Sloppy frame mounts

Has anyone else noticed that the lower front engine mount stud is 5/16 inch (because the engine case bore is 5/16 as is the lower front engine plate bore), but the tube in the frame through which this stud passes is 3/8 inch!? While there are several frame attachment points, the front mount has only two. My fix is to use a two inch section of stainless tube that is 3/8" OD and .020" wall as a shim/spacer in the frame mount tube. This isn't a perfect fit, but takes up most of the .0625". I am amazed that this space wasn't shimmed at the factory.

Yep my lower front mount is also loosey goosey.
I am wondering why as well. Could it have been from the Matchless single cylinder to Norton twin transition? :?:
Cheers,
Thomas
CNN

Could it have been from the Matchless single cylinder to Norton twin transition?

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I suspect you are correct. However, I am surprised that ZDS motors or the Norton factory did not put a bushing in that hole!

Ron L said:

........ My fix is to use a two inch section of stainless tube that is 3/8" OD and .020" wall as a shim/spacer in the frame mount tube. This isn't a perfect fit, but takes up most of the .0625". I am amazed that this space wasn't shimmed at the factory.

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I have never hesitated to stray from originality when I thought there was room for improvement.

Slick

The P11 frame lug holes aren't 3/8" (I know that's what's called out), they're 10mm. I suspect that's because these frames were made at the time that British industry was transitioning from English standard measurement to Metric & it was considered 'close enough'. This accounts for the continuing issue with engine mounts loosening on these bikes. Once they loosen a bit the spacers fret the aluminum frame plates & the issue just gets worse.
My fix, on my 3d P11, was to ream the engine mount holes to 10mm & make M10 studs. In 8 years of VMX competition they've never loosened & vibration was significantly reduced. It did require cleaning some fabrication trash from the frame lug holes, but NOT enlarging the holes & all the studs were a nice tight moderate push fit.

Mark in SC

Perhaps they were 10mm or perhaps they were 3/8 (9.5 mm) and were beat out from the vibration of the 5/16 studs.

Your solution of confirms my suspicion, that this was at least one of the causes for the problems of loose fasteners and vibration cracks.

Ron L said:

Perhaps they were 10mm or perhaps they were 3/8 (9.5 mm) and were beat out from the vibration of the 5/16 studs.

Your solution of confirms my suspicion, that this was at least one of the causes for the problems of loose fasteners and vibration cracks.

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What about making new spacers with a boss that is the same diameter and thickness of the frame fixing lug and the hole for a 5/16" stud? :idea:
Another option would be to just make a short spacer to fit the odd size hole in the frame and drill to the bolt size.
Cheers,
Thomas

If you try the hole spacer or spacer/boss route you'll wind up with a wall thickness of 0.009".
Good luck with that!

Mark in SC

OldForge said:

If you try the hole spacer or spacer/boss route you'll wind up with a wall thickness of 0.009".
Good luck with that!

Mark in SC

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10mm = 0.394" (3/8" = 0.375")
5/16"=0.3125"

0.394-0.3125=0.0815"
0.0815/2=0.0408" wall thickness

You're right. You're talking 5/16" studs, I was talking 3/8".

Mark in SC

OldForge said:

You're right. You're talking 5/16" studs, I was talking 3/8".

Mark in SC

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Ah...that makes sense!