Polyurethane engine mounts?

My jimny has developed a slight knock only does it every now and then on certain corners around my area and every time I hear i can't then re-enact to make it do it again, now my thinking is it's the engine/gearbox/transfer box mounts as they do all look worn and soft, so in keeping with the rest of the bushes on the car I want to change them to polyurethane. As far as I can find there is no off the shelf product so looks like its time for a bit of home manufacturing. Has anyone got any thoughts on poly engine mounts, anyone else made there own, reasons why I shouldn't, what hardness should be used. Any thoughts, opinions or ideas would be nice, thanks.

I recently had a knock going round certain corners and over bumps.

I got someone to rock my jimny from each side successively as that seemed to replicate it. Did that until I felt where the knock was coming from.

Turned out one of the radius arms had become a little loose. Tightened up all 3 bolts on it and then did the other three arms for good measure. Solved my issue. I would check the suspension components as well as they are most likely to require attention.

Engines vibrate, it's part of there charm and the designers attempt to control it my using mounts that restrict this vibration. It is called Noise, vibration and harshness or NVH for short and if you mess with it you will likely find that the NVH has got worse. It is my suggestion that if your investigations prove that an engine/gearbox mounting is failing, then you should replace with a Genuine Suzuki part. If you use a spurious part, then you run the risk of only partially solving your problem, due to the manufacturer not using a material of the same strength/harmonic balance as the original.

Roger

My recommendation, for engine & transmission mounts, is stick with rubber and preferably OEM rubber. Polyurethane engine & transmission mounts are in my opinion a VERY BAD IDEA, and the reason is, as Roger Fairclough has raised, is NVH.

I've run polyurethane suspension bushes back in my "hot hatch" days, the increase in transmitted road noise was huge, I also had "axle tramp" issues with the same vehicle, and resolved that by injecting polyurethane sealant into the existing engine mounts, these "modified" mounts were not as rigid as a pure polyurethane mount would have been, but the increase in transmitted engine vibration was unbelievable, at idle the entire body shell buzzed and resonated.

Personally, I'm not in favor of poly suspension bushings on any vehicle intended for off road use, they reduce flex & articulation, they squeak, the grease the manufacturers provide becomes contaminated with dirt & grit, causing wear, I know people sell them, but I won't buy them, poly shock bushes are fine, there is minimal rotation there, and that's a place where you don't want flex, poly transfer case mounts, if it's a divorced transfer case (which the Jimny is), I would consider, but engine & transmission mounts, only on an out & out competition vehicle.