Crank interchangability

Hi all, could anyone tell me if there are any differences between the Atlas cranks all the way up to the 850 bikes? Cheers.

Hi and welcome to the forum!

I'm no expert but I do know the 850 is a heavier crank and there was a short stroke 850 also. Then there's the 1975 and onward cranks which were also slightly wider than the previous 850's.

I recollect the balance factor changed when they introduced the Commando due to the Iso frame. Better research this one before putting a bitsa motor together.

Mick

Hi there, the atlas crank is balanced to 84% while the commie is at 52% (or 63% depending if you consider wet or dry), so that mean the flywheel of atlas is heavier in the bottom part, and so you can allways modify it to commie balance factor by drilling , the opposite is not easy (putting a cdo crank into featherbed , need to add some weight to the bottom of the flywheel , which is done by some guy ("Basset down" in UK add some heavy round bars of "strontium") same when you fit heavier (steel rods) parts in your cdo crank.........apart of that , they are the same except the rotor keeway which is cut at a different angle.........

Hi and thanks for the replies and welcome. I guessed the balance factors would change, this sort of thing doesn't phase me. and I never realised there was a short stroke. Which model do I look at for that? Now the width issue for 75 on cranks, was this because of the change to superblend bearings?
I've realised that I need to modify my question slightly different too: Are the cases across this time period able to accept the different cranks without further machining, even if different mains are fitted so, for example, does that mean the 75 on cases were machined slightly wider? The Altlas to Commando key-way change, were there any other taper or keyway changes to the end of production or different barrel spigot depths (if used)?

I guess my fundemental question is, if you were designing a crank that you could dimensionally tweek to fit others, which model's crank/motor would you look at first?

Commando cranks are different (longer) at the primary side due to the three row primary, etc. compared to Atlas/N15/P11. The '75 850 crank is different yet at the primary side due to the electric start mechanism. This is in addition to the different balance factors.

I believe the width differences are in the '75 850 cranks and crankcases with all the earlier ones being the same.

The 750 short stroke crank is quite rare and was primarily for racing applications. If I recall this gave 750cc with 77mm pistons and an 850 cylinder.

That's great, thanks everyone, its given me plenty of good directions to continue with.

Blackplate said:

That's great, thanks everyone, its given me plenty of good directions to continue with.

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It might help if you can describe what you are up to and why. There is a fair amount of knowledge around these bikes and more than likely someone has tried just about everything.

Having put an Atlas crank in a Commando i can tell you balance is not the only factor. The key way for the alternaor is not slotted to time a Commando. You will have to set timing with a degree wheel.