High compression pistons for a Dommi 500 plus 20

Chris

Chris

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Hi

Looking at pistons for a Dommi. Basic old fashion sprint bike road class.
Plus 20 9.5 to 10.5 would be ideal. I remember a batch of very high compression pistons probably 11.25 to 1.
I know Jim does something,
Red Lizard sell jp pistons 9.5 ref 18548 plus 20.
Any others?
Please feel free to chime in.
 
I ran 10.5 :1 CR in my Atlas since the late 60's. Back then, 103 octane could be found at any pump. Now days, with fuels available, I have gone down to 9.0:1 CR to avoid the pings, or to avoid octane boosters.

I would limit CR to 9.5 max for a street bike running pump fuels.

Slick
 
10.5 pistons for the 500 Domi are I think, virtually unobtainable now, but if you overbore and use 600/650 10.5 : pistons ……
 
Chris said:
Hi

Looking at pistons for a Dommi. Basic old fashion sprint bike road class.
Plus 20 9.5 to 10.5 would be ideal. I remember a batch of very high compression pistons probably 11.25 to 1.
I know Jim does something,
Red Lizard sell jp pistons 9.5 ref 18548 plus 20.
Any others?
Please feel free to chime in.
Click to expand...
Chris a few years have passed, did you ever find an answer. I ask as I have a Dommi project, domed pistons in it, no idea of CR.
 
Hi Steve's
Its my mate John's sprint bike. We found some 9 to1 & he has been running those. However at this moment in time he is welding the pistons! So far he is very happy with how it's going. I will ask him to take some photos.
 
Thanks Marinatlas. This is where his sprint bike started. Dunstal 3s cam. Machined head. Dunstall racing notes balanced crank.
Stephen yes he is welding the pistons to up the compression & add squish.
 
@Chris
Personally I think welding pistons up are the wrong way to go because it seems to me almost next to impossible to not extend the HAZ into the piston ring region and besides that to weaken the piston considerable or get it out of shape.
I personally had good experience in welding the chambers up (somewhere should be some pictures on the forum)and reshaping them thus also lowering pinging tendency.

Kind regards and happy new year

Christian
 
Hi Christian
I agree with you but my friend is old school. He brought scales & did his own crank balancing. He also through bolted his Atlas sprinter. He can't find pistons so he's happy to give it a go!
I have always loved the Mez bathtub with a Norton head. However it (porting) keeps moving on as you have only to well described.
Keep up you posting & replys.
 
@Chris
Well at least to lower the weakening of the piston ring area through heat, you could tell him that perhaps he could wrap the piston on top with some annealed 1mm copper in order to absorb some of the heat before it soaks in deeper.
Besides that, depending on what sort of Tig welder he has, ha can as well lower the heat energy.

Kind regards Christian
 
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