Common on better bikes from the 1970s to the 1990s, these provide a bigger crank to spindle interface and easier maintenance than the older cottered cranks.
There is some confusion and misinformation about the types of square taper you may find. There are generally two types: JIS (Japanese Industrial Standard) and ISO. ISO is far less common and was used mainly on European brands like Campagnolo and, at one point, TA. It survived on Brompton bikes until fairly recently, causing many headaches for owners of older Bromptons who can no longer buy a 119mm ISO bottom bracket.
Square taper has largely been eliminated on new bikes by splined interfaces, or those where the cranks are permanently pressed to the spindle (Campagnolo and the RH side of Shimano cranksets). There is also a proliferation of bottom bracket standards in different sized shells.
All square tapers have a 2 degree angle but ISO tapers start thinner than JIS tapers. To tell the difference, measure across the flats at the tip of the spindle. JIS is 12.7mm and ISO is smaller (one day I will get one and measure it but, as I said, they are unusual).
A JIS crank, if it doesn’t bottom out, will slide 4.5mm further onto an ISO spindle than would an ISO crank. Likewise, an ISO crank will slide 4.5mm less far onto a JIS spindle than would a JIS crank. This means that Brompton owners can use a 110mm JIS bottom bracket instead of a 119mm (119 – 4.5 -4.5 = 110) ISO bottom bracket provided there is sufficient engagement. I have tried it and it works perfectly well.
Dura-Ace square taper bottom brackets (up to 7400, after which DA changed to a splined connection) have slightly shorter tapers than most other JIS spindles. In fact, taper length does not seem to be totally standardised and I found different length tapers on Shimano bottom brackets of different eras, the longest being on a recent UN55 cartridge unit. All are still 12.7mm at the narrow end. Some people suggest that 7400 was an ISO taper BB but this is not true.
I have fitted Andel track cranks, Sugino XD2 cranks and (yes) Dura-Ace 7402 cranks to the DA 7400 bottom bracket, and none of them bottom out despite the slightly shorter taper. All give the expected chainline based on the 7400 BB’s spindle length of 112mm (English) or 113mm (Italian).
Square taper cranks are subject to wear from fitting and refitting, and the presence of grease on the taper markedly affects how far the crank will slide onto the spindle for a given torque on the crank bolt. My practice for fitting cranks is as follows:
- Grease the taper. I have known ungreased cranks seize and have to be hacksawn off.
- Grease the threads of the crankbolt.
- Fit to the recommended torque.
- Check chainline. if it is much narrower than expected, you may need to pull the cranks, degrease and refit. I will only degrease as a last resort, since the risk of seizure is very great on an all-weather bike.
- Ride for 100 miles.
- Re-torque the crank bolts.
- Remove the crankbolts, degrease the threads in the spindle and on the bolt, and refit to the recommended torque using medium threadlock.
- Do not retighten after this. Repeated retightening will enlarge the crank hole and eventually it will bottom out on the spindle, or even crack.
Moultons and other endangered species 