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The T120 Crank/Flywheel amateur spotters field guide

6628 Views 2 Replies 3 Participants Last post by  rambo
Not sure if this is a waste of time but I am thinking I am not Robinson Crusoe and that other members may sometimes have the same issues as me identifying if parts in their bikes are actually the right ones.

Case in point my crankshaft/flywheel. The bike is a 1971 OIF 650 but is the crank the right one? It looks a bit like in the parts manual but the part number doesn't look right - when I searched for it I came across references saying early 60s, 1969, pre unit etc. :(

When I look on the web for a replacement, different designs are advertised but they often just say fits "T120/T140, TR6 etc" and don't give actual part numbers (eg

So thought it might be of some value to post pics of various items like these to see if there are any general guidelines for better identifying cranks/flywheels (and what part can be used in what - eg can you use a T140 crank in a T120 case? if so do mods need to be made? etc).

I have some random T120 crank photos below. Anyone willing to comment on what might be what, what to look for in identifying them and things to note about them?

Part numbers are the obvious thing to identify them but often advertised 2nd hand ones don't have the part numbers and sometimes the part numbers given don't seem to always match a known one.

Contestant 1 (images are of same item):

Contestants 2 & 3:

Contestants 3&4 (may be same part):

Contestatnt 5:

Contestant 6:

Contestant 7:

Contestat 8:
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Contestant 7 isn't a 650 or 750 crank, it is from something with a bush main bearing on the timing side (3TA / 5TA etc?). Contestant 1 is the light flywheel 650 crank which was fitted from 1966 for a year or so (somebody will know the exact start and end engine numbers).

I THINK but am not sure, that the bevelled flywheels are early unit/late pre-unit, but I'm ready to have my head ripped off for saying this. As far as I think, the late 650 flywheels looked very similar to T140 flywheels, ie: square-edged and with no relived portions around the circumference.

From a certain date (some time in 1970 I believe?) the timing side shaft was changed to metric, meaning earlier cranks won't fit later engines and vice versa, though I think the flywheels may be interchangeable?

Some of the above is kind of informed guesswork so don;lt take too much notice. But I bet, having stuck my neck out like this, whatever's wrong will very quickly be set right!
The bevelled cranks are the heavy type.During about 66/67,they went to the narrow lightweight type which had tickover problems so back to the heavy type.The TR6C crank was about 1 lb heavier than a heavy T120 crank for 69.Metric on one end was about mid 71 due to a bearing shortage.T140 crank has a longer rotor side end for the triple chain.Thats all i know for now.As John says,we are prepared to be shot down later.
I see many engines being fitted with lightweight cranks so maybe a lot survived laying in sheds.
7 looks like a BSA a65 crank
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