1/4" BSF v 1/4" CEI

How compatible is a 1/4" BSF threaded bolt when compared with a 1/4" cycle thread bolt? I understand that the BSF thread is at 55 degrees as opposed to cycle thread being 60 degrees.

The motorcycle that I am working on uses cycle threads on it's cases, but BSF threaded bolts are a lot easier to obtain, so was just wondering if they were a fit.

Thanks, Keith

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.

1958 Triumph 3TA, on the timing cover, the gearbox cover and the clutch cover.

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

**LINK**

1/4 BSF 26Threads Pitch .0385 Depth.0246 Radius.0053

!/4 Cycle 26 Threads RH Pitch .0385 Depth .0205 Radius .0064

This may not be of any help, If you would like a copy of British Standard Cycle Threads i have same

Brian

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 often use 1/4" BSF on motorcycle work. Never had any problem in over 50 years. The class of thread is pretty tolerant of any mismatch.

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

Are you sure it's BSF/CEI? Most old Brit bikes I remember working on used the coarser BSW on case screws going into aluminium.

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.

No, the OP is correct, many, especially BSA/Triumph used fine threads for 1/4" fasteners holding covers onto the alloy c/cases, Triumph used BSW originally, then in the 60's went CEI, and finally back to the better UNC in 1969.

Originally these were CEI, and plenty of m/c specialist shops will sell you a pack for all 3 covers or just a pack for timing cover, primary cover, or gearbox cover.

Choice of BZP, Chrome or S/Steel, the choice is yours on how much you want to spend.

Then there are shops like Custom Fasteners...

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.

1/4 BSF and 1/4 CEI although not the same thread are compatible and fit together nicely, I have never had a problem, your best friend is the pitch gauge when it comes to British bikes, only a few months back I was caught out making a rear spindle for a Douglas, it looked 26 tpi to my eye but 'twas not, it was 24 tpi, live and learn.

Not all makers put BSW threads in alloy, BSF was popular too, in my opinion best supplier of CEI nuts and bolts is Nooky's Nuts, always good quality and for taps and dies Tracy Tools are hard to beat.

Colin

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've found cross head sets in cycle thread, but not allen bolts/cap heads so far.

Yes in the late 60's they changed to unc/unf which I have on a later Triumph 500.

I must confess that I was somewhat suprised when i went to my lockup to find that I haven't changed then to cap heads, but maybe that was due to lack of money 40 years ago when I last rode the 3ta.

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.

I vaguely remember my 750 Bonnie (1975?) used UNF on cycle parts and BSF or BSW on the engine ,requiring two sets of spanners. Long time ago though.

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

Try a 6mill screw very close,.Found this as a mistake Had one in my allen screw assortment screwed in to my BSF thread only to find my AF Allen Key did not fit . Metric Allen key fitted fine screw did tighten up O/k Changed it any way after checking the thread ,because the correct thread was BSF.

Brian