View single post by Primordious
 Posted: 11-17-2013 05:26 am
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Primordious

 

Joined: 08-11-2013
Location: St Louis, Missouri USA
Posts: 9
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 There are a several factors involved with setting the correct piston to cylinder wall clearance, but the most important piece of information comes from the piston manufacturer. Different grades of aluminum have different expansion rates when heated. Just because a set of Brand A Pistons work well with a 0.0020-0.0025 cylinder wall clearance, it doesn't mean Brand B pistons will even run with the same clearance. If they have a higher expansion rate, they could lock solid in the cylinder bore.

Generally when you order pistons for any engine they are ordered by finished bore size, not what diameter piston you want, and the piston size you receive will be marginally smaller based on the recommended piston to wall clearance.

Here's an example;
Let's say you have a cylinder with a 4.00" finished bore and your replacement pistons call for a piston to wall clearance of 0.003". When you put the micrometer on the piston skirt about a half inch below the wrist pin holes, the reading on the micrometer should in theory give you a measurement of 3.997". If your bore is the correct 4.00" you're golden.

Oval, egg shaped, or clover leafed bores can give you fits on ring sealing and clearances, so good machine work is vital.

Side note on new pistons & rings... many years ago I tried rebuilding an engine (my first one) and not thinking I installed the rings with the end gaps basically lined up right above one another. Fouled plugs and high levels of oil consumption led to a second tear down and a lot of mocking by certain friends. On a positive note, I did remember to actually check the ring end gaps to make sure they weren't too tight. What ever brand of rings you buy, be sure to check the data sheet and see if they are pre-gapped, or if they are file to fit rings.