Fitting Urethane Bushings
So you’ve decided that urethanes are the best medium for the form of location that you require. You’ve bought your rod end, bushings and inner steel sleeves so it’s slap it all together, whack a bolt through and tighten up. If it were that simple I wouldn’t have to write a tech tip about it would I now?
The truth is that there can be significant differences in the width of the rod end, thickness of the urethanes and the length of the inner sleeve. If all these tolerances are in the wrong direction you finish up with a solid, squeaking, binding assembly.
To figure out how to avoid this first we have to visualize how it should work. The inner steel sleeve should make contact, and be tightened into the double shear fitting. However this problem can also occur on the much weaker single shear application. The urethane, if correctly fitted, should be tight in its housing and then should rotate around the fixed inner sleeve. To achieve this situation it is usually necessary to fettle the urethane bushings. They should have a clearance of 3-5 thou between the bush and the inner double shear cheek (i.e. 6-10 thou total sideways movement). If this clearance is missing when you tighten the fixing bolt you will actually trap the urethane bush and prevent it rotating, causing binding in the suspension.
Although most bushes are manufactured to about a 5 thou tolerance, the same cannot be said of the rod end, particularly if you are mixing and matching the 3 component parts that make the complete assembly, hence the need to ensure a precision fit. The time spent fettling each assembly as you install will result in a smoother squeak free ride. To thin the top hat off the bush, I normally use a d.a., orbital or finishing sander, together with a micrometer and feeler gauge. Assemble the joint, insert into relevant position and then check clearances with the feeler gauges. Alternatively assemble the unit, push the inner steel bush until one side is flush with the urethane top hat, and check the amount projecting the other side. The urethanes may need thinning down, but in some cases the inner steel sleeve may need filing. Don’t forget production tolerances often mean you can get the required tolerance by mixing and matching components.
When you’ve finished all your modifications and are about to install, don’t forget to lubricate the inside of the inner sleeve with “copper ease” to prevent corrosion and the outside with either “copper ease” or silicone grease. There is a space between the top hats when installed that can be filled with silicone grease to act as a reservoir.
This may seem like a lot of fiddling about for such a cheap and innocuous looking component, but correct installation will bring its own rewards. Besides, less noise and binding, you’ll gain long life and a sweeter riding rod.
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