Frame rear engine mounts, narrowcase (Mototrans)

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Geordie
Posts: 141
Joined: Thu Jan 16, 2020 12:15 pm
Location: Mallorca, Spain

Frame rear engine mounts, narrowcase (Mototrans)

Postby Geordie » Thu Jan 16, 2020 4:15 pm

New here and first post. My second Elite - first was in 1968.

Complete rebuild underway, starting with all the cycle-parts. The rear engine mounts show wear both by thinning of plate at contact points and elongation of bolt holes. Checking the full and half-widths of the casings at the rear mounts, there is wear, mostly to their left side. The distance between the frame plate mounts is less at the contact points than elsewhere. I would guess that loosening bolts was a constant problem during the bike's functioning life and this was corrected by tightening at intervals. The result was the plate mounts were bent in the process. The forward frame tube mounts show little or no wear.

The internal distance between plates where undisturbed is 66.5mm to 67mm and the external distance over the tube mounts is 65.6mm to 66mm.
Discounting paint thickness, I've settled on 66mm to manipulate the plate mounts to. This will give slight clearance to the casing mounts ~ 65.5mm (thanks toni.s on another thread).

I find from others here that this is not a unique problem and I don't want it happening again after correcting it. It is my opinion that the bearing area of the bolt on the plate mount was too little which caused hole elongation. This threw the requirement to stop differential movement entirely onto friction between engine casing and plate mount. This resulted in wear at the contact points and, consequently, the reduction in tension of the bolts.

I am going to weld (outside) bosses to the plates as well as set their gap and at the same time, correct the hole centres and their diameters. I got the hole centres from templating the motor mounts - fortunately those holes were not elongated. If anyone is interested, the boss diameter is 22mm to match the plate mount radius. Attached is pic of a jig to hold all in place for tack welds.

If all goes well, bolts fitted to avoid their threads bearing on plates and bosses should finish things off. Unless someone sees the flaw in my plan?

IMG_20200115_122447795_1.jpg
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jbcollier
Posts: 86
Joined: Sun Oct 10, 2010 2:30 am

Re: Frame rear engine mounts, narrowcase (Mototrans)

Postby jbcollier » Sun Jan 26, 2020 3:38 pm

When I did my NC, I drilled out the cases and frame and fit larger bolts. I fit grade 8, 3/8NF bolts. This was a recommended mod for racing, probably not what you would do for a restoration. Mine is a cafe bitsa so I wasn't concerned about incorrect fasteners.

Geordie
Posts: 141
Joined: Thu Jan 16, 2020 12:15 pm
Location: Mallorca, Spain

Re: Frame rear engine mounts, narrowcase (Mototrans)

Postby Geordie » Wed Jan 29, 2020 7:20 pm

If that works then all is good. I had to weld due to frame hole ovality and plate thinning due to rubbing so a doubler was an obvious mod without being stand-out obvious. I have re-mounted the engine to check centres are OK, they are, and have found some ovality on the front mounts' holes. New plates being made 5mm to match the now 5mm rear mount lugs. They will be in SS and I'll decide on painting once I've decided the route to go on the whole project.


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