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Copyright © 1995-2017
Mechanicsville, Maryland

 

You've disassembled the engine and the pistons are out ... Yay! Now what? If there's no ridges or scoring in the cylinders, do you slap a new set of rings on and put 'em back in? No need to rebore the block??? Whoopie!! Hold your horses, Tex! So you don't have to do all this again right away, here's how you...


Measure cylinders for taper
By Jerry "Hotrod Lincoln" Herbison   July, 2017
 
 

Don't guess ... Measure!!

Now that you've disassembled the bottom end of the engine (removed the main bearings, cam and crankshaft, and turned the engine back over and removed the head, go ahead and carefully push the pistons out -- if they try to hang up on the ring ridge, don't pound on them. You could break a piston ring land. Use a little medium grit sandpaper to scuff away the carbon ring at the top.

If everything else looks fine, the easy way to check cylider bore taper is this:

1. Slip the top ring off one of the pistons and put it into a cylinder at the top of the ring travel, as high as possible without getting on top of the carbon ring right at the top. Make sure it's not cocked in the cylinder (use a piston with the rings removed to push it into place evenly). Use the wrist pin as a leveling guide -- insert the piston until both ends of the wrist pin are at the level of the block, for example.

NOTE: The majority of taper wear to a cylinder happens in the top 1.5 inches of the piston travel, so getting the upper ring gap measurement as close as possible to the ring ridge is essential for an accurate calculation. Taper wear is not linear from top to bottom.

2. Stack up a bunch of feeler gauge blades to measure the ring gap. Write the measurement down.

3. Use the piston to push the ring down near the bottom of the cylinder, and measure the gap again. Record that measurement, which should be somewhat smaller.

4. Subtract the bottom measurement from the top and divide the resulting number by three to get the cylinder taper. For instance:

Top gap: 40 thousandths
Bottom gap: 18 thousandths
Difference: 22- - - -divide by 3- - - -cylinder taper is 7.3 thousandths.

I consider .005" to be a maximum taper for a good re-ring job. Some people will accept a .010" taper, but I've found it makes for a short-life ring job.

If you have a taper in excess of .005, you should consider a rebore.

-30-

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