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ball or roller bearings at midpoints on a crankshaft

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That is one rare bird. It's very likely that these are special production in small sizes. I would guess it runs directly on the crank, so no inner race. RCMK decided this was the best design for their engine and special ordered these bearings and the price reflects that, 44USD for the split cage roller set. and 49USD for the split outer race.

Looking through bearing catalogs I did see some, but mostly in much larger sizes. I saw some that had split shells too. None of this appears to be stock items. One could make a cage to separate the rollers, but the races are much more work if they are to meet the hardness and finish requirements.

Gizmomotors got back to me. The 'Needle Bearing Kit (pair)' SKU: RCMK S26372 dimensions are: width= 0.420" ID=0.670" OD=0.901". Unfortunately, the 44USD price tag for the split cage roller set and 49USD for the split outer race is a wee bit too pricey: I would need five for each of the two engines.   :toilet_claw:


Overbuilt and Overkill:
Napier and a few others may have had some specialized reasons behind using those split needle or roller bearings that might not be quite as obvious as it seems. The exact details are now lost to me because of the 50 + years since I first read about it. One of the more famous racers or engine builders in the U.S. apparently went to a great deal of effort and expense trying to use roller bearings on the crank shaft and cam in there racing engines in an effort to free up more horse power. This was probably in the 1950's - 60's. Mickey Thompson maybe?????? Anyway the effort was a failure simply because it didn't gain any significant amount of HP. They explained it to the writer of the article that they had neglected to factor in just how much friction or drag was being produced once the parts were fully supported and spinning on the oil wedge. And that roughly the same amount was being produced with the roller type bearings. I'm not saying for sure this was the reason, but Napier and others may have had oiling issues for the more standard split sleeve bearings and this was just an easier and to me a bit illogical method around that. With the ever increasing and government mandated fuel mileage increases, if the split needle /roller bearings gave enough of an increase, the automotive engine designs would have been changed to use them for sure. Replicating in scale as closely as possible what original full sized engines used is of course something different. But this might save someone going down what could be a fairly expensive dead end path. For extremely high rpm such as for model and full sized turbine engines, then just enough flow restricted oil injection and to ensure barely adequate lubrication with ceramic ball bearings is helpful to prevent those bearings from over heating or causing too much drag. With those and afaik there always a total loss system once the oil passes through the bearings.

Center split  needle bearings are used in model gas engines.  See my post here:

Lohring Miller

Thank you very much for the Link Lohring  :cheers:

I don't see any seals in the first picture - are the 'Groves' in the middle working as Labyrints ?



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