Whether a classic car or bike is regularly used or not, parts which are presented as bare metal surfaces will be a lot easier to preserve
in good condition if stainless steel is chosen. Normal cleaning activities with water and shampoo are fine, but try to “float off” any gritty residue first, to avoid scratching. After drying,
polished stainless surfaces can be finished off with a chrome cleaner, and/or wax polish. Long term, if scratching has appeared, parts may be removed and polished by abrasive methods, and the
as-new finish can be brought back.
We use a number of different grades, but all are generally austenitic, non-magnetic grades. (but a slightly magnetic “feel” is often detectible with a strong magnet). These grades are in the “300” series, and are the most corrosion-resistant types. The alloy chosen depends on the application, and the manufacturing processes used.
Thread selection can sometimes be problematic, but it is obviously correct to follow what the original manufacturer used. This is a bit difficult with the period after 1967, when the British motorcycle manufacturers made a transitional change from Cycle (CEI), Whitworth and BSF threads, which all used WHIT hexagons. BA threads (very small sizes) also began to fade away. The new threads were UNF and UNC, which interchange with ANF and ANC. (American fine and coarse pitch threads) Many of the bikes from this era finished up with a mixture of threads, and then you needed two sets of spanners and sockets. This is how they generally compare:
||Whitworth (across flats)
||UNF & UNC (across flats)
|7/16”||.710”|| .625” bolts (.687” nuts)|
|1/2” ||.820” ||.750”|
There is more variation in sizes above 1/2”. Hexagon size is most definitely NOT a guide to the thread size. For example, very often, a hexagon size one size smaller than above was chosen, and the BSA handlebar clamp screws, which have a 5/16”- CEI thread, have a "small hexagon” of .445” A/F.
- “Bolts” are the correct term for a hexagon headed screw which is only partly threaded, but one which is threaded up to the head is correctly known as a “set screw”. The length of a bolt or screw is always measured from UNDER the head, so the thickness of the head is not counted. Countersunk screws are the exception, though!
- A screw is any type of fastener with a conventional thread, hex, round, countersunk, etc., but a bolt has particular meaning.
- The old manufacturers made screws and bolts in their factories, so they chose rather specific lengths, (eg: 1-15/16”) but we go in steps of 1/4” or 1/2” so we would offer 2” in that example. It is so simple to cut off any excess with a stainless bolt or screw, (if necessary) because it will never rust like a steel bolt.
Fitting Stainless Fasteners
It is possible for stainless threads to seize together, but this does not apply when a screw or stud goes into a different material, like cast iron or aluminium alloys. Anyway, please adhere to the following guidance for stainless to stainless:
- Make sure that male and female threads fit freely by hand. Take steps to eliminate this potential problem, before using a spanner.
- Some kind of anti-seize compound should be applied lightly to threads. "Copper Slip" is the best known.
- DO NOT overtighten. Keep well clear of stretching the material to the point where the pitch might be permanently increased. If vibration is a serious problem, use a self-locking nut, or two nuts tightened together, or apply a thread locking compound that is designed to allow dismantling when the time comes.