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British Cycle Threads

On the imminent return of my powder coated parts I always have to tap out any holes that haven't been plugged that well, are the majority of the bolts used British Cycle Thread 20 tip and what taps will I need to buy, I.e. between 1/4" to 1/2" ?

email (option): 79aust@sky.com

Re: British Cycle Threads

5/16 and 3/8 are the most common threads and they are 26 tpi.

Re: British Cycle Threads

1/4" is also common. That can be BSC or BSF as it's almost the same at 26 tpi.

Ron

email (option): ronpier@talk21.com

Re: British Cycle Threads

Hi Julian..There's information in the 'technical section' for the threads used for the engine, gearbox and primary drive. Cycle parts are generally (and not surprisingly) cycle thread. This is 26 tpi in it's basic form but a 20tpi variant is used on many parts. This is visibly coarser than the basic form. 'Instruments' such as the carb, the speedo and it's drive, and the lever assemblies are BA... BSP (British Standard Pipe thread) is also used (but not the tapered form BSPT)
It is highly likely you will come across one you are not sure about..a question on here is probably the best solution to that.
Note that BSF threads and cycle threads are only interchangeable at 1/4" diameter...
The threads you can expect to come across on an M20 are BA (British Association), BSF (British Standard Fine), BSW (British Standard Whitworth), BSC (Briish Standard Cycle in both TPI forms), BSP (British Standard Pipe) a few non standard threads for good measure and a 14mm spark plug thread.
It is probably best to buy taps and dies as you need them rather than 'bulk buying' unless you have plenty of disposable income...I can assure you to buy them all costs a fortune (I have)...Ian

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

Re: British Cycle Threads

Thanks Ian for your valuable advice as always.
I've just been cleaning up the studs and some of the frame bolts and trying to fathom out what the thread forms are, this really is a mix.
The BSF, BSW & BSP threads don't cause me any problem as I have these taps as stock, it's the cycle threads or what use to be the Brass thread that I need supplies for.
I'm just going to have to stop being lazy and use my thread gauge a little more to identify all as I go.

I was hoping someone would have an excel spreadsheet with them all listed?

Might be a project for me to attempt?

Kind regards,
Julian

email (option): 79aust@sky.com

Re: British Cycle Threads

|In addition, I have identified quite a bit that seems to measure 7/16", thread forms I've yet to confirm.

email (option): 79aust@sky.com

Re: British Cycle Threads

Hi Julian, I did the engine/gearbox/transmission info on the technical section. I think Robb Nortier has done some work on the rest of the bike but I don't know if it is complete or in a format ready for distribution...Don't confuse British Standard Cycle Thread with British Brass Thread...Brass thread came about because standard brass tubing has the same wall thickness at all diameters and consequently a thread with a constant depth was required. TPI numbers are coincidental. Also Brass thread has an included angle of 55 degrees (a Whitworth thread form) but BSCY has an included angle of 60 degrees. Finally, although the most common BSCY fastener threads used on motorcycles are 26TPI, the TPI for BSCY threads is actually between 20 and 40 TPI depending on diameter and application and some of these are used..Interestingly BSW, BSF and BSCY all use the same spanner sizes...but not at the same diameter!. Ref. your comment on 7/16 dia. threads....If these are coarser than 26TPI they are the 20TPI cycle threads...Ian

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

Re: British Cycle Threads

Many thanks again Ian for your assistance

email (option): 79aust@sky.com

Re: British Cycle Threads

Hi All,

Thanks for the kind reference Ian. I do have a chart of the thread form uses, along with tapping drills for proper clearance.

I'll email this to anyone who asks. I should have Henk post it as this topic comes up often.

BSCY and CEI are the same form. 26 TPI for the small stuff up to 1/2 inch. 60 DEGREE included angle. The whitworth form (BSW & BSF)has a 55 degree angle.

HOWEVER: Both 7/16 and 1/2 BSCY (CEI) also have a 20 TPI form. BSA used 20 TPI BSCY (same as SAE in the States) which is also a 60 degree included angle form.

If you are in the UK, contact Dave at Uni-Thread for taps. dies and inserts (helicoil). Tell him I sent you.........

NEVER............ use a metric thread on these bikes. Well, attaching cycle parts would be OK.

email (option): britool51@hotmail.com

Re: British Cycle Threads

I found this page in a 1944 VAOS spares list, I'd always thought that cycle (C.E.I.) threads were only 26 or 20 tpi, but this list seems to indicate that any tpi was possible and used enough for the Army to stock the appropriate taps and dies.

Is the left and right hand 9/16 for truck wheel nuts?

Rob

cei

email (option): robmiller11(a)yahoo.co.uk

Re: British Cycle Threads

Rob Miller
I found this page in a 1944 VAOS spares list, I\\\'d always thought that cycle (C.E.I.) threads were only 26 or 20 tpi, but this list seems to indicate that any tpi was possible and used enough for the Army to stock the appropriate taps and dies.

Is the left and right hand 9/16 for truck wheel nuts?

Rob

\\\
My friend says its the thread for bicycle pedals.

Best regards,

Simon

Re: British Cycle Threads

Well done, I hadn't thought of that.

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: British Cycle Threads

Julian Hands
Thanks Ian for your valuable advice as always.
I've just been cleaning up the studs and some of the frame bolts and trying to fathom out what the thread forms are, this really is a mix.
The BSF, BSW & BSP threads don't cause me any problem as I have these taps as stock, it's the cycle threads or what use to be the Brass thread that I need supplies for.
I'm just going to have to stop being lazy and use my thread gauge a little more to identify all as I go.

I was hoping someone would have an excel spreadsheet with them all listed?

Might be a project for me to attempt?

Kind regards,
Julian
Brass thread is 55 deg while CEI is 60 deg .
The brass thread is cut deeper than cycle thread so you end up with 26 tpi.
Tapping an internal thread brass will result in a sloppy fit that will undo really easily all by itself.
Next brass being a soft material only need medium carbon steel tooling which can be damaged when used on a thread in a forged lug as you would find on the oil tank mounting lug on the down tube or the front fuel tank bolts.

email (option): bsansw1@tpg.com.au

Re: British Cycle Threads

"I found this page in a 1944 VAOS spares list, I'd always thought that cycle (C.E.I.) threads were only 26 or 20 tpi, but this list seems to indicate that any tpi was possible and used enough for the Army to stock the appropriate taps and dies. "

Get TPI out of your head.

All imperial Threads are defined by the internal angle of the V ,
the depth the thread is cut, either a fixed number for brass or a proportion of the diameter of the fastener for BSW
The size & shape of the root ( and crest ) of the threads.

Now it is difficult to measure these dimensions in the field so we use thread count which is easy to measure either with a gauge or a rule & counting.

Deep threads are stronger than shallow ones but make the bolt weaker by reducing the cross section.

Also remember that in industry threads are cut by a variety of different tools.
You can use a single V point of the appropriate angle with the lead screw set at the appropriate speed. Slow but very accurate
You can use a thread former which looks like a single thread section from a die and this will feed itself along the rod , very fast & can be done on any lathe
You can use a full threader which uses several formers set into a holder so it looks like a normal die except it could have from 3 to 9 cutting edges. Faster still but difficult to set up

And then there was the specialist thread cutter which was a derivative of a capstain lathe.

also remember that most of the threads were cut using high carbon steel tooling which wore quickly.

email (option): bsansw1@tpg.com.au

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