Piston Clearance

[John PurdyParticipant @johnpurdy78347]

A question for the IC engine grus. I have 2 IC engines I am currently working on. They both have cast iron cylinder liners, one of 1″ dia. the other 3/4″. I am planning on using ringed aluminum pistons. What clearance should I be looking for between the piston and bore when cold? Neither engine will be a particularly high performance one. The material I plan on using for the pistons is 6061 ( because I have lots).

John

[bernard towersParticipant @bernardtowers37738]

From days gone by when on full size it was 10 thou per inch of bore dia, one of the things not to be forgotten is to allow clearance behind the ring to allow comp pressure to do its job.

[JasonBModerator @jasonb]

If I were to measure my engines around that size which use 6062 it would be 1 to 2 tho. The 24mm bore midget, RMC type A and Thompstone are that sort of size and I run them with a single Viton O ring

[bernard towers]

On bernard towers Said:

From days gone by when on full size it was 10 thou per inch of bore dia, one of the things not to be forgotten is to allow clearance behind the ring to allow comp pressure to do its job.

10 thou per inch of bore dia seems a bit excessive; I would use 1 to 2 thou per inch of bore diameter with a ringed piston.

Mike

[Peter Jones 20Participant @peterjones20]

I agree with one to two thou per inch of bore.

I think the 0.010″ was for cast iron pistons as I’ve never even heard of that large a clearance with aluminium alloy

For low expansion cast alloy or high silicon forged pistons, (in motorcycles, 68~80mm diameter) I use 0.0017″ on 68mm cast pistons and 0.0022″ on 80mm.

You really need to have some idea of the expansion rate of material if your turning pistons from solid, 6061 is quite high expansion rate, it will also depend on the wall thickness at skirt and the crown thickness where rings are. I would reduce top by at least 0.010″ diameter as it will be hottest part but for such small pistons you only need around 0.0015″ clearance (IIRC, pure aluminium is 0.000006″ per deg C, 6061 slightly less)

[John PurdyParticipant @johnpurdy78347]

Thanks for your replies. When you say two thou clearance is that the difference between bore and piston diameter or is that the annular clearance around the piston, ie piston 4 thou smaller than the bore?

John

[JasonBModerator @jasonb]

Difference in diameter so for a 1.000″ bore piston would be 0.999″ to 0.998″

[John PurdyParticipant @johnpurdy78347]

Thanks Jason, that’s what I assumed, but there’s that word “assume”.  One to two thou is sort of what I thought.

John

[Howard LewisParticipant @howardlewis46836]

The clearance should take into account how the engine will be worked.

If it will only be run unloaded, it may well survive with a smaller clearance than an engine that is going to be loaded.

(An engine for a model aircraft will be loaded by the propellor absorbing all the power that can be produced at a given engine speed, so would need a larger clearance to prevent seizure)

A marine propellor absorbs power at an index of (speed^2.8) so doubling speed would require nearly seven times as much power.

My young greaser rode his little moped from Sussex to Somerset to a pop concert, and returning, had to spend the night sleeping under hedge, missing a day’s work, because the engine kept slowing and stopping. It was never envisaged that it would be driven flat out for mile after mile. Fortunately, the seizures did not appear to be damaging!

One of my brother’s fitters replaced the aged engine in his car, a streaight 6, side valve, with one scrounged from a scrap yard. It was a better engine, until he drove it fast for long. We found that the pistons were cast iron, unlike the alloy ones in the original engine.

Consequenrly, because of the tighter piston/bore clearances, it was unsuited to prolonged high speed high load operation.

Howard

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