Polybushes

[Reckless Rat]

I'm sure most of you (unless you have an engine that is completely oil tight) have had to replace the engine mounting rubbers on the later crankcases. I have lost count of the number of times I have replaced mine, mainly due to the generosity of my engine sharing its contents with the wider world. I have given up trying to make it oil tight and live with it as it is, accepting its foibles as part of life's great tapestry.

However I am getting fed up as it's happened again - the rubber mountings "blow" and split due to the oil contamination, rendering them useless:



Being generally tighter than the windows on a submarine I have thought about alternatives and have made myself some replacements, using polyethylene bar at 90 shore. It is machineable on the Myford providing you use a high speed and a lot of positive rake on the cutting tool, which must be as sharp as possible. It's not the easiest of material to secure due to its pliability but you can get a good enough grip if you use a rotating centre.

Anyway, it's peeing it down here today so I've been busy. I obtained a short length of poly bar from Direct Plastics in Sheffield, but I'm sure there are other stockists near you. Here's the result of my labours thus far:



I'm using a spare crankcase just for fitting purposes. The poly bar as you can see is red but the material can be painted if necessary. I think I'll leave mine as they are.





The job's not yet complete as I haven't yet turned the small inserts for the mountings - due to the flexible nature of the poly I'm going to have to bore it first and then mount the material on a mandrel so I can turn down the outside diameter. That'll be tomorrow's task if it's still raining...

My thanks to Ruairidh for the dimensions.

[Bryan Downes]

Very impressive Reckless - well done!

Bryan.

[Ruairidh Dunford]

I am very interested to hear how this turns out and any differences to vibration you notice. I could trial a set on a journey I am making this coming summer...

[cardiffrob]

Just a thought...3 like that and one in something softer to accept and chassis flexing?

[Reckless Rat]

I would guess that the polyurethane replacements will be firmer than the stock rubber mounts - The poly bar is 90 shore hardness. I haven't got an unused set of rubber mountings for comparison purposes but vulcanised rubber on car tyres is probably somewhere between 60 and 70 shore. The one (and major) bonus is that polyurethane is fuel, oil and abrasion resistant so it should last. It should perform OK because there's not really much movement on a rubber mounted engine when it has been bolted down. I do tend to leave one rear mounting loose to allow for chassis flex.

[Viteran]

Hi
Very interested in the use of poly bar, as well as a Ruby I am rebuilding a ‘35. Hornet which has a single bushed front engine mount. I think I will try turning and drilling out some bar for the mounting.It may also work for the damper bushes.
Would putting in the freezer for 24 hrs make it less flexible and easier to machine ?

[Reckless Rat]

It has been suggested, reading articles on line that freezing polybar can make it harder and thus easier to machine. However they are talking about using liquid nitrogen not just bunging it in a domestic freezer. This is in particular relative to lower shore hardness than 90. I doubt if chilled poly bar out of the freezer would remain cold for long by the time you have set it up ready in the lathe. I have found it easy enough to machine as long as you give it a rest from time to time.

I will post a close up of the tool I ground specially to do the turning tomorrow once I get back down to the garage. I also made a very thin parting tool to start off the parting process, which I finished using a hacksaw.

I found drilling/boring the poly bar was easier using an end milling tool rather than a twist drill.

Another alternative is to buy the polyurethane as a raw product, mix it with the hardener and then pour it into a pre-prepared mould.

[Steve Jones]

I am very interested to hear how this turns out and any differences to vibration you notice.  I could trial a set on a journey I am making this coming summer...

Perhaps better that you trial a set on the way back. That's always assuming they're waiting for you when you arrive....

Steve

[Viteran]

Thanks for the response, I have wondered about using the liquid form but for the small amounts I wanted it seemed overkill. The bar seems simpler.
Unfortunately I no longer have access to liquid nitrogen since I retired!

[Ruairidh Dunford]

I am very interested to hear how this turns out and any differences to vibration you notice.  I could trial a set on a journey I am making this coming summer...

Perhaps better that you trial a set on the way back. That's always assuming they're waiting for you when you arrive....

Steve

Good plan, I'm on it!