Fred, Cylinders

I give man, I checked and your are correct about pistons being larger at the skirt, actually I knew that but was thinking about roundness... I was told by the local shop that the cylinder of an aluminum block (and this was a 4 stroke)will end up oval (again not to the naked eye) on a well used motor, but I didnt realize it was because of the piston, I assumed it was due to inertia, after all they dont move side to side (well not for long anyway) Rog

pissed ons

Well... cylinders do end up wearing oval... for the reasons you mention (the inertia of the rod and crankweight shoving the piston forward and backward)

But pistons come right out of the box... both tapered ..(smaller at the top than the bottom)... and oval (smaller side to side than front to front).

This is part of technology marching forward.

I bet pistons in 1920... WERE round and straight.

Then they learned that the crown expanded so much from heat that they altered the COLD dimensions to make it round and straight AFTER it heated up.

Re: pissed ons

Ron Brown /


Dyke's Automobile and Gasoline Engine Encyclopedia, 1910.

"The piston is usually slightly smaller at the top than at the bottom, because the heat is more intense at the top and expansion must be allowed for."

This, of course, is for a cast iron piston and no mention is made of oval. I guess some things are "new".


Re: pissed ons

Wow... must be a cool book to have.

Re: pissed ons

Ron Brown /


It is, I keep it in the reading room so if I have nothing better to read before I flush, I can see how they did things in "the good old days".


old pissed ons

Check this out.

I was at my local cylinder boring and honing guys place 3 streets down last year.

He had this single cylinder 4 stroke delivery truck engine from about 1912 or something

Some quick tape measurements let me calculate it out to 1700cc.

It had a giant 60 pound (?) flywheel... the driver would pull a compression lever inside ...then jump out... and hand crank the flywheel up... then get back in and release the decomp lever to fire it up.

The wrist pin and crank pin were both 1.5" dia.

There were new (aluminum) pistons available from this company in LA (the Egge co.)...they are not making them any more... but are left from old stock.

Both intake and exhaust 'ports' and runners were pipe fittings.

I thought it was Interesting... but right now I can't remember the name of the truck manufacturer.

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