750 Crank torque settings

Afternoon all
My 1971 750 rebuild is proceeding apace, and I've just reassembled the crank - but I have a question for the forum. I've used a Norvil crank kit which has six studs rather than the original 4 studs and 2 bolts. I'm fine with this but what should I torque the nuts up to?
the early official Norton workshop manual says to ‘make sure they are very, very tight’ – useful advice but not very precise! The 1970 on 750 manual says 35 ft/ld (4.84 kg/m) but that uses bolts as well as studs, the INOA TEch Digest says 25 ft / lbs (3.45 Kg/m) for studs and the Mk III manual says 30 ft/ld (4.15 kg/m) but this uses different studs, so I followed the INOA advice.

What does the forum think - should they be tighter than 25 ft/lb, 'cos I'm going to have sleepness nights agonising over this!

Thanks in anticipation

Matt

Mick Hemmings says this about the crank bolts/studs in his NOC Engine rebuild DVD:



".....I never bother with a torque wrench on these, because unless you've got proper adaptors and things to get at the bottom ones [behind the webs as he has the crank in the vice] I can't see the point of it. So the trick is just to use two good spanners,-and tighten them up."

This is also what I do, although I do use lock tite and peen over the ends of the studs for safety.

I remember when I used to do a lot of work on my bike 30 years ago you knew what 35 ft/lbs felt like with just spanners but now that I'm just starting out again after so many years I've lost that touch. It's going to take a while of working with a torque wrench, doing up, taking off nuts and bolts to get that "Mick Hemmings" feel that most mechanics have.

I'll bet you could put 60lbs on them before you started to stress the threads so I'd go with 35 ft/lbs or even 40. Make sure there are no nicks or blemishes on the mating surfaces.

Rennie, I don't think you could over torque these studs with standard spanners, 60 lbs is difficult even with a 1/2 drive torque wrench.

dave M said:

Rennie, I don't think you could over torque these studs with standard spanners, 60 lbs is difficult even with a 1/2 drive torque wrench.

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Good point! I just tore one down a week ago with spanners and I thought my eyeballs were going to pop out. Made me feel kinda, well ...old.

Good point! I just tore one down a week ago with spanners and I thought my eyeballs were going to pop out. Made me feel kinda, well ...old. I KNOW THAT FEELING TOO! LOL LOL :lol:

I spoke to Les Emery at Norvil today - 25 ft/lb was his recomendation, and he said that any more was not good! So thats good enough for me - 25 ft/lb it is.

This is a case where practice and theory don't match. The 5/16 inch Norvil crankshaft fasteners I purchased had a hardness of Rockwell C 20, not high strength stuff, yield strength around 85 ksi. If this fastener is oiled and tightened so that the stress in the bolt is 85% of the yield strength the applied torque would need to be about 12 ft-lb. How these are tightened to 25 ft-lb or more is beyond me. People do it routinely so it must work but I chose to use some high strength ARP fasteners. The body diamter is a little smaller than what I'd like (.310 vs .311-312) but IMO it's better than using the crap material stuff. Plus you can get nice 12 point nuts that are much easier to tighten.