Rebuilding a later "Circlip Type" Jampot Rear Suspension Unit.

        First, lets get the obvious stuff over and done with. You will, of course, need to remove the unit from the bike. It's easier if you do them one at a time as like us, the bike will stand quite happily on one leg but will fall over of you take them both. There are a couple of "special" tools that will make your life easier but I'll come to those as we need them.
Picture001.jpg        The thing on the left is a spring compressor that I put together when I was building my basket case 350cc Matchless G3/LS. It's evolved a bit and now uses a couple of lengths of M10 studding to compress the spring where it originally used a couple of sash cramps that I inherited from my Dad. It works very well and makes removing the circlip that holds the spring retaining ring in place very much easier. It's made from some timber that I had knocking about in the shed, a couple of lengths of 1/2" diameter steel and two lengths of the aforementioned M10 studding. With the Jampot located on the two bolts, the nuts on the right hand end of the studding are done up. Picture002.jpgThis causes the two pieces of 1/2" bar to bear on the retaining ring and compress the spring. The circlip can then be eased out of its groove using a small bladed tool like a penknife and the spring pressure can be released.
        The circlip can be spread open far enough for it to be eased over the lower clevis casting. If you're lucky, the retaining ring will slide over as well and you'll be able to remove the spring and the two spring covers at this point. There should be two large leather washers, one inside each of the spring covers. They will be hard and brittle, maybe even broken and should be replaced when the units are rebuilt. If the retaining ring won't go over the lower clevis casting, then you'll have to unscrew the lower clevis casting first, which is the next task anyway. The clevis will be tight and unlikely to give up without a fight. Don't be tempted to use a pipe wrench on the aluminium tube, you'll probably end up ruining it and replacements aren't easy to find. I made up a simple clamp from a piece of hardwood (American White Ash) and put some rubber strip on to hold the tube securely. The clevis cap still needed some heat to persuade it to undo. That was applied with an electric hot air gun. Picture003.jpgI used a large open ended spanner in the clevis slot and with a bit of effort, the clevis unscrewed. With that cap off, the unit was relatively easy to disassemble completely. I had to use heat on the top clevis casting to persuade that to unscrew. You will need two different peg spanners. I used an adjustable version to unscrew the ring that retains the oil seal and bearing bush. The smaller one that is needed to unscrew the collar on the top end of the damper rod I had to make. That was easy enough. Two 3mm dowel pins set into a short length of 3/8" square mild steel bar. The dowel pins are set into the bar on 0.660" centres. Incidentally, the units I'm rebuilding here are the "competition" version that are fitted to my 1956 Matchless G80CS. They differ from the units fitted to the touring bikes in a number of ways. The springs are different and they have adjustable damping but the dismantling and re-assembly method is the same for both versions. I did take the damper rod assembly out of the inner tube but I didn't take it apart any further. There was no need to do anything to it other than a good wash with brake cleaner. All the parts were washed and inspected. As there was still a little oil inside, the internals were in very good condition for a 65 year old shock absorber. I will only need to replace the bearing bush, oil seal, sealing washers and the leather washers that cushion the main spring.
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Picture006.jpg      The new parts have arrived from AMC Classic Spares so I'm all set to re-assemble the Jampot. The first step is to put the damper rod assembly back into the damper tube and lock it in place with the "C" clip, making sure that the clip is correctly located in the groove half way along the plunger sleeve and the cut-outs in the damper tube. Slip one of the two red fibre sealing washers over the damper tube and assemble it in the steel inner tube. [If you forget to fit the fibre washer, you'll have to strip the unit right down again to fit it. Go on... ask me how I know.] Then screw the top cap onto the plunger rod and secure with the locking nut. You will need the small peg spanner to hold the top cap. That assembly can be put to one side for the moment and we'll come back to it in a minute or two. Before we can go any further, the bearing bush and oil seal need to be fitted into the outer aluminium tube. I'm refitting a bronze bearing bush in place of the plastic item that was fitted originally. Additives in modern oil can affect the plastic causing it to swell and jam the suspension. The bearing bush should be a light push fit into the outer tube but the oil seal may need a little persuasion.
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        I'm one of those old school engineers that believe a hammer should only be used for knocking in nails, so I have an aversion to using one to install oil seals. I much prefer to use a controlled force to achieve that end. Picture009.jpgA length of M12 studding, a couple of nuts and a suitably sized socket will achieve the same result without damaging the oil seal. In this case, a 1/2" square drive, 3/4" Whitworth socket was just about perfect. With the oil seal installed, the retaining ring can be screwed down with a suitable peg spanner.
        Back to the inner tube. Make sure that there are no sharp edges or burrs on the thread end as that will have to be pushed through the new oil seal without damaging it. The rebound buffer spring and collar are installed and the inner tube assembly can be assembled into the aluminium outer tube. The oil seal and inner tube should be liberally coated in clean 20W fork oil. The inner tube can now be eased into the outer tube and carefully through the oil seal.
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Picture012.jpg        The rubber buffer ring is now pushed on to the inner tube. The next job is to replace the rubber sealing ring in the top clevis casting. It's located in a groove at the far end of the internal thread. The old one can be "winkled" out with a sharp tool like a scriber or a small penknife blade. Coat the new ring in clean oil and work it into the groove. Your finger is the best tool for this job as you don't want to damage it with anything sharp. With the new ring inserted, the top clevis casting can be screwed onto the inner tube. A little grease smeared onto the damper rod top cap will help it go through the new sealing ring without damaging it.
        At this point, if your retaining ring will NOT go over the lower clevis casting, assemble the spring, inner and outer spring covers (not forgetting the new leather washers) and the retaining ring to the outer tube assembly. If it will go over, then leave the spring etc. until after the lower clevis casting has been screwed on. Before you can do that, however, the unit needs to be filled with 85ml (or 3 fluid ounces.) of fresh 20W fork oil. Hold the assembly vertically upside down, and lift the inner tube until there is room to pour the oil in. You may need to wait a few seconds to allow any trapped air to bubble out. Now place the second new red fibre sealing washer on top of the inner tube and screw on the lower clevis casting. If you haven't already done so, replace the spring etc., and compress the spring enough to replace the circlip. That's it... job done.

Last updated - 14/03/2021.