1934 Blue Star fork assembly

[David Samways]

Just reassembling my forks and wanted to check on the position of the spring washers. Do I take it that under each bolt head is a spring (Thackery) washer on the off-side (rider's right) with a single nut on the nearside to lock at the desired tension? This seems logical, but I don't understand where the fourth Thackery washer goes.

Also, should the bottom headstock link axis be shimmed to make allowances for the wear to the endes of the friction links and headstock pivot tubes themselves? This would seem to be a sensible adjustment.

David

[Bicycle repair man]

Shouldn't the washers be fitted under the nut rather than the bolt head? The bolts control the link end float so aren't tight and I don't think that the lock washers would work here.

[David Samways]

Mmm... I don't think the function of a Thackery washer is to lock anything. They are frequently used in rocker assemblies or with carburettor flange fixings in order to allow a limited degree of movement while applying pressure in one direction.

If they were intended to lock the nuts a normal spring washer would be used. In any case, the nut is fitted to the threaded link side and thus used in the same way that one would employ two nuts locked against each other - no washer needed in that case. I had assumed their use was to apply pressure rather than lock anything.

[Bicycle repair man]

I understand that this type of washer can be used to control movement on a shaft but in this case there is no benefit in allowing end float of the fork links. In fact, I would say the opposite!
The parts book lists it as a spring washer. Although the picture shows a double turn washer, like a rocker spindle shaft, this washer crops up in a few places. Part number 24-6035, according to the model range parts book is used on a battery carrier, pillion footrest securing plate, front mudguard bottom stay bolt and legs shield brackets as well as the fork link bolts.

[David Samways]

That's true, but with the foks the friction discs actually determine the effective width between the bottom links, meaning that the headstock end must be allowed some axial movement - otherwise as the discs wore they would become inneffective or the forks would lock. The same is somewhat true for the top links since the distance between each pair is unlikely to be exactly the same on the fork end and on the headstock assembly end - hence the washers are needed. The design of the fork is very poor IMO. I note that the later fork employed on the M20 etc. moves the damper assembly outside the links thus not affecting the axial distance between each pair.

I think they are included in the other places you mention for the same reason they are used on carburettor flanges - to allow some movement and prevent cracking or warping. Although, some of the places mentioned do seem somewhat unlikely in this respect.

[Bicycle repair man]

I have looked at the other applications for this washer and none would really fall into the category you mentioned.
It's interesting that my bike is a 35 with the Bakelite damper. This only uses 3 of these washers.  Wear of the dampers doesnt factor here. The type with the wing nut damper uses 4. It's also interesting that for other applications of this washer, it shows a conventional spring washer. On the fork page the illustration is shared by part number EB150. Wrong picture maybe?

[David Samways]

It is a bit of a mystery. I certainly can't think of any reason to include these Thackery washers other than to allow axial movement. As I say, they would be pointless and even counter-productive under the nuts. I'm not sure anyone could produce a definitive answer to my question unless someone can turn up a factory photo or assembly drawing. The washers I have are rather weak (I suspect they've been heated and have lost their temper) and I'm not sure they will do what I think they were intended for so I've ordered some replacements. I'll see how it all seems, the function is the critical part - BSA may have not got it right and a mod may be in order.

[Y13 Alan G]

I'll add my twopenny worth to this discussion. I have re-built/restored several pre-war BSA motorcycles some of which I still have. I have never used this type of spring washer on any girder fork links. Any side play in the fork links is very undesireable. The method of setting the minimum amount of side play is to gently tighten the long through bolt and then undo it about half a turn and lock it with the nut on the other end of the long bolt. Plain washers under the head of the bolt & nut are sufficient. Regular greasing will ensure long life of the link pins.

[David Samways]

I can understand your reasoning Alan. But the fact remains that old forks wear axially at the pivot points both on the fork itself and at the headstock assembly. Moreover, the friction links wear and so too the friction discs themselves. When the damper assembly is adjusted to the desired position it is inevitable that there must be side play in the other links. Even if the width of the fork and the headstock assembly were manufactured to fine tolerances - lets say two or three thousandths of an inch (which I very much doubt btw) the friction links and discs will still have been subject to wear which would have led to seizure.

Having been involved with very high precision engineering for over 30 years, I can say with some authority that this is just plain mechanics. It appears that BSA's compromise was to introduce the Thackery washers as a means of addressing the side play issue. To me, this seems like a pretty poor idea, since, as you say, side play is undesirable. My preferred solution is to shim the links to eliminate side play, but I suspect BSA did not care for this solution as it means that constant vigilance will be necessary as the friction discs wear. Beyond a certain point thickness of the disc, adjustment will cause a bending moment in the bottom links which will misalign the parallelism of the pivot pins and cause binding, wear and even possible fracture. It seems that BSA agreed with this analysis and hence the Thackery washers were employed. Fortunately, they came to their senses and redesigned their forks later in the 30s.

As I say, this is conjecture based upon my mechanical analysis of the fork and the listed components, I'm perfectly open to being shown to be wrong about BSA's design principle and execution.

[Steve.S]

I completely agree with Alan G. I can also confirm that both Percival Brothers and Webb, and Jake Robbins don't use these washers, just flat washers. If BSA were concerned about the nut coming loose, I think a single spring washer would have been more effective here than a thackery washer. I have never known these nuts to come loose  (just using a flat washer). It wouldn't surprise me if BSA had mistakenly ordered thousands of thackery washers, and had to use them up somehow. The only place I use them, is under the steering damper knob.

[David Samways]

Thanks for all the replies. I'm certainly not happy about these Thackery washers. If the ones I have are the originals they wouldn't do the job I thought they should do. But none of this addresses the issue regarding the effective width of the fork pivot points and the potenial for binding and excessive wear. I think I'll shim mine, but it really is an absolutely abysmal design.