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Discussion Starter · #1 ·
All, I haven't posted in a while but I'm still slogging along on my '76 Bonneville. The latest issue is with the swing arm. On removal it appears that there are some issues. I'll try to use the proper terms though I find it confusing.

The swing arm itself has two heavy steel sleeves, called bobbins in some places, each supported by two bimetal bushings. In this case it appears the bushings on the inward facing sides were either not fully pressed into the sleeve or somehow worked their way out resulting in them wearing against the face of the shouldered bits that fit into the sleeve that is mounted to the bikes backbone. Hope I'm making sense.

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I have pressed them out and bought the LH Harris swing arm kit with bobbins, bushing and dirt excluders. My first question is, how does one reassemble this in a way to insure the bushings don't protrude? The shop manual is a bit light on instructions, use tool 61-6117 and a bit of grease and pull them into place.

Next question, there are two shouldered, what do I call them, steel distance pieces?

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These go into the steel tube mounted to the backbone. These are obviously trashed but looking at the parts book I can;t identify a part number. Can I get some help identifying them by part number and also some input on how this happened and pointers on reassembly?

Thank you.
 

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Hi Rick, The tool in shop manual uses a tube & two stepped thick "washers" Placing small washer into bush & large on tube, with tube placed at end of fork, you pull the inner bushing to the outside & hits outer bushing, pushing it out into tube. So both bushings pulled at once. Going in you pull bushing in one side without tube using small & large washers. You must stop when bushing is flush with end of swing arm. You have to look as tool doesn't stop it. Then use tube to pull the 2nd bushing in from other direction.

Since you have bushings out, you just need to press them back in supporting the arm at each end of housing. You'll need to jig up press to do this unless you have wide press. Don't set one housing on press & press bushing in other housing. You'll risk bending swing arm.

Shop manual says you could drift them. I bet I'd beat them silly with drift. Factory tool 61-6117 not so easy to find in stock in USA. Do a search.

So normally on a well serviced bike where swing arm was greased regularly the Bobbins (pivot) 83-2691 should just push out with your finger or a wood dowel etc. They are supposed to be a sliding fit. Mine are not tight in bushings.

The stepped bushing in the 2nd photo is normally pressed?? or welded?? into the frame tube. I've not seen them loose like yours. Mine are quite tightly fitted. I've not seen part # for this stepped bushings. Never seen them out before.

It looks like your swing arm was not greased & bobbins/pivots froze to the bushings such it was rotating the bobbin & it spun stepped bushing in frame??

Could you sand swing arm side of bushing ends flat, use red loctite to hold them or are they too loose in frame? then use some machine bushings thin flat washers from Ace hardware for spacers to fill what was removed? If you were local I'd make some to fit frame.

I don't have much experience with T140 swing arms so hopefully you'll get more complete answer. I just took my pivot bolt out last weekend as I'm welding reinforcement tabs to my frame. Not cracked yet, but replicating the '78 & later reinforcements to prevent future problems.

On my bike the swing arm pivot bolt nut was very tight. Shop manual says tighten until swing arm falls under it's own weight. I don't know what shop manual really means. I don't think you could tighten bolt any tighter. Swing arm moves freely. This has been my experience on two other T140. Bobbins are long enough that they pinch tight to frame side plates & the stepped bushings on back bone, slight side play in swing arm. A few thousands to .010" maybe. So the shop manual has always puzzled me.

I've always wondered if the root problem of back bone cracking at pivot tube was dry swing arm bushings twisting on back bone?? Still wonder.
Don
 

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Discussion Starter · #3 ·
Thanks Don. You confirmed what I was thinking. I was trying to understand how it is intended to work and it seems that the bobbin is meant to be slightly longer than the tube it mounts into on the swing arm and that allows the swing arm to move. The bobbin gets essentially pinched in place and doesn't move, the swing arm moves by the bushing slipping on the bobbin. My issue is likely from poor maintenance. There was some grease in each but there was corrosion on the bobbins so they probably bound up. I was speaking with the shop that did my motor, he can get new shouldered bits, that mount into the tube on the backbone, machined. They apparently want to be an interference fit, he reccommends warming the frame sleeve, freezing the shouldered bits and using green locktite. The catch is I need to supply accurate measurements since the originals are pretty beat up. Inside diameter of the sleeve on the backbone and then thickness of the outer shouldered bit. I'm thinking this might be an iterative project.
 

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Hi Rick,
This was a bit of clever design from the people at BSA when they planned this frame.
The setup is designed so that all relative movement is between the bobbins and the bushes, and never on any frame, spindle or swingarm surfaces, this way neither the frame, spindle or swingarm would wear out.
Replacement wear parts came in a kit (bobbins and bushes) making fixing worn out pares cheap and easy.

Looking at your photo, I think the previous owner must have been a Marvel fan and watched a Thor movie before attempting to fit the bushes, what a mess.

There is a thin spacer for the chain side and a thick spacer for the thin side, when these are installed with the swingarm and the bobbins, they are a close fit and only just slide in. The spindle goes in and the bobbins, frame and spacers all pull up tight and lock together solid as the spindle nut is tightened, leaving the swingarm to rotate on it’s bushes.

I am wondering if the stepped bush is part of the frame and has been broken out. On the back of the downtube there is a welded on swing arm tube, these have a flat surface each end, if the smaller part of the step fits inside this tube, leaving the flat surface to control sideways movement of the swing arm.
I thought these were welded in to position to determine the correct width, but they could be pressed in, I have never taken these out.
I can see in your photo someone has thought attacking them with a chisel was a good way to get them out.

Your machinist will need the swingarm with the new bushes in to be able to position the stepped spacers, the tube/spacer width is critical to getting no sideways movement in the swingarm.

If Don has any photos of the 1978 frame mod that he is doing, then it is worthwhile also doing this at the same time. It strengthens a weak point where the tube is welded to the frame.

Good luck
Peg.
 

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Hi Peg, I don’t have meaningful photos yet. Just of the thick paper templates.

I see a rounded groove where the stepped bushings abut frame tube. I see welding spatter on the bushings, but it doesn’t really look like they are welded. Maybe some spillover weld around back side where fender bracket is welded. But not welded all around for sure. I’ll try to post photos tonight.
Next week I’ll be mostly off line with very limited web access. So l haven’t abandoned you all.
The bushings could have been induction welded into frame tube. Again I didn’t know these were separate.
Peg, looking at later reinforced frame.
How is the forward reaching fender bracket welded? My plan was to wrap my metal around the bushing area to more closely replicate later frame.


I would wager dry, binding bushings spun the bushing in back bone.
My son is skilled welder. Owns both MIG & TIG. Plan at moment is TIG weld to bracket & tube with steel filler. Weld front to backbone with silicon bronze filler due to less heat needed & still very strong. Inside of backbone must be super clean to reduce pull through of oil in the puddle. More critical with steel filler, but still oil wants to pull through on red hot metal. I’m no real welder!! Non chlorinated cleaner must be used. Not a problem to get here. My fear is to introduce a leak. It’s bone dry now so no leaks.

I’ll just post photos from phone. They often don’t enlarge well though.

First photo is right side, 2nd left side. You can see the weld going slightly on bushing.
Don.
 

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Hi Rick, Can you post some photos of pivot tube ends? Is both bushings out and or damaged or just one side?

Did someone drive bushings out that you know of, or did they fall out after removing swing arm?

Another thought my son had is if pivot bolt was loose the center tube would not be supported by the frame side plates. Was the bolt tight?
He’s mechanical engineer, no motorcycle experience at all. He doesn’t care for bikes. Thinks I’m nuts. Him & mom are right!
Thanks, Don
 

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Discussion Starter · #9 ·
So both of the shouldered bushes just fell out on disassembly. No weld spatter on the that I can see though a fair amount on the weld attaching the tube to the backbone. Post pub lunch welding or a later repair attempt? One of the shouldered bushes is deeply groved as if it was grinding away on the backbone tube. Here are more pics. PXL_20210723_003412650.jpg PXL_20210723_003440454.jpg PXL_20210723_003733067.jpg
 

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Hi Rick, Without seeing in in real life, hard for me to say exactly what I'd do.

Probably attempt to face off the bad end of tube square. Machine bushings custom to each end & face them to get correct end play of swing arm. I'd make them with the small end much longer, moderately tight press fit & use green loctite (sleeve retainer). We used sleeve retainer at work to very good results, but the shoulder on original bushing is too short. I don't feel there is enough purchase for the sleeve retainer to do it's job. I'd draw new bushings in with threaded rod.

The shop will need frame & swing arm to fit the new stepped bushing end play & press fit. Expect about 3hrs or so shop time. Finding shop to take on job may be challenging. Really this is simple turning, drilling operation. Do you know if any local general machine shops or anyone that has a lathe? Anybody you know that knows anybody with lathe? I just brought up google maps & entered machine shop. I was surprised to see a few locally to me. I thought they were all gone. If you could find a hot rod shop that fabricates or find machinist that likes old bikes & cars. Lots do. They would "get it" & be on your side & hopefully do a good job. Most machinists are pretty smart.

Or maybe someone in the RAT group is willing to take it on. There's a handful that have machine shop at home.
.
Has anyone in this group seen this problem before? I've not heard of it.

Don
 

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Hi Rick
The welding around the tube looks strange, I am wondering if there was a leak and it has been repaired, and this was the start of the problems with these stepped bushes.
When fixing this, make sure that any assembly is done with the swingarm spindle in place, there are 4 holes to be aligned over a wide distance, these must be in perfect alignment before any permanent fixing is done.
If they are fractionally out of alignment you will not get the spindle in.

good luck
Regards
Peg
 
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