help needed shim cylinders or not after head work

[gorskined]

i just got my 76 row turbo heads back from the machine shop the motor is pretty much stock running sc cams carrera tensioners with a half bay andile long neck intercooler. european high flow headers and a ss steel exhaust with a k-27 7200 turbo .
SO it took them two cuts to get the heads perfect. they removed .25 mm taking the head down to the min 84.25mm ..
what should i do ???
i can simply use a .50 cylinder base gasket to compensate for the cut and this will bring my compression and piston height back to original. .
or
i would like to use the standard .025 mm base gaskets to add a little compression as the flat top piston comes about 1/4 inch from the top of the cylinders anyways from what i understand the 76 turbos are stupid low compression
like 6.5 to 1
if i use the .25mm gasket will this cause a valve clearance issue ????? or have i read to many performance books that don't apply to this engine ?
also if i do use the .25mm gasket will i have to machine .25mm off the cam chain housings or will the o-ring pick up the slack?
if i use the .25mm gasket what will the new compression ratio be ?

Thanks in advance
Regards Ned

[356911930]

Ned,

My experience as a simple DIY guy is not as great as some of the experts we have on this board but, I think either of your plans would work.

If you go with the 0.50mm base gasket you should theoretically be back to stock.

With out an intercooler or EFI you might want to stick with the 6.5:1 static compression ratio.m Higher dynamic compression could cause detonation. Equal length headers and an SC cam will be way better than a risky compression IMO.

If you go with the 0.25mm base gasket you will be increasing the compression but I do not know how much. Since you stated that the minimum height is 84.25mm, and this is where you currently are, I ASSUME that this is the minimum due to moving the cam tower closer to the intermediate shaft resulting in some extra slack in the timing chains. Your chain tensioners should be able to take up this slack and still have some room to stretch as the chains wear.

Others with more engine building experience will chime in and we will wee if I am correct.

Best of luck with your rebuild,

Rahl

[gorskined]

Originally Posted by 356911930

Ned,

My experience as a simple DIY guy is not as great as some of the experts we have on this board but, I think either of your plans would work.

If you go with the 0.50mm base gasket you should theoretically be back to stock.

With out an intercooler or EFI you might want to stick with the 6.5:1 static compression ratio.m Higher dynamic compression could cause detonation. Equal length headers and an SC cam will be way better than a risky compression IMO.

If you go with the 0.25mm base gasket you will be increasing the compression but I do not know how much. Since you stated that the minimum height is 84.25mm, and this is where you currently are, I ASSUME that this is the minimum due to moving the cam tower closer to the intermediate shaft resulting in some extra slack in the timing chains. Your chain tensioners should be able to take up this slack and still have some room to stretch as the chains wear.



Others with more engine building experience will chime in and we will wee if I am correct.

Best of luck with your rebuild,

Rahl

thanks for the info i did up date my original post to mention a few critical points with the engine

[lite75]

Just a consideration, milling material from the sealing surface and reducing the CCs from the combustion chamber and adding to the cylinder height isn't a 1 to 1 ratio, your compression will actually be less. Here's a good link.

https://www.summitracing.com/popup/calcsandtools/compression-calculator

[T77911S]

with the thicker gasket i thought it would reduce the CR because now the piston deck height will be lower.
so now the cylinder will effectively "fill in" what was removed from the head thus increasing the chamber size

the thicker gasket will also line the cam back up to the chain housing. I don't know if the thinner gasket would make a difference though.

just how I understand it.

[356911930]

Ned,

It looks like you good to go with the 0.25mm cylinder base gasket and a small bump in compression due to your intercooler, I would think.

Rahl

[spuggy]

I believe the deck height is the critical thing that should be checked and adjusted to be in spec with any machining or base gasket changes. But that's about the extent of my knowledge

That said, the only oil leak I've ever had on my 930 motor was a base gasket.

About 15,000 miles after rebuild from a dropped factory con rod bolt (915 money $$hift), noticed a couple of oil drops on the garage floor one day. Under the car, a good 3/16" of base gasket was poking out from the cylinder base on #5 near one of the corners - really thin; like a feeler gauge or paper.

Everything else seemed fine. Just looked like that one cylinder moved and destroyed the gasket.

Shop (different one) thought it was really weird; but suggested we didn't use really thin gaskets... Sealed with 1mm base gaskets all round and it was never a problem again. No clues as to the original issue found during disassembly or re-assembly either..

[gorskined]

looks like i will do a mock up and see exactly what my clearances are the general consensus is that the original style .25mm gasket should work just fine . The fact that if i go to the .5mm gasket would put my compression even lower than stock tipped the scales for me ..
now off to replace the piston oil squirters i have two stuck open and 4 that wont spray any words of wisdom on that..... off to search some old posts for tips.

Regards Ned

[jwasbury]

Originally Posted by spuggy

I believe the deck height is the critical thing that should be checked and adjusted to be in spec with any machining or base gasket changes. But that's about the extent of my knowledge

^2nd this. You must measure up the deck height, and select a base gasket that will achieve the desired spec. 1mm is a good target. Its possible to run a tighter deck, but you have to carefully check for interference (valve to piston, and piston to head). Too much deck height is also not good.

[David]

I do a mock up with solder on opposite sides of the piston at the cylinder wall to check the squish band clearance then set the base gasket to get the required clearance. Rotate the engine over to squish the solder, take it apart, and measure the solder thickness. It's what I did when building kart engines so I just do it on 911 engines too.

Too much or too little clearance can both cause detonation so best to be spot on.

[Ollies930]

If you do the math, then you will see that the compression change is insignificant. You are going from 6.5-1 to 6.6-1 (assuming that the original stated compression ratio is correct, usually lower).

[Ken911]

you might run into a stack height problem, especially if you also got the cam towers cleaned up. when the cams are installed make sure they turn freely after the seals are installed on the ends of the shafts. They will rub on the camshaft flange cover if too much is taken off. At that point either the cylinders will need shimmed or some material removed from the chain boxes or the inner diameter of the camshaft flange cover.