P11 front engine mounting studs

Ron L

Feb 27, 2004
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Has anyone else noticed that the lower front engine mount stud on the P11 is 5/16 inch while the frame hole is 3/8 inch? It apparently did not concern either the guys at ZDS or the factory, but as much vibration as the big twin genetrates and the mounting plates being aluminum with toothed lockwashers on the stud nuts, it seems like a prime opportunity for the stud to loosen.

My quick and dirty solution has been to cut sections of 3/8 inch OD stainless tubing (0.030 inch wall) and use them as spacers. I'm sure a proper spacer made from a section of 3/8 inch bar stock bored to 5/16 inch ID would be better, but I had the tubing in the shop.

P11 front engine mounting studs
I never have fully understood that one either unless there is not enough meat under the crank in the cases at that location to drill all the way through for a 3/8" stud. That frame wasn't made for a 750 Norton engine was it? Old rumor in the 70's was the Norton twin engine was stuffed in a singles frame. You would know. I'm not a Norton historian, or restorer.

I just went out and looked at the stud I replaced that was under there for 50+ years and it is bowed from the load a little.

The new engine mount bolt kits from Andover have thick flat washers on all the studs and don't include the star washers. I took the star washers off last time I had the motor out and back in using new hardware.
The frame was built for the Matchless 500 single (G85), so I'm not surprised to find a mismatch. I am surprised that neither the ZDS guys or the Plumstead works thought to fit spacers. But then again, I would have thought that by the time the factory started assembly they would have made the engine mount spacers out of alloy and tacked them to engine mounts. That would have saved time and aggravation in assembly. They did this with the N15/G15 hybrids (although the engine plates are steel), so there was a precedent.

As for boring the early 750 cases for a 3/8 stud, I think it could be done but it would weaken the boss significantly.