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Does anyone know if the crank cases have to be changed as a pair or can you just replace the bottom?

The bolt lugs for the gene cover have broken and I have a spare bottom crank case

This should actually work but as you haven´t got a real gasket in between (only silicone fluid) they´d better match tight. Put them together and check possible gaps with a gauge.

Are you sure that you can´t repair the old crank case?

Yeh I can but I'll have to send it too the welders .
Easy fix just weld new lugs and tap & die.

Just thought since I have the case lying around I may as well use it

im not so sure, if your top and bottom crankcases use the same colour/number bearing shells then they are basically identical, and you just need to check them for a good case-to-case contact, if they are from different engines, with differently sized holes that were drilled for the crank then i expect you may have some oil clearance issues when you put it back together.

i would explore getting a welder to make the necessary repairs before i would look at matching two different crankcase halfs, just my opinion mind you, feel free to correct me.

failing that, crankcases come up on ebay from time to time ~£80 for the pair?