Twinc Piston Clearance

[JonnyPlus2]

Hi All,
I'm about to get my tall block bored out to fit some 85mm Cosworth pistons (Ooooh Kids, that's REALLY tasty !). Please don't tell me how that's dangerous etc. as I've read all the threads. If you want to discuss how big to safely bore the block, please, kindly, start (yet another) thread devoted to the subject.

Saint Vizard suggests that I need a piston to bore clearance of .0045 whilst the shop manual suggests closer to .0035. I'm planning to used a high lift can and rev to a max of 7000 rpm (if I have to put a Porsche driver in his place) otherwise 6500 will be the usual limit. I am inclined to follow St. Vizard's recommendation but I would like to hear from others who have built up engines and find out what you think. Your comments will be read with much interest. Thanks.

Best Regards,
Jonny +2

[StressCraxx]

Johnny,

It depends....

What kind and alloy of piston? The pressure cast ally hepolites did not expand as much as the newer forged pistons. It depends on the alloy and its rate of thermal growth. Ask your engine builder or get the recommendation from the piston supplier. BTW, 0.001" isn't much different in clearance and bore tolerance are likely +/-.0001" anyway.

[JonnyPlus2]

Hi Mr. Craxx, (may I call you Stress ?)
BIIK what the alloy is in the pistons. It' the alloy that Cosworth used in their forged 85mm twink pistons back in the 60s. I could email Cosworth and see what they have to say about it all, maybe they could recommend a clearance. Thanks for this.

Best Regards,
Jonny P2
P.S. German Playboy Johannes Thurn-und-Taxis was known as Johny TNT ! A lot cooler than Jonny P2

[rgh0]

Yes I would recommend talking to the piston manufacturer i.e cosworth. There are generic guidelines you can find and apply but the right number is specific to each piston design and material and application.

cheers
Rohan

ps good luck with the 85mm bores