E: Engine Comet Mongrel

vibrac

vibrac

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Low ex pistons and 6 thou is as big as I would go and that's with the reborers recommendation of 4 thou ringing in my ears anyway that's what the racing comet always had
 
davidd

davidd

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I would think that the .0065" might be a good number. It would keep you within the recommendation, but also recognize that being a Comet it is more subject to uneven heating than the twin.

David
 
passenger0_0

passenger0_0

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I 've previously used 0.0065" with Specialoids on a race twin with no signs of nipping up. The cylinder liner/muff fit is probably more important to get right. If you're worried about it, why not use a single-cut file to hand 'cam' the opposing sides around of the gudgeon pin bosses to give more side clearance?
 
Cyborg

Cyborg

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passenger0_0 said:
I 've previously used 0.0065" with Specialoids on a race twin with no signs of nipping up. The cylinder liner/muff fit is probably more important to get right. If you're worried about it, why not use a single-cut file to hand 'cam' the opposing sides around of the gudgeon pin bosses to give more side clearance?
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Can't I just once install a part without opening the file drawer?
 
Cyborg

Cyborg

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vibrac said:
Low ex pistons and 6 thou is as big as I would go and that's with the reborers recommendation of 4 thou ringing in my ears anyway that's what the racing comet always had
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The Richardson book makes reference to the "N" (which this piston does not have) which he states means low-clearance.... which I assume means low expansion. The way I'm reading it, the .006 - .0065 is being recommended for the earlier non N pistons. To me .0065 sounds like plenty, but I spent most of my life around stuff from Hamamatsu.
 
Bill Thomas

Bill Thomas

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Cyborg said:
I'm likely going to use this piston, because I have it and it will fit. The bore is STD and shows some light scoring, but would rather bore it to .010 over and have everything straight and round. The long term plan is for something larger, but figure it is best to get the engine together and start figuring out things like what frame I am going to use. It is a Specialloid H92, so 7.3 to 1 CR. +.010 and no "N" stamped on the crown. According to Richardson, Shadows used them fitted with .006" to .0065" clearance and I know some folks suggest more. I guess the question is how much more. Road use only and riding fast is not recommend in my neighbourhood....

View attachment 18388
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Even though you say you won't ride it hard, Poor old Comet's have a hard time !, Well mine does !.
The rev's at 70 mph are a bit high, I tried a twin rear sprocket, But with my size and the stuff I carry, I needed more Power.
Just seen in MPH, Graham is soon doing a bit on wonky barrels, I had a bad one on my L/ning when it nipped up at 7 thou clearance, So now 10 .thou. Good Luck, Bill.
 
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