A blog on threading? Obsessive, or what?
Well, no. Take a Moulton F-frame to a well-equipped bike mechanic, or a framebuilder or repainter not familiar with them, and it could well end up ruined. The threads on these bikes are mostly archaic imperial standards, and three different standards to boot. Force a metric bolt into a hole, or run a metric tap through it, and the bike will not be the same again.
Imperial threads – yuck!
Well, not really. Metric threads are convenient and work well for most purposes. You can buy metric fasteners anywhere, and the sizing is very straightforward: M6 has a 6mm outside diameter (well, nominally; an M6 bolt will drop through a 6mm hole nicely), the thread angle is always 60 degrees with flattened crests, and the pitch varies according to diameter to keep things in proportion. For most applications, a metric thread will give good holding power, not be too prone to stripping, and won’t vibrate loose.
Imperial threads, on the other hand, come in a plethora of different standards depending on the intended use. BSC is for bicycles and motorcycles as it is more resistant to loosening through vibration. BSW is the standard coarse Whitworth thread, BSF is for smaller threads with a finer pitch, BSP is for pipework to give sealing properties, and there are still others, like British Brass. Then there is BA (British Association), a British metric thread (really: 0BA is M6 but with a different thread angle and shape), often used in small assemblies and scientific instruments.
In many cases, the imperial thread does a better job or is available in a more suitable size. Metric fasteners are rare in some odd-numbered sizes like M7 and M9. Nor is metric threading especially foolproof: just as we have BSW and BSF, metric fasteners often vary in pitch. An M6 bolt is usually 1mm pitch but it can be M6 (fine) which has a 0.75mm pitch. Generally, “M” then an integer means the thread is the “standard” pitch. If the thread is finer or coarser than this, it should be specified M6 x 0.75 (or whatever) but this isn’t always done. There are at least three pitches for M12, as I once found when trying to get a replacement Nyloc nut for a Mazda steering wheel, where the manual just said “12mm nut”. In the end I bought all three and used the one that fitted. So all that BSW and BSF stuff doesn’t look so silly now.
Nevertheless, imperial threads aren’t used on new bikes – except for some BSC threads, which have been adopted into ISO – and your average mechanic won’t come across them very often.
So here are the threads on a Moulton series 1 or 2 (and a cautionary note or two about series 3), based on my own research and measurement. Disclaimer: threads wear and corrode over the years and may measure up a little differently to specification. I have looked at all the likely candidates and these are the best possible matches. Where possible, I have tried a new bolt or nut of the size to confirm.
The threads
Headset threads: 1″ BSC/ISO, 24 threads per inch (tpi). Series 3 bikes may have Raleigh threading, which is 1″ 26tpi. Test first with an ISO adjustable race, carefully.
Fork rebound stop retaining screw (if you round it off or have to drill it out): 2BA. However, the only other use of a 2BA Philips head screw I know of is for the distributor mount on a Triumph motorcycle. You may have to settle for a slotted head, rendering your long Philips screwdriver redundant after taking the fork apart.
Mudguard eyelets and stay-to-blade fixings: 2BA. 2BA nuts have their own series of across-flat dimensions for hex heads and none of your AF or metric spanners will fit well on the backnuts. A 2BA socket is useful to have.
Front hub axle, if original, 5/16″ BSC.
Bottom bracket: BSC/ISO, 24 tpi. Often erroneously called BSA. Still used on most new bikes. Series 3 Moultons have Raleigh 26tpi threading which can be a real problem, since no new cups have been made in this threading since the 1980s.
Front rack head tube mounting bolt: 1/4″ BSF.
Rear hub axle (Sturmey-Archer), 13/32″, 26tpi.
Rear hub axle (Sachs Duomatic 102), FG10,5. This is a German bicycle thread (Fahrradgewinde), 10.5mm diameter with a 26tpi imperial pitch!
Rear hub axle (Sachs Duomatic R2110), FG9,5.
Seat pinch bolt and rack clamp bolt (where rack has an integral clamp that must be slid over the frame “spike”): 5/16″ BSC. An M8 seat pinchbolt will fit very nicely as a metric alternative, although you can usually only squeeze M7 into the rack clamp. The original bolt in the rack clamp was so tight a fit that it is usually found to be bent when removed.
Rack clamp (very late 2-bolt fixing using a separate clamp and a rack made from U-shaped pressings): 1/4″ BSF. It doesn’t appear that Philips head bolts are available in this threading any more, so you will need to settle for hex heads.
Rear rack light bracket boss: 1/4″ BSF.
Self-tapping screws for the “foot” of the lower rack strut (also attach the suspension block to the rear frame): No.10.
Self-tapping screw underneath the very rear of the rack, attaching it to the “spike”: I confess I’ve never been able to extract one in good enough condition. Possibly no.8, although I just run a drill through it to enlarge the hole enough for a no.10. It’s not as if it shows, and a bigger screw helps, as it is the only thing that positively locks the rack in a horizontal position.
Moultons and other endangered species 