Which motorcycle is it? I very much doubt if any British motorcycle ever used cycle threads in anything but engine to frame bolts or wheel spindles and possibly steering head bearing adjusters.

Motalia sell fasteners in various sizes and threads, also they are a good place to look at thread charts.

**LINK**

I would imagine BSF & CEI are interchangeable if the TPI's /diameters match, many years ago I worked in a factory where BSW & UNC were frequently used together, apart from 1/2"

Tony

I had a 1961 BSA A7 500 many years ago, that had cycle thread fixings all over the engine. The barrel to crankcase studs were 3/8 Whitworth into the alloy cases and cycle thread where the stud passed through the barrel base flange. I cleared out the local bike shop's stock of 3/8 nuts during the rebuild.

Dave

I have worked on many old British bikes & it was always whitworth into the crank cases & BSF most every where else. The only cycle threads I came across were the wheel spindles & the front forks.

A myriad of threads have been used by the British motorcycle industry. These include BA and specials like 25 TPI on early Douglas machines. BSA used Whitworth/CEI with mainly Whitworth spanner sizes but threw in some Imperial A/F as well. Tread carefully on all pre war stuff and look out for kitchen table bodges, when these machines where worth very little anything was done to get the machine working and take the rider to work next day.

Steven at AMC Classic spares can supply 1/4 BSC(26TPI) nuts and screws/bolts

I am constantly working on both BSF and BSC and the 1/4 have the same pitch but are still not a good fit. Steven will supply all the nuts you need and quality is excellent. ( I have no connection other than as a customer. AJSMathless club will also supply these but you will need to become a member)

Hooper wrote "Most old Brit bikes I remember working on used the coarser BSW on case screws going into aluminium."

The motorcycle must have been very old. I have never seen the courser Whitworth thread in any motorcycle I have owned. AMC studs are BSF into the alloy and BSC for nut. However I notice similar studs on Velocettes are BSF along the full length.

BSA (and presumably Triumph) switched to UNC threads late 60s early seventies when their main market was USA.. They seemed to have changed gradually as i have a 1969 A65 which has both.

I had a brief "go" on a Triumph 3TA many years ago and what a lovely machine it was, probably the nicest Triumph I've ever ridden. Unlike the 750 twin, which was like riding a mobile jack-hammer.

Keith, have you tried Dragonfly motorcycles. They have bought up some old triumph and BSA stock and also supply spares for Triumph and BSA. Although they might not have the allen heads you want

Edited By Gerard O’Toole on 04/02/2020 10:59:57