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Series 'A' Steering-Head Bearings

A_HRD

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VOC Member
I was messing around with my A restoration the other day trying to determine how much wear/ovality might be present in the headstock, etc. (It's not that easy to do consistently/accurately with callipers). Anyway, by coincidence a colleague contacted me asking about the same bearings. He wanted to know what grinding work would have to be done, and what dimensions to work to, to convert postwar cups and cones for Series A use.

It seems from my discussions with 'A' enthusiasts, that the predominant 'A' system was originally "cup to cup"; ie 4 x F128 ground in a simple 'U' shape. I have a pair of worn ones on the desk in front of me. They are much reduced in height compared to a postwar pair with balls in place, moreover there are tiny differences in OD and greater ones in ID when compared to post-war ones.

Surely, there's someone out there who has grappled with this conversion and has some information that they can share? Here's hoping. :)

Peter B
 

Marcus Bowden

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VOC Member
I have done it but can't remember what, nowadays I normally write things down and take pictures. Sorry, Peter.
 

greg brillus

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VOC Member
I did it to the Comet I am restoring, I think I only had to make up some spacer shims to pack the bearings out to the correct height. I don't remember it being too difficult to carry out, unfortunately the bearing races are expensive. That is for 2 inners and 2 outers..........
 

A_HRD

Well Known and Active Website User
VOC Member
I've measured postwar cups and they are 1.9880" OD. I suspect the nominal required OD in the steering head was 1-63/64" = 0.9844". In which case, 0.0036" sounds like a lot of interference fit for steel in steel that remains at ambient temperature in use.

Anyone have a view on that?

Peter B
 

Dinny

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VOC Member
Hi Peter,

The original A cups were missing on mine. I got the later setup from Steve Hamel here in the US and tried to measure if everything would fit height wise and in the end gave up as every measurement was different.

I found someone who could grind them and did the following mods.

The top yoke bearing housing was machined to a good press fit for the top cone, the same was done for the bottom cone onto the bottom yoke. I don’t remember the exact clearance but I think it was around 1.5 or 2 thou. They were still tight when fitted using heat and the freezer and will not be going anywhere.

I then fitted the cups into the frame, after cleaning up the frame I had a nice tight fit without any mods.

When assembled I only had about 3 threads on the top nut. The upper and lower bearings as mounted are shown below which was not good and the outside faces of the cups were proud of the frame. I then calculated that if I ground off 40 thou from the rear faces of the cups that would allow everything to seat correctly without the balls showing and large gaps. This also gave me enough threads on the top nut and the cones fitted nicely just below the surface of the frame. There is plenty of material on the cones on the post war setup.

I hope this helps and it all works fine.

Cheers
Mark
 

Attachments

Dinny

Well Known and Active Website User
VOC Member
I've measured postwar cups and they are 1.9880" OD. I suspect the nominal required OD in the steering head was 1-63/64" = 0.9844". In which case, 0.0036" sounds like a lot of interference fit for steel in steel that remains at ambient temperature in use.

Anyone have a view on that?

Peter B
I have just checked my emails, my cones were 1.9870 OD from Steve. If I recall correctly the clearance on the frame was 2 thou. 3.5 does seem a lot to me.

Thanks
Mark
 

A_HRD

Well Known and Active Website User
VOC Member
Mark,
Great stuff that, and I concur with the thrust of your findings. I too went down the postwar route and had the same problem with the races and balls out in the airstream. I've since found a pair of worn-out A original cups and they are much narrower height-wise. From various measurements I've been taking I reckon the OD on the Series A cups was about 4 to 5 thou smaller than postwar bearings. That amount could have been lost in the 85-year time-warp, hence the head-scratching now!
Peter B
 

Dinny

Well Known and Active Website User
VOC Member
Hi Peter,

I agree that in 85 years anything is possible and maybe the were never that accurate in machining the frames back then. I’m happy with how mine turned out but it was a slow methodical approach, only modifying one thing at once and it turned out great. I didn’t want to screw up the expensive new parts.

Still a ways to go but it’s now on its wheels and progress is being made. I also found a bth headlight, the correct Amal handlebars and the P&H rear light so I’m now trying to get as many of the original 35 parts on it which is slowing me down. There is very little info about.

Cheers
Mark
 

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