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Hi all I'm looking for some help here regarding the fateful narrowcase weak point.
The engine is out, the cases are split and all the internals are spilled out. Now how best to permanently repair the cracking boss where the motor mounting bolt passes through the casting? After a professional aluminum repair using...what kind of aluminum? Should one then sleeve this boss to mitigate the problem from arising in the future? Advice and experience would be greatly appreciated. Pete

I haven’t seen many broken cases at the motor mounts but it does occur, not due to inherent weakness in the cases but due to crash damage, loose motor mount bolts or abuse. Weld with silicon alloy rod. I don’t think sleeving is necessary unless it’s the most expedient approach to effect the repair. Assemble the bare cases in the frame and verify that the motor mount bolts can be installed without having to force any of the bolts. You should be able to insert all the bolts in place without prying on anything to line up the bolt holes. If you cannot, the frame is bent. Also, any ovalized bolt holes should be addressed.

Matt

Thanks Matt.
Yes, I suspect that the frame has seen some "interesting exercises" in its time.

On opening-up my (Mototrans) crankcases I was surprised to find that the mounting bolt holes are tapered. The result is that the bearing surface of bolt onto casing is only at their outboard ends. It is no wonder that the holes elongate and the engine moves about. The race prep advice is to bore and fit 3/8" bolts. That would then have the effect of increasing the bearing area to around half of the casing depth. As I don't want 3/8" bolts in a road-only restoration, I am going for plain sleeves to a depth of 12mm to give greater bearing area and to take out ovality and the same but as top-hats where the casing width has also been reduced by wear-down from friction. (Note that the rear lower bolt has a locating dowel tube, the others don't.)