Compression ratio question

[brs127s]

I could have asked this in my gasket post, but I though I would keep it separate.

Last night, my buddy and I milled the head on my 83 GS850 to make it true. He took off .004 of an inch. Can anyone tell me what kind of effect is this going to have on my compression ratio? Is it a small enough amount that it will have no affect on it? How will I be able to tell what the effect is?

Thanks,

Brandon

[Motorcyclemodeller]

.012 thou decking raised my 10.25:1 weisco kit to about 10.75:1, this is a rough estimate.

Therefore .004 should raise you from 9.5:1 to about 9.7:1 very minimal.

Make the horsepower where you can. You can check by doing a compression test after assembly it should raise the PSI up from stock by a few pounds.

8) Andre 8)

[JJ]

You could measure the combustion chamber volume and calculate the CR yourself. I don't remember the formula right offhand, but it should be easy to find on the web. Taking off .004" could practically be ignored, unless you were running a pretty high CR beforehand. It *MAY* raise the CR .2 points, at the most.

As Andre said, you can take a compression test afterwards and compare to what it was before.

[SqDancerLynn1]

.004 tho off would hardly clean up the head surface. I would double check with your buddy and make sure it wasn't .040 which is possible. If so I would check the valve to piston clearance

[brs127s]

It was .004. Basically, that is what it did. It cleaned off the old stuff left over from the original gasket. I assume it was an asbestos coating. We checked the head at each of the four corners with a dial guage on the milling machine, then ran the guage over the center of the head. It zeroed at each of the four corners, but was .004 at the worst spot when we ran a line across the center.