681F or 711M Crank?

[fjbm]

Hi all

I'm going to rebuild the engine and use a tall block to do that.

I have a 711M crank and another from a 681F with the smaller counter weights but that has been tuftrided.

Both are in very good condition.

I've read somewhere that the one from the 681F blocks can produce vibration around 7000 rpm and therefore can cause problems with the flywheel and the rest of the engine moving parts. Is this true?

Which one do you recommend for my engine?

Cheers

[CBUEB1771]

Which one do you recommend for my engine?

All of the "Kent" production crankshafts with 77.7 mm stroke have a torsional resonance in the region of 8000 rpm or a bit lower. That is why "starter motor failures" are so common in Formula Ford. The crank fails near the rear main journal allowing the errant flywheel to force the starter motor to part ways from the bell housing. I am sure there are minor differences among the various 77.7 mm cranks that change the characteristics of this resonance but will still be there. Racing variants as used in the Cosworth BD series are fully counterweighted, much stiffer and having 12 bolts to try to keep the flywheel attached a bit longer. I don't think there is much to differentiate the 681 and 711 crankshafts in this context. Are you planning to use more than 6500 rpm for extended periods, say in a track day car? If so you might want to consider a fully counterweighted crankshaft.