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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #781  
Old 2 Days Ago
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I hope you mean .018.
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  #782  
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Yes, sorry, .018". FSM says .015-.025", but is it OK for my engine?
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  #783  
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I should add I'm using Dytch/Axtell forged pistons and Hastings rings. Corrected CR should be around 8:1.
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  #784  
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On Hastings site, they recommend a minimum gap of .0035" per inch of bore diameter, in my case it will be 3.375 * .0035 = .0118". But that recomendation is probably for automobile engines and probably 3 ring pistons. Found some other recomendation on .0050" for forged pistons, in that case the gap should be 3.375 * .005 = .0169".

Another thing that should be taken into account - that will be a kick only engine. I was reading the forum and came across Dr Dick's post that he was using Total Seal rings in his 5" kick only stroker and had to go back to regular rings because it was really hard to kick start the motor.

So a small gap is bad anyway - there's a risk of ends of the ring touching each other due to expansion from high temperatures and it also makes kick starting the bike harder due to increased cranking pressure.

So what gap size is optimal in my case?
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my experience is with S&S.their end gap specs are so close to the fsm specs. you posted,it's not worth posting.
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Just make sure you have plenty. .018" is not a bad number for a 2 ring piston.

One compression ring means that ring has to do the job of 2 when it comes to carrying heat away from the piston, i.e. it's going to grow more.

There's no measurable performance difference between .010" and .020" so make sure you have plenty.
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  #787  
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Thank you, Don!
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