Top fork stanchion's inside thread

RGM  say 1" 1/8  x 20 tpi   I recon that is  BSC

Regards 

Katherine Scott 

BSC was pre Commando, Commando was UN special, tpi for both is 20 so only difference is thread form.

In practice just use the cheapest of the 2 for your tooling, I use carbide inserts on an internal threading tool of both thread angles 55 and 60 on a lathe, so both are same price to me and no need for tapping dies.

https://www.gewinde-normen.de/en/20-un-thread.html

... are 60 degree thread angle so if the tpi are the same then there will be minimal difference between the two. Purists may disagree......

The tool I use is like a small boring bar but with the insert angles set for thread cutting not boring, the inserts are 3 pointed and come in both 55 and 60 degree flavours, so covers all the Metric, BSW, BSF and Cycle threads. I need this setup as I cut a lot of custom threads, last one was M30 x 1.0 pitch for a yoke spindle. 

The only difference between the BSCy and the UNF thread form is the UNF form is slightly truncated (the top of the thread v form is rounded off.).

If you are looking to just clean the thread, a tap is much easier. Tracy Tools list a tap for a reasonable price. Much cheaper than a new leg!

Tracy tools list 1 1/8 × 20. They have the HSS ones but are out of stock of £18 carbon steel ones. I bought an 1 1/16 × 20 carbon steel one for another project and it was fine.

I used it on steel. I have used their carbon steel taps before and they are fine on mild steel but I  wouldn't try stainless.

This is Interesting info for a novice many thanks.

I actually prefer carbon steel for occasional use, even on stainless, as I think it cuts slightly better than HSS till it's worn. For a one-off it's fine.

I can't say that I have noticed much difference.  As you say, for occasional use or on softer materials carbon steel is fine. I use a lot of small taps and dies for my model engineering which is brass, bronze an mild steel and these are often carbon. I made a lot of the nuts and bolts for my pre war Norton and BSA and purely because of the materials used, opted for HSS. You get what you pay for.