Alright, so I have a B17, left it std 81mm bore (Or so I hope), got it honed and hottanked and got it back from the shop. Bought NPR std 81mm piston rings. Bought std 81mm CTR NPR pistons.

Right now, ive only checked the top ring gaps, will be editing this post within 30 mins with the rest of the specs, but so far this what im seeing, and this sounds to be too loose..

Going to be a N/A DD setup.

Top Rings: 0.020"
Second Rings: 0.019"
Oil rings: 0.020"

Doesnt make sense why my 2nd rings would be TIGHTER than the top ones? Seems it should be other way around, and im certain im looking at the right ones..

The formula for figuring ring gap would be 81mm = 3.1889 x 0.006 = 0.019 for the top right? And then 0.019 + 0.004 = 0.023 for the 2nd ring. I may be wrong... So if this is true, im not to bad out of shape, so really need a answer from a pro on this to have piece of mind.


Is it possible that the rings just come like this, I mean I can take some off but I cant put some on.. The reason I said Or so I hope at the top is because I knew I needed a dial bore and this gives me a good reason to get one, gonna order one tonight & a micrometer and check to see what size the cylinders are, he said they were left std size and just honed though, though he did mention they were in bad shape and needed the honing pretty bad, so i dont know if he went to far over or what and if I need to bore it to 81.25 or what or if this gap isnt a terrible thing after all.

 

the forumula doesnt have you add to get the gap for the second ring, you just multiply bore size using a different factor. .020 and larger seems big for an n/a 200ish whp motor @81mm bore. honestly if you are not planning on spray or anything id use like x.0045 for the top gap and .005ish for the second gap....so maybe around .015 top ring and .017 or so for bottom ring. anything over .015 is good for oil rings



what was the piston to wall clearance that your machinist came up with after the hone?
for you goals i think it should be around .0015 or so.


when measuring the gap you've got the ring down around halfway into the cylinder and square to the bore right (parallel with the deck surface)

 

that's one mistake of not owning a decent .0001" bore gage and mic set when assembling and measuring these motors...

before you start gapping and filing your fits the bores have to be measured out come back to us with bore measurements and go from there my friend...further help can only be given with proper information...there are no psychics on here...

 

Minimum end gap is much more important than the maximum.A few thousands to much gap will not effect performance or reliability.It has been generally accepted to gap the top and second ring the same for many years now.Some builders actually gap the second ring larger than the top.This has been proven to make a better ring seal than the old method.The thought is that pressure build up between the rings lifts the top ring off the bottom of the grove allowing pressure to get around the back of the ring and between the two rings.This causes the top ring to "float'.
Anyway those gaps will be fine as long as your piston to wall clearance is reasonable.

 

All I did was get the damn block honed by a reputable shop, and put the rings in the bore how they came out of the box? Why would I NEED to have a bore gauge before hand if at the most I would have to just file some rings, but instead it looks like more than that.

The bore I have, and the rings untouched already being 0.020" on the top, how are you going to be able to help me if the machine shop ****ed it up? If I get a bore gauge and measure it and its not 81mm but the ring gaps are to big but its not atleast .25 over, then what the hell can i do? bore it to .25 and sell the pistons and rings and get new pistons and rings right? that would be my only choice from what i see unless its just slightly over 81mm on all cylinders and the rings just happened to come gapped a tad loose from the box.

 

Either way, tomorrow im gonna place a order for a dial bore and a micrometer to get the piston to wall clearance checked out. What exactly should I be shooting for on this? Ive read all kinds of different things on this particular sort... From what i've seen, .0025 seems to be around what i'd be wanting? Once again all motor N/A DD, no spray or nothing like that, occasional track runs.

 

Absolutely agree with the above. Big ring gaps are no big deal because even when several times over the supposed maximum, the gap area through which pressure can escape is very very small relative to the quantity of pressurised gas, and considering the very short time period in which the gas has to escape.

The effective gap area is found by multiplying the gap X piston to bore clearance, not the gap X ring section depth or width (i.e. the 'escape' area is the gap X the distance by which the ring protrudes past the ring land to the bore, which will always be very small, unless something strange is going on...).

Can't recall where but I've read a report on tests conducted with increasingly oversize ring gaps, and according to the results of this testing the gaps were something like 5X manufacturer specified max before any significant change in compression or performance was measured (and then were very small losses).

Very commonly oversize gaps are found in engines that have lost compression, and are then often blamed for the compression loss. However this is guilt by association, the compression loss will always be from another cause, most commonly from rings sticking in the grooves (carbon, varnish, general sticky crud build up), in which case hot gasses get past the ring which then further loses contact with the (relatively cool) bore and then as a result overheats and loses tension...

When this happens the ring loses it's seal around it's full circumference, so compression loss is substantial (and failure often quite rapid), as opposed to a ring gap, which can never flow substantial amount of gas (even when significantly oversize).

I've heard this argument, and it makes some sense to me (though I'm not 100% convinced that pressure between the two rings is likely to become great enough to cause a problem...???). In any case I can't see that it would hurt to increase the second ring's gap just in case, especially since significantly increasing the second ring's gap can't hurt compression to any significant degree...