On Cycle Threads

One of the things about working on bicycles are the different threads that you can encounter. Why is it important to know the type and size of thread? Firstly, if you know the type and size of thread, then you know what replacement part you need. Secondly, if you are fortunate enough to be able to afford the tools, then you can often repair damaged threads or clean (chasing) the thread prior to refitting the part. In Britain there used to be different thread standards, B.S.W., B.S.F., B.A., and B.S.C. British Standard Cycle (B.S.C.) replaced the earlier C.E.I - Cycle Engineers Institute who set the standard in Britain for cycle threads up to 1938. The cycle thread angle was 60 degrees, as compared to 55 degrees for B.S.W and the depth of cycle threads were shallower than either B.S.W or B.A.

Size	T.P.I.	Application	Comments
3/16”	B.S.W.	Grease nipple	Bottom bracket shell
2 BA	B.A.	Mudguard	Mudguard eyes on dropouts
1/4”	26 TPI	Handlebar	Retaining bolt for bars
5/16”	26TPI	Headclip headset	Retaining bolt for top race
5/16”	26 TPI	Seat bolt
5/16”	26 TPI	Front hub axle	British Standard Cycle
5/16”	26 TPI	Pedal cone thread	British Standard Cycle
9/16”	20TPI	Pedal axle	British Standard Cycle L & R I.S.O.
31/32”	30 TPI	Fork steerer tube	B.S.A. headset
1”	24 TPI	Fork steerer tube	British Standard Cycle I.S.O.
1”	26 TPI	Fork steerer tube	Raleigh
1 3/8”	24 TPI	Bottom bracket	British Standard left & right I.S.O.
1 3/8”	24 TPI	Fixed Sprocket	Thread on rear track hub
1 3/8”	26 TPI	Bottom bracket	Raleigh left & right

The table above gives some of the threads found on older British made steel bikes and the list is not exhaustive. I have marked the ones that are commonly encountered  with the highlighted I.S.O.  Modern bikes tend to use metric thread – M3, M5, M6, M8, M10 along with some of the above. If you are working on French made bikes manufactured before 1980, you are likely to encounter French metric cycle threads. Italian bicycles also use a different metric thread. Tandems often use larger cycle threads than a solo bicycle. Worn tandem headsets on old tandems can be problematic especially if you have a French made tandem.

How do you identify a thread? You can use a Vernier calliper to measure the outside diameter to give you the nominal size and then a thread gauge to measure the threads. The other thing to note is that C.E.I./B.S.C. threaded bolt heads on older bikes usually require a B.S.W. spanner to remove. It is worth arming yourself with a set of Whitworth spanners or wrenches in the smaller sizes for working on old bicycles. A set of metric spanners or wrenches are useful for bicycles made after 1980 and a set of metric allen keys would also be useful. (British bikes from the 1970s tended to use a mixture of imperial and metric hex allen keys bolts just to cause more confusion). Most sets don't include a 7mm hex allen key, but this size was used for the expander bolt on some French made bicycles pre 1980.

Cutting taps and dies in the correct sizes are the tools you will need to repair damaged threads. Metric taps and dies in M3 – M10 are easily available. Buy the best quality you can, use oil or cutting compound and go carefully. A taper tap is best, as it will tend to find the existing threads. Be careful that the tap is square to the hole, so you don't cross thread and end up doing more damage than you set out to repair. If you are not sure, take it to a bike shop that can repair it. British Standard Cycle taps and dies tend to be more expensive as they are less common. Steerer dies, bottom bracket taps, and pedal taps, are specialist cycle workshop or framebuilder tools and are not cheap. They are sold through the cycle trade by specialist cycle tool manufacturers and usually come with the die holder or tap handles. 

British Standard Cycle bottom bracket threads are handed left and right, so you need to know what you are doing. Manufacturers usually sell bottom bracket taps in Italian thread sizes to fit the same handles.On the subject of Italian threads, Campagnolo Gran Sport or Nuovo Record hub thread is a mixture, 9mm or 10mm by 26 TPI as compared to metric 9 x 1 or 10 x 1.  Zeus was another anomaly as the axles and cones were initially Campagnolo copies but the thread was subtly different to Campagnolo. Campagnolo cones would fit the hubs but not on the Zeus axles, so you had to replace this too, if you didn't have the correct Zeus spares!

Where you will struggle, is to get taps and dies in the older steerer and bottom bracket sizes. It is worth visiting auto jumbles, to see if you can pick up some of the tools in the sizes you need, as early motorcycles tended to use cycle thread. Make sure that you know the sizes you need and check the taps or dies you find are sharp. If they are loose in a box, covered in rust and filth, be careful as they more than likely have had the cutting edge knocked off. Taps and dies should be stored in such a way, so as they don't rub together, to preserve the cutting edge. 

The same is true for tapered and parallel reamers. Older taps and dies, if still sharp are good for chasing and repairing damaged threads. Do not try and use them to cut threads where none previously existed. This is a job for a new cutting tap or die which has to be very sharp along with cutting compound.