Piston Ring Tips

Piston rings will rotate in the cylinder when the ring side clearance
to the ring land is excessive. A new ring in a new groove has 1 1/2
thousandths of clearance built into the ring. Once the piston
grooves wear the ring is not held properly and will rotate. In a
horizontal engine the force of gravity will work the gaps to the top.
With proper clearances this will not happen. If you see all the
gaps lined up you had better check for ring groove wear because
trouble is just over the horizon. The greatest old wives tale about
piston rings is that there is great importance to the gap whether in
size or orientation. The gap is only about 5 percent of the
equation that includes the ring side clearance. When there is
compression leakage past a ring 95% goes past the sides of the
rings and never enters the gap. If you bother to check the end gap
why don't you check the most important thing - the fit of the ring to
the groove? Most people don't. Then when they don't get any
compression they call me. You can live with as much as 5
thousandths of side clearance but 6 or more means you will not be
happy with your results. Machine work will be needed. Pins in the grooves will force a ring to seat in one spot but there again when the ring grooves wear the seal will fail. Gap size should be a minimum of 3 thou per inch of bore for easy break ins. Engines that will work hard and hot immediately after overhaul need 4 thou minimum end gap per inch of bore. As much as 8 thou per inch of bore is acceptable.


The other thing people overdo is hone. They want to see a
pretty 350 chevy cross hatch pattern but that is not necessary in
our old engines. You are grinding away precious cylinder wall
material. The extra piston to cylinder wall clearance will allow the
piston to cock around in the cylinder and will break the seal of the
rings. If you can't resist the extra honing get a piece of emory
paper and do it by hand. When your arm is tired you know that
you are done. Leave the power honing to the professionals who
have modern marvels like oversized pistons in their bag of tricks.

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