Main bearings

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Oct 19, 2005
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I recall there was a lengthy discussion on main bearings in a post quite some time ago and Foxy kindly posted some information on the difference between brass caged 'super blends' and polymide caged ones, where he stated that brass has a better heat and vibration resistance. I have just been at a friend's place, he has a modern KTM single which uses the same kind of main bearings under discussion, his bike revs to about 9,000 RPM and the bearings have a polymide cage. Has anyone built a Commando bottom end using the polymer caged main bearings and if so what were the results. The polymide caged bearings are significantly cheaper than the brass caged ones and all other specifications would seem to be the same.
In a recent magazine article here in the UK, it discussed this issue, but without a definative conclusion :?: What it did say though, is that Steve Maney, the well known Norton performance parts manufacturer apparently uses the polymide caged bearings in his racers, but the article does go on to say that obviously these engines don't do the milage of a road bike and regularly get stripped down etc.

I have had polymide caged mains in my Norton which are fine, but having said that, I have only done about 3,000 miles since I bought this bike and don't know their history prior to my ownership :?:

Would the worst case scenario be that they need replacing sooner than brass caged or could they disintegrate and cause significant damage to the engine? Apparently (according to the article if I recall correctly) the polymide caged bearings do not have any information suggesting that they are not for use in an internal combustion engine.

I await the more learned amongst us to give a more definative informed opinion.
Speaking for a friend who has many years of experience in building and servicing Commandos, he has always used polyimide caged Superblend main bearings and has never found a fault in one yet.

Reggie and Mick, Thanks for your replies, I think the issue is that the brass-caged bearings can operate at a higher temperature than the polymide ones, but I don't know if this temperature difference is critical given our application, however it is sometimes the collective experience and wisdom of people on this site that can give a definitive answer as to whether something actually works in the real world. I would also have to think that a Steve Maney machine will get a harder prolonged thrashing (with consequent higher temperatures) than I am able to give my bike, even when I morph into Peter Williams at The Isle Of Man, for the last few miles of my trip home on a closed road!
It was suggested to me by my friendly bearing supplier that the cages react differently to vibration and he was regualy suppling polymide ball races as a replacment for performance motors, he slso said steel cage roller bearings fail very quickly in certain applications,rock crushers, were the brass cage holds together. What are they trying to achieve, is it less bearing mass :?:

This is from the FAG Standard Program. It's ambiguous, but here it is:

Cages of glass fibre reinforced polyamide 66 withstand temperatures up to 120deg C for longer periods. In case of oil lubrication, any additives contained in the oil may adversely affect cage service life if the temperature exceeds 100deg C for longer periods. Also aged oil may have an impact on cage life at these temperatures so that the oil change intervals should be strictly adhered to.

I don't know what they mean by lasting "longer". Longer than the ones cast in a nice Parmesan ? As you said, you can't beat the real world for a definitive answer.
I can see why folks are confused...bearings in most cases are going to run in some sort of lube and the centigrade confuses me more. 100 centigrade is about 220 F isn't it? That is right about the temp you want the oil to run when warm.
Lets discuss all the times we have heard poly rollers broke up. We use them in a few applications on buses and trains, I've not heard of problems.
I'm a bit conflicted about this as I also wonder if the poly rollers might not store better and a lot of Nortons sit a while.
Not had reason to fit new mains to a Norton, but have done a few BSA Unit Singles and always fit polymide caged Superblend mains and not had a failure.
I just bought a pair of NJ306e main bearings with polymide cages for about $20 US each, this is considerably cheaper than the $80-plus that the brass caged ones seem to go for. I intend to put these in a 750 with 4S cam that I will be riding in a spirited fashion, so hopefully if anything is to go wrong it will do so with this bike then I will know whether my parsimonious approach to building a Norton bottom end is clever of foolish. I will report on my findings, although this particular project hasn't even been started yet, so patience is required.
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