MOBI > Stronglight Crank Extractor Threads

Most square-taper cranks use an extractor with M22x1 threads — that is, 22 mm nominal diameter and 1 mm pitch (distance) between threads.

Crank bolts often have 14 mm or 15 mm wrench flats, and 15 mm bolts are big enough you need a thin-wall socket wrench to fit in the 22 mm hole.

An obvious approach is to use bolts with 14 mm heads, although 15mm is a more common wrench size.

A slightly less-obvious approach is to use M23x1 extractor threads, giving an extra 0.5 mm for the socket wall. That is the approach used by T.A. on many of their older cranks.

Until 1982, Stronglight used 23.15 mm diameter threads — which is 0.91929 inches, or between 29/32" and 59/64" and thus not a metric conversion of a fractional-inch size.

Actually, it is unclear what is the size — variously quoted as 23.15, 23.2, or 23.35 mm diameter. None of those correspond to a common fractional-inch size, and all suffer from the problems below.

23 mm is so much larger than 22 mm it is hard to damage threads by mistakenly using the wrong extractor. However, 23.15 mm is so close to 23 mm that a 23 mm extractor threads in and appears to be a good fit, but far enough from 23 mm that the 23 mm extractor pulls out most of the threads and ruins the crank (or at least makes it impossible to remove with a standard crank extractor).

It is hard to guess how Stronglight arrived at 23.15, since it corresponds to no obvious units, to no other threads, and since making a special thread means making special tools to make the crank threads, to make the dustcap, and to make the crank remover.

Further, whatever the clearance advantages, at least some Stronglight crank bolts have a 16 mm head — also a Bad Idea™, as 16 mm is an uncommon wrench size anyway, and the clearance between wrench flats and crank extractor threads is so small that (as with the more common 15/22 combination) a thin-wall socket is required, further limiting the choice of tools.

Further, it is not uncommon for crank bolts to come loose while riding, and riding with a loose crank will ruin it. So it is advantageous to use a common wrench that will be in every tool box.

If Stronglight really wanted "bigger than 22 mm", 24 mm is hard to confuse with 23 mm and causes no new problems. Today, M24x1 and M24x1.5 are standard threads, though I do not know if they were standard when Stronglight started using 23.15 mm.

In retrospect, it seems like an obvious choice is 15 mm bolt heads, since 15mm is a standard size, and M24x1.5 extractor threads, since that is a standard size with clearance for many 15 mm socket wrenches and 1.5m mm thread pitch should still give enough engagement to support the extractor load.