WD Motorcycle forum

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Jul 4, 2015 - 2:53PM

M21 thread help

After 9 months of dismantling seized components, removing 57 years worth of caked and dried grease and dirt, and poring over parts books I finally got the frame and tinware back from the powder coater's and I can begin the rebuild :-)

I've only ever done Bantams before and I learned that BSA seemed to use whatever fastenings were available and there was little consistency even within the same model.

Most of the threads on the M21 frame are 20 or 26 TPI and I had assumed would be BSCY thread but it seems that many are not. Can anyone tell me what thread was used on (for example) the footrest retaining stud and the engine mounting studs, are these Whitworth?

Gary.

email (option): gj.owen@hotmail.co.uk

Ian Wright

Jul 4, 2015 - 9:05PM

Re: M21 thread help

Hi Gary...Generally threads into aluminium are Whitworth...

Handlebar controls, carbs, speedos and electrical equipment are mainly BA...

Cylinder head bolts and the main oil unions into the crank case (at the crankcase end) are BSF...

Oil pipe and petrol pipe nuts and petrol taps and fittings are BSP (the parallel form)..

Nearly all cycle parts are Cycle thread, either in the 20 or 26tpi form...Also the threads on the carb studs, timing plug screw etc.

Whitworth on the footrest bar would be something that someone has added..

The original thread was 1/2" Cycle...Engine plate studs/frame studs are 3/8"..7/16"..1/2" Cycle depending on application..Ian

email (option): ian@wright52.plus.com

Gary Owen

Jul 5, 2015 - 11:13AM

Re: M21 thread help

Ian Wright

Thanks Ian. It was just that the new nuts on old studs seemed a bit tight and I wanted to be sure before I apply too much spanner pressure! There seems to be a variance in the quality of modern day BSCY thread quality, has anyone else experienced this?

Gary.

email (option): gj.owen@hotmail.co.uk

Ian Wright

Jul 5, 2015 - 1:50PM

Re: M21 thread help

Cycle thread fasteners, which are an obsolete thread form really, come from multiple sources and manufacturers..

Some are even produced in China..

This also applies to some of the other thread forms which are not in regular use currently but can be found on old motorcycles...

Consequently variations in specs. and tolerances do occur at times as many manufacturers are not necessarily held accountable for the accuracy of what they produce and may not have the resources/inclination to manufacture and inspect in a sufficiently controlled way....

It is therefore important that a working knowledge of the various thread types is acquired along with the relevant reference material...

Further, if restoration is to be a long term pastime then the accumulation of the correct tooling used (thread gauges,taps, dies and tap and die holders) should also be considered a very worthwhile exercise...Ian

email (option): ian@wright52.plus.com