O.K. so here goes; This is not Honda related, I need some help from some people with experience with stroking/destroking and who can crunch some numbers for me. The engine will be used for F.I., tho!

I have an engine with a 3.5812" stroke. (7M)

I have a crank that will fit this engine that has a 3.3448" stroke. (5M)

Assume both blocks are identical for both engines.

The 5M rods are .125" LONGER than the 7M rods. BUT, the 7M wristpins are .125" lower in the 7M piston.

What I want to know is:
Can I use the 5M crank with the 7M pistons and rods?

If not, please tell me what will be "off"

Like, "the piston will sit .2364" lower in the cylinder"

and, what will this do to compression? Stock compression is 8.4-1

Random info you might need:

Rods: 5M=6.105" 7M=5.98"

Stroke: 5M=3.3448" 7M=3.5812"

Assume deck height and everything else to be the same.

I'd want to use the 7M rods and pistons because of their much improved strength/beefyness, and better ring area design...

Please help me out Honda-Tech!
Thanks for your time,

Alex

Cliffs: 5M crank + 5M rods + 5M pistons=works (2.8l)
7M crank + 7M rods + 7M pisons=works (3.0L)
5M crank + ? rods + 7M pistons=teh win?

 

c'mon now...

this is cake for someone in here... I know it is...

I just can't wrap my head around it...

 

Bah!

 

To tell you that most people will need some more information.

Deck height, compression height on the pistons, etc.

 

As I stated, assume everything is the same(a non-issue) Anyone with some experience?

 

Well I can't tell you how far its going to be sitting in the bore unless I know the deck height.

 

 

I'm not sure if this is 100% accurate...but hopefuuly it will provide you with whatever you need to figure this out.
Thanks.

 

So if the difference in the stroke is .2364" (6.00456mm), all else being equal, the piston will sit .2364" lower in the bore. Right? All things being equal.
Was it that simple?

What will this do, all things equal, to the compression ratio? (originally 8.44-1)
This would be worst case scenario, as there will be lapping of the head and block, and I can run a thin MLS head gasket.

Anyone?

Anyone...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JalopySiR &raquo;</TD></TR><TR><TD CLASS="quote">
The 5M rods are .125" LONGER than the 7M rods. BUT, the 7M wristpins are .125" lower in the 7M piston.
</TD></TR></TABLE>

ok let me get this straight

5m rods are longer by .125 because of the stroke difference. but the 7m pistons have the pin lower by .125

here i see a problem.... rods being longer = farther up in the bore(if that was the only variable) but since the storke is shorter... maybe that will even out. you have to do the math on that and check.

next the 7m piston pins are lower by .125 meaning the piston will be higher in the bore at TDC then the 5m


if these engines deck heights are the same then most likly you wont be able to use the 7m pistons. on 5m rods

simple version..... from what im reading

5m rods longer to make up for the stroke difference(maybe that evens out maybe not, thats what needs to be figured)
7m piston pins are lower, raising the piston in the bore. if everything else is the same this will be the interesting part, too high?

***second thought***

so far i dont think the hybrid rod/piston idea will work

if both these engines are the same, i.e. deck height. the fact that the stroke is shorter on the 5m, you would have to run the 5m rod and pistons because of the sizing differences, thats what makes up the difference to TDC between the cranks.

short version

5m crank = shorter stroke = longer rod to get to TDC in same block = 5m piston to match 5m rod to make it to TDC.... otherwise the 7m piston would be .125 too tall

so from what my brain is wrapping around... i would say no... sorry. unless you got custom rods

so far the calculations dont make sense the pistons and rod measurements should make them close to even..... but then factor in the crank stork being different and it wouldnt work

i have to venture to guess the deckheight on the blocks are different... that is the only way it would make sense.

if we assume they are the same it will not work with these measurements.

im getting confused by this i need to stop reading it....


Modified by pornstarSR at 7:52 AM 6/30/2005


Modified by pornstarSR at 8:08 AM 6/30/2005

 

If I figured this right we have:

Stroke of: 3.3448"
Deck Height: 9.075"
Rod Length (7M): 5.984"
Rod Length (5M): 6.109"
Piston CH (7M): 1.299"

So, if we divide the stroke by 2:

3.3448" / 2 = 1.6724" above crank centerline at TDC

Add in the rod length:

1.6724" + 6.109" = 7.7814" to piston pin centerline

Then, we add in the compression height of the piston:

7.7814" + 1.299" = 9.0804" to top of the piston.


Modified by civicross at 11:27 AM 6/30/2005

 

I got confused myself. I had to dumb it down alot to grasp "what I think"

I "know" the blocks are the same...

I know an all 5M rotating assembly would work, and I know an all 7M rotating assembly would work.

What I WANT is a 5M crank, with the 7M rods and pistons. (as they are stronger)

What I ~think~ will happen is: the piston won't quite make it to the tippity-top of the bore. It will sit a bit below the deck. What I don't know is: how much lower it will be, and what the effects of this are. i.e. compression ratio, etc...

I also think the post above this answered the question, but I really suck with the old numbers...

Civicross, please dumb this down a bit for me, and spoon feed it to me.

 

i talked to a few others today at work.. and we came up with this..

the 5m and 7m piston and rod are the same height(side to side)... cause of the .125 differencees in different areas.

so this leaves the crank as being the variable....

with both pistons and rods being the same height... the stroke difference would determine the height in the bore that the piston would go. So with that being said.... the blocks cant be the same.... the deck heights have to be different

other wise the 5m pistons and rods combo would be lower in the bore because of the stroke difference(from the number we had we got .125 lower, might be alittle different than your numbers but its close enough).

this is what we think... it would work if you wanted to use the 5m crank, 7m pistons with the 5m rods in a 7m block.

the blocks cant be the same if your numbers are right....

 

I don't think the blocks are the same...

 

I'm 99% confident in the rod/stroke info in my original post, as it did NOT come from SupraForums. The spreadsheet pic is from SupraForums, so it is thrown into question, as they are retarded over there.

It has been ~stated~ as fact that the blocks are identical, and people have "said" that 5Ms have been stroked with an all 7M rotating assembly, and one source said it was common in Japan to swap in a 5M crank to "improve the feeling" of the motor... You know those wacky Japs.

Wierd fact: stock 7M pistons stick out of the bore ~.012-.020 on the 7M. I have actually seen pictures of this.

For all intents and purposes of this discussion, let's just ~say~ the blocks are the same, and that all the other numbers are right (which I'm not 100% sure of).

The 5M crank + 7M rods and pistons would have the pistons sit lower in the bore, right?

What does this do to the c.r., etc...?

I'm going to go see what else I can did up...

 

you will lose quench.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JalopySiR &raquo;</TD></TR><TR><TD CLASS="quote">I'm 99% confident in the rod/stroke info in my original post, as it did NOT come from SupraForums. The spreadsheet pic is from SupraForums, so it is thrown into question, as they are retarded over there.

It has been ~stated~ as fact that the blocks are identical, and people have "said" that 5Ms have been stroked with an all 7M rotating assembly, and one source said it was common in Japan to swap in a 5M crank to "improve the feeling" of the motor... You know those wacky Japs.

Wierd fact: stock 7M pistons stick out of the bore ~.012-.020 on the 7M. I have actually seen pictures of this.

For all intents and purposes of this discussion, let's just ~say~ the blocks are the same, and that all the other numbers are right (which I'm not 100% sure of).

The 5M crank + 7M rods and pistons would have the pistons sit lower in the bore, right?

What does this do to the c.r., etc...?

I'm going to go see what else I can did up...</TD></TR></TABLE>

If the stock pistons stick out of the bore .020" then the setup you want to do will work, because they sit only .008" out of the bore.

 

So, your math says they will stick out?

Why do I think they will be lower?

Can you break your math down for me?

If PornstarSR's comments are accurate, and the differences in the piston pin locations and rod length cancel each other out, then the shorter stroke would cause the piston to sit lower in the bore, correct?

 

It does sit lower in the bore. You said the stock pistons in a 7M block stick out of the bore about .02". The 7M pistons/5M rods and crank will only stick out of the block .008", so this would be less than stock.

 

O.K. so I did some math.

There were 2 rod lengths for each rod that people did math on.
Check back in the post to see the variance.

Anyway, ALL 5M assembly using CivicRoss's calculations:
(1.299 as compression height)
9.0804
9.0764

ALL 7M:
9.0736
9.0696

The only things I changed were the rod lengths and strokes.
I did not change the piston compression(pin) height from the chart.
Is this number (1.299) affected by the differences in pin height on the pistons?

If so, I got 4 numbers by adding or subtracting .125 from the 5M numbers from above.

If I add .125 to the 5M numbers I get: (1.299CH+.125)
9.2054
9.2014
If I subtract .125 I get: (1.299CH-.125)
8.9554
8.9514

Here's where things get interesting.

Using CivicRoss's calculations with a 5M crank and 7M rods/pistons I get:
8.9554
8.9514

These are exactly dead on to the all 5M assembly if I subtract .125 from the compression height.

If the 5M pistons have the pin .125 HIGHER, than the compression height would be that much SMALLER, correct?

This math leads me to believe you can simply swap the crank.
I'm sure I screwed up somewhere, so please go back and check my math for me.

 

O.K. so the all 7M assembly is damn close to what the deck height was supposed to be for that motor.
So by using that as deck height, and subtracting 8.9534 (avg. of the 5M crank/7M rod/7M piston combo) from it, you get ~ 3mm.

I forgot the exact numbers if you are asking why we switched all of the sudden to mm.

Anyway, sitting 3MM below deck height doesn't seem like that big of a deal to me, as even HKS supplied a 3MM headgasket, granted that was for compensating for the surface grinding of the head and block to prep for a MLS headgasket.

Milling those sufaces and running a thin-assed MLS headgasket should account for the difference for the most part. I really don't mind lowering the compression a bit as I ~plan~ on running over 25psi...on 93 octane...yeah, I'm crazy, I know.

Can anyone tell me what's going to happen to the compression ratio?

 

Ive helped several people with their 7mgte's and you simply swap the crank and either shave the head or get a thinner headgasket to compensate for the lower compression ratios.

or leave it and raise the boost.

They swap for the 5m crank because it has a smoother rotational feel with the tansition into higher than normal boost pressures. have fun!

 

Is anybody other than myself finding a piston sitting more than 1/8 in the hole pretty hard to swallow?

You mentioned decking the block at least 1/8 and running a thing gasket, what timing belt did you plan on running. Taking that much material off of the block and running a thing gasket = no way in hell the stock timing belt will work.

Your compression wont be as low as your counting on if you remove that much material from the block and run a thin gasket so your kinda doing a lot for nothing.

With out decking 1/8, sitting in the hole roughly .123, yeah your compression will be low but thats going to suck *** for making power. Im also having a hard time swallowing 25 PSI on stock pistons as well, just because your compression is or desired to be low doesnt mean the pistons can handle that much boost.

As stated before, sounds as if your trying to run a 5 mile 1/4 mile

 

Please re-read the whole thread.
Then repeat.
You are missing many key, valid points.
I also find it rather immature that you dug up this thread, and tried to bring it down, as you did with the one in the all-motor forum.
You've since put in your .02 in two different forums.
You don't get it. I understand that.
You can leave now this thread now.

 

Explain your EXACT intentions and reasons then??

Explain your "key valid points" as well...

Because as I see it, its a pretty dumb idea