheel from another CB750C!


-The bodywork doesn't all fit back and forth between all three models, but the tanks swap back and forth for the most part. The CB750C & CB750K have some pretty nice seats for which there are some pretty nice replacement covers, so why would you even WANT the 'F bodywork? Ha-ha.


-For REAR-SETS there were indeed some "Sport-Kit" peg brackets made for the 'K and even the 'C which are possibly the rarest of all types - far better just to drill & bolt through the frame tab which the passenger pegs are mounted off of. Simple generic universal rear-sets would work. I suppose the CUSTOM folks like to go with "forward controls" too, but that's just like fitting rear-sets they just go in a different spot so WTF.


-There's a European version aka "Sport-Kit" throttle & switch-pot which has cables pointing forward rather than down, so if you wanna fit lower bars and you're worried about denting the tank or even thinking of denting the tank DELIBERATELY, you should look at these "Sport-Kit" throttles. I'm trying to mod alloy pots with new threaded bosses on the front/top for the European style cables.


-The rear disc brake is pretty heavy, so if you're thinking it's a performance upgrade just think about how much you USE your rear brake and rethink it! It really complicates the rear pegs more than anything else.


-The CB750C or CB750C SWING-ARM is actually a desirable part for the CB750F 'cause you can use it to fit an UNDER-SLUNG CALIPER which can be gleaned from GL1000 & GL1100 models (though you might want to drill & tap another bleed hole for bleeding the brake properly) Always worth having these things braced by your welder friend if you're gonna keep your stock steel swinger. For the CB750F the 900 had a stronger swing-arm - I guess they look the same but it's thicker tubing or something. The CB1100RB & CB1100RC had an even stronger version, even thicker tubing, but pretty damn hard to find. You might be able to acquire a CB1100F one and swap it with somebody who wants to upgrade their '81-'82 CB1100R to '83 spec. I know I'd rather have the tubular version than the square one from '83 CB1100F/CB1100R....


- DON'T under any circumstances ditch your air-box and replace it with POD filters!
(((Not unless you're replacing the carbs themselves with some type of straight-pull NON-Constant-Velocity type carbs or better still fuel injection system!))) Because pod filters will cause a huge "flat spot" in the middle of your RPM range, where you're supposed to be making some really great power.


-Four-into-one exhaust pipes, pretty much any of 'em 'cause they'll save a butt-load of weight. If they help make more power then so be it. Saving weight should be the greater concern!


-Get a lighter gel battery when you replace your stocker - by all means don't spend good money on a ridiculous heavy one when a couple more bucks will ditch that much weight off the bike!


-Look up "The GM coil mod" on the CB1100F.net 'F-orums, 'cause they're an improvement and you can find 'em in just about any junk-yard.


-LED signal lights and running lights will save a huge amount of juice!


-Don't screw around with squeezing that extra 10mm worth of tire width onto a too small rim. My CB900F came to me with a 140/80-18 on a 2.50" rear rim, the P.O. thought it was sooo much better. But the tire's completely PINCHED in that rim, so it's worn down way too quickly. Ridiculous. There's absolutely zero discernable improvement in the ride, other than the fact that the 140-spec tire has gotta be three or four pounds heavier than the 120 or even 110 that I would normally have fitted to that size of rim. If you want the wider tires, go with wider rims. And if and when you DO that, try & offset some of that extra tire weight by going with a lighter type of wheel. Comstars have some pretty nice light alloy rims in thinner construction than their cast/mag competitors, but the rear hub is a beast. Some wire-spoke hubs with alloy rims would be a heck of an improvement. Heavy-duty spokes to keep 'em nice and stiff, and watch what brand of rim you go with 'cause some of 'em are really thick and just as heavy as steel rims anyhow. 40-spokes is pretty standard on wheels for bikes this size, but go with higher spoke counts and the rims are more prone to crack between the spokes and valve hole. Hence the manufacturers put even thicker rims into the wheel. Bought some KZ1000CSR & KZ650CSR rear wheels for a project bike and found their 3x16" rims weighed more than double what the 3x16 Borrani & even 3.5x16 Super-Akront weighed.


-Either which way - enough about rims, but the point being the fatter rear tire thing can work if you do it right but trying to take short-cuts or fake it, only makes it worse!


-And the main thing to do with the CB750K or CB750C or CB750K or CB900F models, ANY of the DOHC-4 Hondas, is to put 'em on a crash diet and lose some friggin' WEIGHT. Never mind about the fat this or heavy-duty that - just ditch the boat anchors, get out the bailing bucket and dump out some water.....


- My original point: If you're gonna rebuild a DOHC-4 motor or have one rebuilt FOR you? Pick one that's worth your while. For the same price in replacement parts (or in this case, probably a lot less 'cause you'll find a motor that's less damaged!) you could have a real hot-rod engine.


-ALL of these engines are inter-changeable, you can swap 'em from frame to frame. So you see, instead of boring out a 750 to 836cc's you could have a bone-stock 901cc engine straight from the junk yard. Though, rebuilding a CB1000C engine with CB900F output shaft, or CB1100F engine - both of these are getting scarce. As such, there's definitely some value in rebuilding a 900 as a 985cc! Wiseco 985 kits were really cheap on eBay recently.


-Back to wheels again for a minute. There used to be companies like "Kosman Industries" which widened stock bike wheels with a welded-on section on either side, cut from a billet wheel blank OR a standard wire-spoke alloy rim. But I guess they're not doing it anymore. There are a couple of folks who still do this type of mod.


-Far better to go with wire spokes, as I was saying. However the selection of HUBS which are suitable is pretty thin, and most of 'em are really heavy. Well, DRUM brakes have a pretty good selection, actually. It's the DISC brake versions of wire-spoke rear hubs which are so limited. These bikes use a heavy-duty "CUSH-DRIVE", 20mm rear axle, use 40-spokes, and have a disc-brake. If you wanted 20mm axle, disc-brake, and 36-spokes to suit a 17" SUPER-MOTO rim in 3.0x17" or 3.5x17" (Supermoto front) or 4.25x17"-5.00x17" (Supermoto REAR) you'll find only NON cush-drive versions of hubs in 36-holes with a 20mm axle and cush-drive. There are some light-weight versions on 17mm axles from the likes of Yamaha TDR250, there might be some way to make that work. 'Cause lots of the later swing-arms people swap onto their DOHC-four Hondas actually HAVE a 17mm axle. Even so, that's a pretty tiny rear rotor and only has three bolts. Not too many other discs that would fit it. The Yamaha TZ750 rear hubs look great, but have no cush-drive and cost an arm and a leg. So while DRUM HUBS have some great selection and plenty of options, the DISC hubs are limited to SOHC CB750F, GS1000, KZ1000A/KZ750B, and later KZ1000LTD or even KZ1000CSR etc. Of these, the KZ1000A/KZ750B is the lightest of the bunch, seem the most versatile for brake rotors too (fits a front comstar rotor and one bolt hole lines up perfect, just needs three more bolt holes drilled for a five hole disc on a four hole hub) But it's a composite construction with a steel flange glued to an alloy hub center, which makes me nervous. One OTHER option is "the front hub trick" used on a lot of CB750K '69-'78 drag-racers, it's just a front hub in a rear rim with a sprocket adapter bolted to it. Well there are some very cool ways of adapting a cush-drive to it, where you'd wind up with a very light-weight rear hub. And late model HARLEY REAR HUBS are Aluminum and some of 'em have an integral cush-drive which normally comes with a belt-drive pulley however there are chain-drive conversions for Sportsters etc. Seen a few rear wheels built that way. Seemed VERY light-weight. But hey, seriously on a CB750K or CB750C I'd KEEP the rear drum. If they made one big enough, I'd want a good 4LS drum up front as well!


I've got a 39mm CB900F fork here which I've adapted with a Suzuki GT750J 200mm 4LS drum, but it's going onto a KZ440LTD, a project for my kid called "KZ440LOL" as in "laugh out loud" - it uses 3x16" Borrani alloy rims with maxi-scooter tires in 110/70-16 & 140/70-16, all NOS belt-drive, etc. It would be interesting to stick it on a CB750K for her to ride on the highway, but I'd much rather keep this dinky little 200mm version on a smaller bike, and use at least a 230mm 4LS hub on a Honda four, preferably a 250mm Fontana or 260mm Yamaha racing hub on the 750 if I had my druthers. 230mm would probably cut it though.


There are folks these days using the CBX550F internal-disc hub from comstar wheel, converted for wire spokes. That one might be adequate for a 750 - but for my 900 I wanna build something similar from GL1500 Goldwing and PC800 Pacific Coast parts, with DIY rotor shrouds..... A heck of a big project, but it's a fake drum brake with 286mm rotors and twin-pot floating calipers in a 41mm TRAC anti-dive fork, integrated fork brace & hollow 20mm axle - pretty good upgrades. The OEM rotors on the 900 are only 276mm so this would be an upgrade too. However I've already got the 296mm rotors from CB1100R on my bike, so I'd wanna have some custom rotors made for this fake drum hub project, and that would make the whole project another degree of magnitude more complicated. Even so - it would be pretty damn awesome to keep a DRUM BRAKE look on the front end of my CB900F Bol D'Or café, homage to the '65 CB450K0 Black Bomber, aka "CB900K0 Bol Bomber"........
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Old 04-09-2015, 08:33 AM
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Default Re: 1981 CB750 "DOHC" ISSUE

That's a fricking epic post. Seriously, nicely done. As an '82 CB750C owner, you've gone through about 90% of the stuff I've already been considering and investigating and provided more background and details than I've been able to glean from weeks of searching. Saved for later reference and many thanks!!
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