This can be a frustrating job. Not because anything is seized – you’ll be fitting new parts with plenty of grease – but because the design of the rear suspension means that the rubber block is always in compression, and you will be fighting the rubber as you try to insert the pivot bolt.
Firstly, obtain a pivot bolt kit. There are a couple of suppliers on eBay and Moulton Preservation sell kits if and when they are operational again.
The critical dimensions are 7/16″ for the OD (outside diameter) of the sleeve and 5/8″ for the ID(inside diameter) of the pivot tube. The bushes must be just over 7/16″ ID when pressed in and just over 5/8″ OD before pressing in. Check the kit on arrival to ensure your bushes are slightly over 5/8″ OD, using calipers. 0.630″ is just about ideal. They must be a tight press fit into the pivot tube, which means they need to be larger than the tube itself. It is critical that the bushes are not loose in the tube.
In theory you could manufacture your own kit quite cheaply if you can’t obtain a good one from anywhere; the kit consists of two nylon bushes, a long M8 partly-threaded bolt (or an 8mm spindle with threaded ends), nut(s), washers, and a steel sleeve of precise length. The sleeve is easy, as 7/16″ OD steel tube costs next to nothing. The bushes are the tricky bit, as no-one seems to sell off-the-shelf nylon bushes in the correct size, although this type of bush costs peanuts from a bearing supplier. Bushes with a 16mm OD and a 10-11mm ID are available, and could potentially be reamed out to the correct ID in situ (16mm is an excellent press fit into a 5/8″ ID tube).
The tools you will need are as follows: the trusty mallet, a long fully-threaded M8 bolt (150mm is good) with three large washers and a nut, two 13mm spanners, a pop rivet gun with 4 x 3.2mm rivets, and a ratchet strap. Molybdenum disulphide grease (CV grease) is the usual lubricant. A long 7/16″ reamer is very useful when dealing with pivot kits of indifferent quality.
Rivet the suspension block to the rear triangle, top side first. You may need to carefully run a 3.2mm drill through the holes to remove any new paint from the inside of the holes. Now do the bottom rivets, carefully tapping the tab back into place with the mallet so that the holes line up. A suspension block from one frame will generally not fit a different frame without some metalwork; the holes were not drilled consistently at the factory.
I de-rusted, primed and painted the metal parts of this suspension block before re-riveting it. These were originally bare galvanised steel but they blend in with the rubber part better if they’re painted black.
Clean the inside of the pivot tube and lightly grease the bushes on the outside. Put one washer on your long M8 bolt, followed by the first bush, flange towards the bolt head. Put the threaded end through the pivot tube then, on the other side, add the second bush (flange outwards), two washers and the nut. Hold the bolt head with one 13mm spanner and wind the nut in with the other 13mm spanner, ensuring the bushes go in squarely. Ensure the bushes are fully seated, but don’t overtighten as this will deform the flanges.
Grease the sleeve liberally and check it for fit. It should go in under firm thumb pressure, or with very light taps from the mallet. If it is too tight, you have a number of options:
- Get a bush kit that fits correctly
- Run a reamer through both bushes to make the ID correct
- Remove the bushes and clean any thick paint from inside the pivot tube
- Reduce the size of the sleeve slightly with abrasive paper. Putting it in an electric drill as a makeshift lathe is a good way
Once you have a sleeve that is a good sliding fit, thoroughly grease it, insert it, and ensure it protrudes from the bushes slightly on both sides. This is essential. If it doesn’t, there may be thick paint on the faces of the pivot tube, or you have been supplied with a sleeve that is too short.
Offer up the rear swingarm to the frame, ensuring the “shoe” of the suspension block slips squarely into the main spar. You will notice that none of the holes quite line up, because the rubber block needs to be compressed.
To do this, get a ratchet strap and pass it around the base of the head tube and the brake bridge of the swingarm. Protect the paint with soft cloths before tightening the strap. You will be able to pull at least one hole into line. Insert the well-greased bolt or spindle from the kit (not forgetting the anti-turn washer, if you have the bolt type) and push or lightly tap it in. It will generally become stuck as it reaches the “ear” of the swingarm on the opposite side. Some manipulation of the triangle and tapping with the mallet will now be necessary to get the bolt to pop out of the other side.
If using a bolt type, tap the bolt fully home, add the other washer and nut to the other side, and tighten to about 30Nm while holding the bolt head steady with the other 13mm spanner. if it is the spindle type, centre it up in the frame, then add the washers and two nuts before tightening as above.
Now check that there is a sliver of daylight – it doesn’t matter how small – between the bush and the “ear” of the swingarm on at least one side. It is critical that the ears are pinching the sleeve and not the bushes. If there is zero clearance, you need a longer sleeve. This can happen due to a poor quality kit, or because a previous pivot has seized and allowed the old sleeve to chew into the ears.
You can see that the swingarm ears are not pinching the bushes in this shot
Fit the three no.10 self-tapping screws that secure the “shoe” of the rear suspension block to the frame. New stainless steel screws are strongly recommended, as for all small fasteners on the bike. The bottom one will benefit from threadlock at this point, and you can use threadlock on the two side screws when you come to fit the “foot” of the lower rack strut.
Some kits come with an over-length chamfered bolt, which is easier to encourage out of the opposite side of the swingarm. If you have one of these, you will need to hacksaw off the chamfer when installed, and smooth the end of the bolt with a file. Watch the paint when doing this.
Moultons and other endangered species 