I have a rm 85, Has a crack at the oil drainage plug what should i do?

A cracked engine case should be welded. You can attempt to put a helicoil in it but you will be wasting your time with that temp fix.

Take it to someone that knows what they are doing and not just any random welding shop. Try to find a machine shop in your area that deals with aluminum.

It is hard to find someone that is good enough to weld an engine case and I have seen some pretty good JB weld jobs that have lasted a good while.

I will say JB Weld has worked great on my RM125. I do not know how bad you crack is but it sounds like the same problem I had. If you put a little time into it you can make it come out really nice and make it rather unnoticeable.

After you JB Weld just remember when replacing the drainage plug to not over tighten it. Word of advice go to a hardware store and pick up an rubber O-ring to put on plug. Pros-Keeps yo from over tightening it as the rubber O-Ring creates friction, and acts as a gasket. But in all actuality you do not need to really wrench on the plug....just a little more than finger tight.

Ben has it right, the only permanent repair is to have the case welded and the thread retapped to the original size and not just doing away with the plug completely. Use a copper washer on the oil plug and tighten firmly. I haven't heard of an insert being made but if it is available the cracked case would still have to be repaired anyway.....

Ben and Bedouin are correct.

Depending on your case material, weld it, JB weld it (I have used this in the past, it works really good) or replace the case half that is damaged.