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Mk 3 crankcase on mk 2 engine - any issues?

Submitted by nigel_duffield on Wed, 24/02/2021
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Commando

I've stripped my mk 2 '74 commando engine down and it seems someone in the past has fitted it with mk 3 crankcases - see attached picture. Everything else including the timing chain cover, primary case, crankshaft etc. seems mk 2. I understand the mk 3 crankcases had studs the inner primary located on and the holes for these have been blanked off with what appears to be cut down bolts.

Is anyone aware of any problems using the mk 3 cases may cause or any further mods I should be looking for or doing? Also are the crankshaft bearings any different in the mk 3 or can I/should I use mk 2 bearings? 

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20210224_163843_compress71.jpg

Submitted by steve_constable on Wed, 24/02/2021

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Interesting engine number? Do others have the SS at the end?

 

I believe that is the engine number of the mk 3 the cases came from. I haven't noticed SS on the end of any other numbers on the bike. The paperwork I got with the bike when I bought it in New Zealand states the engine number as being 304227. 

Submitted by john_holmes on Wed, 24/02/2021

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The SS probably has some connection to the use of the replacement MK3 cases, either the mechanics initials, a local licencing requirement etc.

Not sure why they did not use grub screws for the stud replacements, the cut off inserts look ham fisted and possibly bits of thread taps not cut off bolts. At least it all gets covered up.

Submitted by stan_thomas on Wed, 24/02/2021

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Mk3 crankshaft is wider, primary chaincase is mounted on 4 studs with a different flange. Not a straight swap.

Mk3 crank cases are stronger so there is some appeal but the ones Andover make now for the earlier 850s have the improvements.

Bearings, timing side etc are the same.

Thanks Stan, the different flange would explain the vast amounts of silicone sealer that had been used to seal the inner primary case to the crankcase!

Am I right in thinking the mk 3 engine's longer crankshaft is an extension on the primary side to accommodate the electric start with the distance between the crankshaft main journals being the same as the mk 2, i.e. the mk 2 crankshaft assembly will fit into the mk 3 crankcases without any issues or misalignment?

Submitted by stan_thomas on Thu, 25/02/2021

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The Mk3 crank has wider (stronger) webs and as a consequence the bearings are further apart. This is in addition to the very obvious extra section on the drive-side for the electric start. I could have made that more clear, sorry.

The upshot is that an earlier crank does not fit correctly in Mk3 cases (and vice-versa). I have heard of people machining the Mk3 drive-side to match the pre-electric start pattern. It looks like the d/s crankcase has been machined at the primary chaincase mounting flange, so maybe you've got a full Mk3 bottom end.

Once upon a time I had the measurements for the two types of crankshaft. I'll try to find them.

Edit: It's just occured to me that you can easily identify whether you have a turned-down Mk3 crankshaft - the timing side crank web will have a groove machined in it for static timing of the engine.

 


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