Technical Articles

Oil Leak – Crankcase not even

Checking crankcase mouth for step.
Checking crankcase mouth for step.

Checking crankcase mouth for step.


In the technical world we live in today imperfections are rare. In the day when these bikes were made they were common place. One that catches many mechanics out is a step between the two case halves. Sometime in the 1970’s someone at the factory decided that could speed things up by abandoning the time tested machining of the crankcase mouth with the cases bolted together. This gave us a perfect surface for base gasket to seal. 
Checking with feeler gauge finding a .011" step.

Checking with feeler gauge finding a .011″ step.



What it did give us was the potential for more oil leaks and distorting the cylinder bore. The first time the crankcase halves saw each other was at final engine assembly. As the pictures illustrate you can often find these 1970’s 650’s with a step between the case halves. This is guaranteed oil leak and/or a base gasket that is broken and sticking out of the joint. The only cure is to mount the case on a milling machine and fly cut the crankcase mouth. This will also cure another problem common on these twins: The crankcase mouth not parallel to the crankshaft center line. If you in search of some performance gain this is one of the first things you need to do to these 650-750 twin cases
Checking with micrometer finding the step is .015" at joint.

Checking with micrometer finding the step is .015″ at joint.



While you are checking things:
Be sure to remove, and seal, the two studs where the stud hole goes right int0 the crankcase. Failing to do this oil will wick up the stud and leak oil next to the tappet guide block. With the common practice of over tightening the base nuts some aluminum around the stud can be drawn up above the gasket surface. This will also prevent the base gasket from sealing. Removing the studs and using a bastard file about 12″ long carefully draw file (google it) the protruding aluminum flush with the gasket surface.

Copyright John Healy – Vintage Bike Magazine 2015-2017