.020 off cylinder heads is htis a problem?
 

I am having my heads rebuilt and to get a good surface on the bottom of the heads where it meets the cylinder 20 thousandths had to be taken off. The engine is a 1980 SC 3.0 with 9.3:1 compression. Is it necessary to shim the bottom of the cylinder or machine chain housings?

thanks-Matt

Half a millimeter is quite a bit, you'd probably have to shave the chain housings for that much. Would require checking piston/valve clearance as well. Benefits - you'd have a nice bump in compression.

Or you can just stick a 0.50mm shim on your cylinder bases and forget about it.

Thats a lot, I think the spec is only .010 max, they must have really been bad, usually I can clean them up with 2 or 3 thou. Were they pitted or grooved? Was it just one or all, usually you take the worst one then match the rest.

Ollie's had to take similar off mine for some taper, and I've got .020" sealing rings between cyls and heads to compensate. Or, you can shim at the bottom as Dave mentions.

Yeah does seem like a lot, just got a set back that was shaved .008" and they said that's double what they usually do.

they were grooved pretty bad, I guess my best bet is to get a shim. Does anyone know what my compression increase would be and if my valves would be too close to the piston?

You really need to manually check the piston-valve clearance. I'd think that 1/2 a mm may get you into the danger zone. But there are others here with more knowledge about this.

You'll need to check the clearance between the head and piston as well as the valve to piston clearance. Hopefully they chamfered the heads to keep the relative clearance from the pistons. If you use a .5mm base gasket you'll still have .25mm improvement in compression and not have to machine the chain housings. Clearance will probably be good this way also.

-Andy

also, the cam chains get "longer" not sure in this exact case but I think it can be an issue running out of travel on the tensioning

Lemme try to calculate your new CR:
bore=95mm, stroke=70.4mm

V1 Swept volume = pi*9.5*9.5*7.04/4 = 499 cc
V2 Head volume = 90 cc for a 3.0
V3 Piston dome volume ~ 40 cc (to be verified)
V4 deck height volume ~ 10 cc (to be verified)

Then old CR = (V1+V2-V3+V4)/(V2-V3+V4) = (499+90-40+10)/(90-40+10)= 9.31:1

If the deck height is reduced by 0.5 mm, deck height volume is reduced by 4.75*4.75*pi*0.05=3.54 cc, so new V4=6.46 cc

Then new CR = (499+90-40+6.46)/(90-40+6.46) = 9.83:1

If you add a 0.25 mm shim, you have:
V4= 10- 4.75*4.75*pi*0.025 = 8.22 cc

Then CR = (499+90-40+8.22)/(90-40+8.22)= 9.57:1

That is a ballpark estimate of the CR change, if I did not mess up the math...

Aurel

i'm impressed; but are the calculations correct ;)
munchovie

First time I do it. Someone "in the business" better check them. ;)

Aurel

This is what I used because I am not smarter than a 5th grader.

http://www.csgnetwork.com/compcalc.html

or this one too:

http://www.rbracing-rsr.com/compstaticcalc.html

Increasing the compression ratio is good for increased torque and mileage; NG if there's inadequate clearance between the moving parts.

The minimal labor method would be to insert cyl. base gaskets equiv. to the amount removed from the cyl. head to re-establish factory internal clearances.

To take advantage of the slightly increased compression ratio requires more checking and potential metal removal:

- Check piston to head clearance: Place modeling clay on the top of a piston, install head with used gaskets, then carefully rotate engine (w/o valve opening). Machine accordingly.

- Check piston-to-valve clearance: Place modeling clay on top of piston, install cyl. head with valve train. Use rocker arm adj. screw to open valve until it hits the piston at TDC. Clay determines clearance area. Compare with lift specs and min. recommended clearance. Machine accordingly.

The procedure should be in Wayne's rebuild book (among other pubs).

The add'l machining could be expensive, minimal or not required.

Chain boxes must also be shaved an amount equivalent to that removed from crankcase, cylinder and cylinder head to ensure the cam is centered in the chain box opening. Otherwise, an oil leak is a possibility.

Sherwood

I think my best bet is going with a .5mm base gasket. That will be a decrease of .010. I will have a tiny bump in compression and I should not have to worry about clearences although I will check them. I also don't think I will have to machine the chain housings. When I origianlly started the post I was unaware that Pelican Parts sold the thicker base gaskets.

Hi Sherwood,
Where would the oil leak occur?

Thanks,

Dave,
Not to steal Sherwood's thunder - but this leak occurs at the cam seal, due to the cam tower and chain housing being offset. The cam is no longer centered with the thrust plate and seal, allowing oil through.

No problem Dave. Saved me a few words.

Sherwood

A stock "9.3:1" will actually measure out less. Never use the factory compression numbers as a basis for calculations or decisions on what is "safe" as the factory numbers always seem to be high.
-Chris