Crank Whip

[Paul N-M]

Looking through past threads on this topic there is a fair amount of chat, a lot of which is based on original Austin cranks so rather than looking at piston to head clearance at high revs in general terms such as 20 thou bellow deck height for 2 & 3 can anyone be a bit more specific bearing in mind modern EN40 cranks.
I'm building up a competition engine and would appreciate any help on this.
So with a Phoenix 2 bearing 1.5" splash crank revving to 7000 + what would be the recommended final piston crown to head clearance gasket fitted be. i.e. the actual running clearance regardless of gasket type/thickness. And would this differ for pistons 1 & 4 and 2 & 3?

Paul N-M

[Charles P]

As it happens I lifted the head on my car this afternoon.
It runs a 1.5" Phoenix crank with Tony Betts' slipper pistons, a Ruby head and a high compression gasket. I've not reduced the height of any pistons and they come right to the top of the block. No difference in any of the markings or carbon on the 4 piston tops and no witness marks visible despite revving to 6,500- 6,750 at times.
I suspect that some people (especially on social media) parrot what they've read without first hand experience.

Charles

[Paul N-M]

Thanks Charles, 
Any idea of your compressed gasket thickness?

[Chris KC]

In the heat of the moment it's not unheard of to "miss a gear" (or even stomp the wrong pedal), which can send revs through the roof. The forces acting on crankpins under those circumstances are staggering.

[Paul N-M]

Hi Chris,
Indeed, hence asking the question aimed mainly at those with racing experience.
Also I will be fitting a rev limiter.

If anyone can chip in with their thoughts please do.


Paul N-M

[leaf200]

I ran an engine with a 1.5” crank and a ruby head and when I removed the head there was clear witness marks in the carbon from 2 and 3 hitting the head. I did not see any damage to the engine from this so it may have only been a light touch. I take a bit off 2 and 3 now if running a ruby head.

[Alan]

I’ve been using 1.5in Phoenix cranks since the 1980s and they’ve had a hard life. I’ve always set piston height level with the top of the block and as long as the gasket doesn’t overhang the bore, never had any trouble.

[Paul N-M]

Hi Alan,
Thanks for that. Any idea of the compressed gasket thickness? And what revs are you going to?


Paul N-M

[Alan]

It’s a standard HC gasket. Revs up to 7000