Making POLY bushes
First attempt but not the best result.
Spot my mistakes! (But I get there in the end)
Many, many classic car enthusiasts are faced with replacing rubber bushes when they wear out. Some of the more popular classic cars are very well catered for and new rubber is available and inexpensive. But there are some cars for which new bushes are either unavailable or very expensive. My old Subarus require new suspension bushes and new spring-damper mounting bushes. After much time on the net and in contact with a Subaru parts department it became clear that even if all were available (they weren’t) they would be expensive; about £300 per car. A huge sum for a few small rubber parts.
In this ‘How to’ I’ll go through the stages of making my own using easily available craft materials. But be sure to read to the end because the first attempt did not work as planned and I ended up using a slightly different type of mould.
(Above) both of these bushes are part of the top mount of the rear spring-damper assembly. The one on the left is only slightly worn but feels slightly brittle. The one on the right looks OK from this angle but…
(Above) …seen from the other side it is badly worn and has started to break up; indeed, a large chunk fell off it soon after this shot was taken.
While some people buy polyurethane rods and turn them on a lathe to make new bushing, I decided to cast mine in a mould using a two-pack liquid pouring polyurethane. I chose a ‘shore 70’ hardness rating as a compromise between a hard-riding spec of 90 or a soft road spec of 60. Though available in ‘hobby’ quantities, the Polycraft product is of industrial quality so I’m reasonably confident it will work almost as well as a commercially made bush: it will not be as good as ‘the real thing’ because my home-poured product will have fine bubbles and various other flaws that proper manufacturing processes can avoid; curing temperatures can be critical.
(Above) I also bought some casting plaster and some modelling clay (not shown) from the same supplier.
Having used plaster moulds for poured vinyl and papier-maché when I was a prop-maker many years ago, I decided to use the original bushes as plugs for a poured plaster mould. Hopefully, the mould would be tough enough to cast 4 sets, two for each rear side for each car. My first step was to make a strong wooden tray to hold the mould.
(Above) The tray had to be strong enough to ensure the plaster was well protected and would not be easily fractured, so I decided to screw and glue it. Some scrap 12mm plywood was ideal, the sides of the tray being approximately the depth of the bushes.
(Above) I drilled the base for screws and for the centre spindles of each bush. A good thickness of PVA would ensure the mould assembly would not leak and stand up to rough treatment if, for example, the finished items needed to be levered out.
(Above) The tray was then coated inside with PVA. While that was drying…
(Above) The spindles are made out of the scrapped shock-absorber so it’s going to be a reasonably good fit. Here I’m using a mole-wrench as a saw guide – the first few cuts on a hard chrome finish being hard to start otherwise.
(Above) The spindles were then nudged into the holes with a hammer; I’d chosen a drill-size slightly smaller than the spindles for a friction fit.
Now there’s a slight gap in the visuals because I got so interested in the task I forgot to stop and take pictures! The next stage was to ensure the old bushes were ‘repaired’ enough to be able to be used as plugs for the mould. The broken bush was glued back together and missing bits were replaced with modelling clay. To avoid ‘undercuts’ in the mould that might stop the finished pieces releasing easily, I ensured that the narrower end of each bush went face down in the mould tray. Gaps between the worn old bushes and their spindles were filled with clay to stop the plaster from rising up the gaps when poured.
To ensure the original bushes would release from the mould, I rubbed fresh grease into the rubber where it was textured and sprayed both with WD40.
(Above) With snow falling on my tin-roofed shed, I moved indoors for the next stage. Not only was I warmer but so was the plaster and polyurethane – they take forever to set if cold. Here we see both of the old bushes narrow-end down in the tray on their spindles with clay filling the gaps between spindle and bush and plaster poured around them.
A warning about this brand of plaster. It is fine but the label is incorrect: it tells you to use water to plaster in the ration of 100:35 but that is wrong.
Indeed, it should be 35:100 water to plaster. Because of my experience using plaster, I guessed the label was wrong and did some small test mixes to make sure. Due to the cold weather (I was indoors but in a room not more than about 15˚C) the plaster took a while to set. But there’s always other things to do – don’t be tempted to take out the plugs before the plaster is properly hard.
(Above) with the spindles knocked out and the plugs eased out, the mould is cleaned of any loose dust and bits of clay and is ready for greasing.
(Above) Here’s the mould with the spindles put back. For a release agent I used a mixture of grease and WD40 and made sure there was plenty of it brushed inside the mould and on the spindles.
Using an electronic kitchen scale, I weighed the original bushes; 57 grams. As the two pack PU required a 1:1 mix I mixed together 30grams of each to make a 60 gram batch.
(Above) The mixed polyurethane starts to go off quite fast at room temperature so its very difficult to have a bubble-free mix. Here is it just after pouring. The little plastic pot in the background has a little bit left in the bottom of it so my weighing of the objects worked out well. I hate wasting materials!
At this stage I was hoping all I had to do was to slide out the spindles and pop out the new bushes and all would be ready to cast another set. But….
(Above) I tried everything except a metal lever (I did not want to damage the bushes) and even took the tray apart so I could push the bushes out from the bottom. With a great deal of persuasion the spindles did let go eventually. But the new components would not pop out. Maybe the grease/WD40 mix was not a sufficient release agent or perhaps the polyurethane simply was not squishy enough to cope with the unevenness of the surfaces – whatever, they would not budge. It must be remembered that I had not ever cast this material before and I was copying a technique I had used for casting a softer, more flexible vinyl. So…
(Above) …I decided to break the mould. Indeed, the release agent had not worked and you can see plaster embedded in the surface of the polyurethane. But the good news was that the components looked perfectly usable. But I wanted a mould that was easy to use and very much re-usable. For casting rigid materials I have in the past used split moulds, and while polyurethane is not rigid it is not all that bendy either. So I decided my next attempt would use a split mould. I also decided that I would seal the surface of the plaster with PVA and use pure grease as a release agent rather than the mixture. I could have bought some proper release agent from the supplier but I’ve never bothered with it before, even for larger castings.
(Above) I’ve reassembled the tray and marked it for sawing it down the middle. This technique works because the spindles are part of the mould and will keep the dimensions reasonably true in spite of the width of the saw cut.
(Above) The tray halves are put back and clamped around the spindles.
(Above) with the gap between the tray halves filled with clay, a clay wall was made to divide one side of the mould off from the other. The bush on the left is undercut so clay had to be carefully arranged under it and then ‘squished’ down by the bush. The bush was then lifted and excess clay removed. The side to be filled with plaster needs a nice flat ‘wall’ – use a screwdriver or small knife - but the other side need not be so carefully made. Both the bushes and the spindles have been thoroughly greased but not to the extent that the grease would alter the shape of the mould.
(Above) Here I’ve poured plaster into the mould and left it to set.
(Above) After the plaster has thoroughly set, the mould is then split and disassembled but not before marking the position of the plugs – they are not perfectly round or symmetrical so will have to go back the same way.
(Above) Next, after the plaster is properly set and no longer damp, I used a flat sanding block to make the side of the mould nice and smooth so that the two halves would separate easily.
(Above) With this half of the mould cleaned up, I gave it a good coat of PVA to seal up all the edges to help with a smooth release when the mould is used and split.
(Above) All cleaned up and with the PVA dry, the two halves are reunited. The face of the plaster, where it will meet its other half has been well greased as have the plugs and spindles. The gap between the halves of the tray has been filled with clay from the outside, any plaster that seeps between the halves can, after it has set and been disassembled, be cut off as ‘flash’ or simply left – though being thin it will probably break off anyway.
(Above) With the second half poured right up to the edge of the first, the join should be perfect. Hopefully, the PVA and the grease will allow them to separate just as perfectly! There’s then the wait for it to set – by this stage I’m getting impatient for a result.
(Above) With the second half set, the mould is split. The next job is to fill the width of the saw cut in the tray.
(Above) A couple of layers of thin card are glued to the edge of one of the tray halves and the gap is filled. The newly cast half is cleaned up and, like the first, is given a good coat of PVA, allowed to dry and then greased.
(Above) Though exactly the same size in theory, the two halves might not meet up exactly because the plaster halves might not set at the same rates and at the same temperature. Here, the two halves are reunited but there are small differences in their relative sizes that will produce joint marks and/or ‘flash’ on the finished items. On a rubber bush, such imperfections do not matter at all. On a rigid component requiring more precision, one might have to make good with a file afterwards or machine from a larger cast size to the correct size.
(Above) Here the new mould has been filled with rubber. Note that I have failed to check the spindles are both properly centred and the smaller of the two looks a bit off. Again, for a rubber or polyurethane bush, it might not be a critical problem. In this case, the bush will fit over the narrower threaded section of the damper spindle with the gap being there to take up compression of the bush so a slight off-centre doesn’t matter. If it were a suspension bush, I would not use it if I thought it would affect the suspension geometry unduly.
(Above) Success! The mould split very easily and the bushes popped out without a hitch.
(Above, left) the smaller bush has more flashing and is slightly ‘on the piss’ so next time I’ll ensure I put a spirit-level on the mould before it sets. (Above, right) After they have been cleaned up, I think they are both quite usable. The mould also seems to have stood up to the process though some definition will be lost. I’ve filled the mould again and, given the cost of bushes from pucker manufacturers, the materials have already paid for themselves. 90% of the plaster is unused and I’ve used a tiny amount of clay. I’ve used about 20% of the polyurethane to make 4 bushes. So I can make 20 bushes from a kilogram of material. The Poly cost about £25 and lets say the other materials total £5. So, for £30 all-in I can make 20 bushes - £1.50 each.
Of course, if I factor in my time, it’s a different story. But I made the moulds and cast the bushes on a Saturday evening and a Sunday afternoon; I’d most likely be watching telly or reading a book otherwise so, as far as I’m concerned it’s worked out well and the next time I need to make a mould, I’ll do it right first time. As I said, I’ve used a ’70 shore’ rated material. If that proves to be too soft or if it is not tough enough for the job and wears out too fast, I’ll let you know.
Disclaimer: These pages are purely for entertainment and sharing and do not constitute an instruction for your vehicle. I undertake these tasks for my own vehicles at my own risk and no promise or guarantee of quality pertaining to methodology or materials or function is made or implied concerning work by other parties on other vehicles.