fork wear and limits

[shadowfan]

I'm just looking at the forks on my 1951 A7. The fork stanchions are 1.25" diameter. The ones in my bike are within tenths of this figure and I think they are the original legs as the bike has done a genuine 50k miles from new. Most manufacturers should be able to grind the legs to within a tenth or two, and to get spot on bushes only requires a 1.25" reamer, and any basic machine shop will have one of these. In summary stanchions and bushes shouldn't be a problem from a manufacturing point of view. A trickier problem must be the variability in slider wear and the difficulty in rectifying this and maching the o/d of the bushes to match the remachined slider.
Don't forget the top bush outer surface doesn't move in the slider so shouldn't wear (this assumes the shims and wire circlip restrict shuffling between the slider and bush o/d ). The wear is between the stanchion and bush inner surface.
 The bottom bush is the opposite i.e. bush outer  surface slides in the slider and there is no wear between the bush i/d and the the stanchion (they are locked together by the nut at the bottom of the stanchion).
What would be useful is to know the factory wear limits and acceptable clearances. Has anyone this information?

Regards

[A10Boy]

Hi

Welcome to the forum. Someone will come up with actual wear limits no doubt.

My own view is that even with new bushes and stanchions you will feel some movement, the wear limit is when you or you tester feels thats its got to loose.

[MikeN]

Most manufacturers should be able to grind the legs to within a tenth or two,
Regards

I would have thought so to .But when i bought the stanchions for my A10 (ground chrome) One had a 3 thou taper on the working area!  I dont know what limits BSA set but I remember reading the dimensions for a 500 triumph which has a similar set up and the clearance was (from memory) 3-4 thou. Its not the stiffest of set-ups. Reaming a bush with a 1.25" reamer to fit a 1.25 stanchion wont be much good. Anyway, the bushes are sintered bronze .Are you talking about making them from bronze bar? Ive done this a fewl times and i bore them to size on the lathe.
 The steel sliders last well  although you may have to compromise to obtain an acceptable working fit.
Mike

[muskrat]

G'day Shadowfan, welcome to your new skool.
                                                                        I have only done the bushes a couple of times and found 2 thou to give the best results. My rule of thumb is if i get more that 1/8" movement at the axle it's too much.
Cheers

[BSA_54A10]

Worth remembering that the lower leg also wears and in that case it is a line boring job to get it back to parallel again.
One of the "roundtuit" jobs when we get the mill set up ( haven't touched it for a year ) is to bore some of my lower legs and try tefflon lower bushes as they should wear in prefference to the leg.
Leaded gunmetal ( because it machines beautifully & I am not a fitters shoe lace ) for the uppers and as Musky noted around .002" clearence

[shadowfan]

I've just found the fork dimensions and tolerances for the later 1969 to 1970 Lightnings etc which use the same type of fork but with increased stanchion diameter and side holes in the seal holders. The factory working clearances i.e. new forks leaving the factory were .0035 to .005" clearance for the top bush and .0035 to .0065" for the bottom bush. I guess there are two reasons for the clearance 1) manufacturing costs and 2) possible jamming of the forks (due to deflections of the parts) if built with tighter clearances.
Conclusion - If you've got up to .005 working clearance don't waste your money on new stanchions, bushes or slider repairs.

p.s. the stanchion diameter for the above model is 1.3025 to 1.3030 inches. All the bushes have a 1 thou tolerance. Slider tolerance is 1.498 to 1.500".

[muskrat]

Ah, so that's why they handled like a wet sponge. 6 1/2 thou is like a c**k in a sock 
Cheers