Author Topic: 1966 A65 frame dimensions at swingarm pivot  (Read 211 times)

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mikegt4

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1966 A65 frame dimensions at swingarm pivot
« on: 30 May, 2022, 04:05:52 »
I retrieved my Spitfire Mk11 frame from it's decades long home in the attic space over the garage the other day in my effort to get it back on the road for my 50th anniversary of ownership.

The swing arm pivot tube had seized making the bike a hardtail resulting in my completely disassembling the bike in an effort to remove the firmly stuck swing arm. After trying everything I could think of to free it I finally bent the tabs on the frame outward enough to get a hacksaw blade in and cut the pivot tube completely through on both sides of the swing arm which allowed it to be removed from the frame. Fast forward to today I have bent the tabs back to pretty close to their original position using a 3/4" threaded rod and nuts run through the same holes that the pivot tube fit in.

My question is does anyone have the original dimension between the insides of the tabs, overall width of the swing arm with the bushings in place and the clearance between the tab inside and the swing arm when fitted in the frame? Currently I am about 9-3/8" inside of the tabs which seems to be the same as where the tabs are welded to the frame tubes. I am in my 70's so I won't be dragging my knees around any corners and don't require dimensional perfection but I would like to get it as close to factory specs as I can.

Bess

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #1 on: 30 May, 2022, 06:35:50 »
Hi,
    9.373 - 9.383", internal dimension. Hope the image helps:
« Last Edit: 30 May, 2022, 06:38:03 by Bess »

Spaceman

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #2 on: 30 May, 2022, 14:04:58 »
As long as you can get the re-bushed swinging arm back into position between the frame brackets, tightening up the swinging arm pivot bolt should pull the brackets back to where they need to be, I would have thought?.

mikegt4

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #3 on: 31 May, 2022, 04:23:41 »
Thanks BESS, that is exactly what I needed. The 9.375" indicated on the plans is what I have after bending the tabs back straight.

Today I searched through my original parts and the replacement parts that I purchased for the repair circa 1980. The original "silentbloc" bushings were about 1/2" longer than the replacement part (68-4362), together the originals spanned the distance between the mounting tabs less about 1/32". I will have to be careful to get the proper protrusion at each side of the swing arm when I press the bushings in or maybe make a spacer to fill the gap between the bushings. Surprisingly the rubber ends of the original bushings were at the center of the swing arm instead of out at the ends. My "new" parts were still nicely preserved in cosmoline and in plastic bags from GP Cycles in San Francisco, I don't think they still exist but I bought parts from them back in the day after all the Brit bike dealers closed.

Here are a couple of photos, the original pivot is on the bottom. The camera doesn't show it but there is a line in the center where the 2 "silentbloc" bushings butt together. The rough surface is what is left of the rubber that was melted (torched) to get the pivot tube and inside tube of the "silentbloc" (both still fused together) out of the swing arm. I pressed the outer tube of the "silentbloc" out of the swing arm a few weeks ago, unfortunately I didn't save the parts.

Spaceman, there is no way that the pivot tube and nut would ever bend the 3/16" thick tabs, the shallow fine threads on the tube would just strip out. I had a 12" ratchet cranking the 1-1/8" nut on the 3/4" threaded rod to get the tabs to bend back straight. The nut does little more than keep the tube from backing out should the small anti-rotation bolt on the other end fail or fall out. With my pivot tube seized solidly to the inner tube in the bushing I am surprised that the little bolt didn't just shear off. It's the same design as front loaders on tractors and construction equipment use at their pivot points, without the rubber of course, and the pivot pins are usually solid steel. I have made a couple of replacements for my front loader. Photo of a loader pin, they always have grease fittings like the BSA pivot should have been.

Bess

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #4 on: 31 May, 2022, 09:47:06 »
Hi,
    The dimensions of the bushes should be checked before installation and the 0.0120 left protruding. Use lots of lubrication cool the bushes and warm the swinging arm. Make up a bush installation tool.

When the bike is complete sit on it, ask a friend to tighten the swinging arm spindle nut to securely trap the inner sleeve against the tabs in a loaded position, ensure there's some clearance between the swinging arm and the tabs to allow the arm to 'swing'.

If you want a needle roller bearing conversion instead I have some plans you can have.

Best wishes...


« Last Edit: 31 May, 2022, 09:49:07 by Bess »

Mike Farmer

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #5 on: 31 May, 2022, 14:43:56 »
 :) :) :) :)

I put mine in the freezer overnight. Makes a lot of difference.

Mike 8) 8) 8) 8)

mikegt4

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #6 on: 31 May, 2022, 16:00:31 »
My "new" bushes #42-4362 are 4.152" overall, the outer bush is 2.25" with 0.110" of inner sleeve protruding from the end of the outer bush. The original bushes butted together in the middle, as best I could measure they are about 4.672 overall but the ends were probably damaged during the removal process. I could make a spacer to compensate for the bush length discrepancy, I have a lathe.

Bess

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #7 on: 31 May, 2022, 19:55:27 »
Hi,
    I believe there were a batch made up awhile back that were incorrect, seems you have a pair from that batch. Worth checking the diameters as well, 0.965" and 1.250" that's an average as the tubes aren't perfectly round. The spacer is an option, when fitted I have found an issue pushing the spindle through where the bushes meet, with a spacer it might be more of a problem.

Is it an option to send them back for a refund and buy a correct pair from here:

https://sites.google.com/site/priorymagnetos/rear-suspension-footrests-stand

Best wishes...

mikegt4

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Re: 1966 A65 frame dimensions at swingarm pivot
« Reply #8 on: 01 June, 2022, 05:08:13 »
I don't think I could send them back, I purchased them circa 1980.
When I cut out the swing arm from the frame I did so by spreading the tabs enough to expose the pivot tube enough to get a hacksaw in the gap and cut the pivot tube flush with the ends of the swing arm. Once the swing arm was free I burned the rubber which allowed the pivot tube with the inner bushing shell (tube) to be driven out of the swing arm.

The photo is of the pivot tube seized inside of the inner bushing(s), where the 2 bushing shells meet in the center can be easily seen. The marker lines I drew on show where the remnants of the burnt rubber are. It sure appears that the bushings were installed incorrectly with the rubber at the center of the swing arm instead of the ends next to the frame. I never did anything with the swing arm and I don't remember my friend whom I got the bike from ever disturbing the swing arm either. Between us that would cover the bike's history back to 1969 so odds are the factory installed them that way.