The Stovebolt Site Forums The Shop Area Engine Shop 307 bore tolerances

I'm getting ready to check the bore on a 1970 chevy 307. What is the acceptable wear reading?

Cylinder taper of more than .005" from top to bottom of the piston ring travel is my personal limit for a re-ring job. As a cylinder wears tapered, the rings must constantly flex to maintain contact with the cylinder walls. at 2500 RPM, that means the rings are flexing 5,000 times a minute, or 83 times a second. In a rebored, straight cylinder, they flex- - - - -ZERO times a second. Which one will give longer ring life?

You can get a pretty good check on cylinder taper by inserting a used piston ring into a cylinder, and squaring it up against the ring ridge at the top. Then stack up feeler gauge blades and measure the ring gap. Push the ring to the bottom of the cylinder and measure the gap again. Subtract the bottom measurement and divide the difference by Pi (3.14). The result is the cylinder taper in thousandths of an inch. For instance:

Top gap = .090"
Bottom gap = .065"
Difference = .025" Divide by Pi = .008"- - - -too much taper to re-ring!

Jerry

Thanks Jerry for the input on taper, how for out of bore diameter tolerance would be considered ok.

If the cylinder is round, and the taper is less than .005", hone the cylinders, clean them up THOROUGHLY with a stiff-bristled scrub brush and HOT soapy water, and re-ring the engine. If the cylinder is egg-shaped anywhere or the taper is more than .005" bite the bullet and rebore it. The most cylinder wear occurs in the top 1 1/2" of piston travel where the combustion pressure is the greatest- - - -it doesn't progress gradually from top to bottom, so where the rings need to seal the best, they're least likely to be able to do so in a taper-worn cylinder.

I have used a rigid hone to take out 3-4 thousandths of taper, and then put a knurl on the piston skirts to take up the extra clearance. That was an oldtime procedure we used back when a re-ring job only lasted 30-40K miles, and some engines got one or two in the frame overhauls before getting rebored. Perfect Circle used to sell a piston-knurling tool called a "Nurlizer" that could be used for that purpose by small shops without a lathe. It's possible to expand the diameter by .002"-.003" with a knurl on the thrust face of the piston skirt to avoid piston slap after a hone job.
Jerry