E: Engine Piston Clearances

erik

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I start to rebuilt my rapide and two cp Pistons should be used.Terry Prince recomends a Piston clearence between o,oo25 and o,oo35 inches. this is about 0,08 to 0,09 Millimeters. other sources saying 0,05 and up to 0,15 mm. what should i do?Erik
 

Oldhaven

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This discussion addresses your question to some extent:

 

Albervin

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I have a pair of CP pistons (7.5:1) with 4 thou clearance. No oil burning and no problems after 20,000 miles. Just like the balance between rich and lean fuel mixture I always go for safety. In Australia, where it can be very hot and the fuel can be suspect I thought I would rather buy more oil than pistons.
 
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Martyn Goodwin

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I support Alyns comments. With the generally warmer weather in Australia my Comet with a freshly fitted CP pistons (8.5:1) and a clearance of 0.003" as recommended by CP, the motor tended to 'nip' up.

Go for safety and give 1 to 1 1/2 tho more clearance than specified by the piston maker - remember piston makers these days assume their products are going into liquid cooled motors that have less thermal expansion/contraction than our air/oil cooled motors - thus the need for us to have a larger piston clearance.

In my motor I increased the clearance to .005", which required a re-hone of the barrel, and no further problems.
 

greg brillus

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More critical on a single that is working harder, most all twins with either Cp's or Omega's I give them about 3.5 thou no less. just as important are the ignition timing and fuel mixtures. Less full advance and slightly more mixture for our modern faster burning fuels is the general rule now.
 

vibrac

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I would go with marting's suggestion I always ran the racers with 6 thou. Back when these bikes were used daily vastly wider clearance was not uncommon through wear and closer staring clearance gave longer life and people were accustomed to running in an engine. When did you last see "running in please pass" on a tin box?
 

erik

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I live here in Germany in the wine area Rheingau and in summertime rit can be very hot up to 40 degrees Celsius . so may be 5 thou ,0,127 mm would be a good choyce for a twin ?
 

oexing

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Erik, that is way too much clearance. That number would be OK for a big piston of 85 mm in an ALL cast iron cylinder with less heat expansion. Greg´s number of max. 3.5 thou is about right. You can compare a Vincent cylinder with that of a Yamaha SR 500 or XT, same size of piston, shrunk in cast iron liner, reaching way down into the case and not much more oil flow. The SR oil pump is small, all roller crank, no oil jet for the bore, so no factor here. They go for 3 thou clearance. Same story with two valve /5 or / 6 series BMWs , iron liner in alu cylinder.
I am running 5.5 thou clearance in the iron block 3.8 Jag E-type with that sort of pistons so, no, 5 thou is ridiculous. You just have to see the very different conditions in iron or alu engines while looking at the same types of pistons without any extra features like slots or cast-in steel inserts for expansion control.
All statements are to be seen on a basis of correct engine work, meaning perfect geometric honing process and no distortion of the liner in poorly fitting , too tight engine case.

Vic
 
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