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Front wheel bearing problems! 1950 M21.

On my rigid tele fork M21 with the 7" front brake, I decided as long as I am replacing the brake shoes, I might as well while I am in there replace the original bearings with modern seal type. The 71 year old original bearings still feel OK and are kept clean and greased but I thought I should replace them. Big mistake!

Disassembly was easy and all looked OK. The new bearings I noticed were a tighter fit on the inner axle and the brake hub. All went together OK but the bearing were way to tight and took some force to spin the wheel. I checked for side loading on the bearings but this was not the problem. So what could it be?

The original bearings are Hoffmann LS9 which are 7/8" x 2" x 9/16". This same bearing is available under many different part numbers depending upon who made them. RLS7 and 1640 being the most common ones available today. I bought 1640-2RS in standard clearance as that was how the original ones were marked. They were too tight! So I bought 1640-2RS in C3 clearance and still the problem was there. Now many hours have been wasted on a simple job.

Time to measure everything.

The original LS9 bearing ID measures at .8753" so is .0003" big. The OD measures at 1.9987" so is .0013" small.

The standard clearance 1640-2RS bearing (made in China) ID measures at exactly .875" and the OD is 1.9992".

The C3 clearance 1640-2RS bearing (made in China) ID is also exactly 7/8" (.875") and the OD is 1.9997".

With modern bearings the press fit on the axle and hub is too much together and removes all the clearance in the bearing. To use them I would need to remove about .001" from inside the hub. With the original bearings I have .0016" less total press fit which is a lot.

So did Hoffmann make the bearings this dimension on purpose to fit the BSA wheel or was this the lever of accuracy that was achievable in 1950? Perhaps my hub was machined a tiny bit too small? The original bearing have been cleaned, re-greased and back in the wheel and all is OK.

Anyway, I just thought I would pass the word to those who may run into this problem.

Re: Front wheel bearing problems! 1950 M21.

In 1950 bearing manufacturers were capable of equivalent degrees of accuracy to modern manufacturers...'C3' clearance relates to the internal clearances of the bearing (measured in microns), not its outside diameter or inside diameter so the degree of fit to the hub and/or spindle would be unaffected by the change from the standard specification. Only the internal fit after installation would be affected....Tolerances for the hub and spindle produced by BSA would have been such that a consistent fit was achieved...

Personally I never use bearings from Chinese manufacturers due to concerns regarding their quality so my first inclination would be to check the originals against a 'branded' bearing from a recognised European, Japanese or American manufacturer...The bearings originally used were 'stock' bearings with no special characteristics dimensionally and you shouldn't have replacement problems.....Ian

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Re: Front wheel bearing problems! 1950 M21.

Hello Ian,

Yes I am aware that C3 refers to the internal clearance and nothing to do with the bearing dimensions. For a bearing this size a C3 would have .00005" more clearance than normal. I was hoping this would allow the bearing to spin more freely then the regular clearance. It did not!

And I think you are missing my point that the OD of the original Hoffmann bearing is supposed to be 2" but is 1.9987" which is a lot smaller (.0013") than it should be. It was measured with two different calibrated micrometers.

Why is it smaller, I don't know?

The China bearing took a lot more force to press it in the hub than the original one and it was for all practical purposes exactly 2". Even with only one bearing pressed in you could feel it was getting stiff to turn. I don't have another hub to measure so don't know what the bore dimension should be.

Were this bike a Velocette, I would expect these kind of problems as they were famous for not scrapping out of tolerance parts and doing what was necessary to make them fit.

But I have a problem believing BSA would do this.

Next time the bearings need to be greased, I will try to find some not made in China. Probably won't be easy to find sealed ones not made in China as being a imperial size bearing there is not a lot of demand for this size anymore.

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