normally the lower races are subject to most wear and the upper races are still quite usable, even in rather grotty looking Raleigh headsets.
The threaded upper race is normally quite difficult to 'damage' since IIRC the whole thing (threads and all) is hardened. The locknut is softer and can be damaged more easily.
There are still thousands of old bikes knocking round with these headsets on; I would say that you ought to be able to find a headset on a scrap bike at your local recycling centre, for example.
You can instead renew just the lower races which can give a good steering action in many cases.
At least one LBS near me just forces a 24tpi steel headset onto a 26tpi fork these days when they are faced with this problem and don't have a 26tpi headset to use. It is absolute butchery but it seems to work reasonably well. There are a few headsets in which the locknut is a Zinc die-casting; I would imagine that these can be mounted more easily to a steerer with the wrong pitch thread on it.
NB it is as well to note what the threaded parts actually do; when the locknut is tightened, the threaded race is forced down onto the tops of the thread form on the steerer, and the locknut is forced against the underside of the thread form on the steerer. At this stage the loads on each part (with low bearing preload) are equal. In service the threaded race sees loads that reduce the vertical load between it and the steerer, but increase the tilting (rocking) load as the forks twang back and forth. The vertical load between the steerer and the locknut is increased in service.
When a headset is run loose, the rocking of the threaded race tends to wear and strip the threads on the steerer, local to the threaded race. Eventually the threads will strip badly enough that the headset cannot be tightened any more. The threads above that, where the locknut engages, are usually OK. I have repaired many steerers by adding locally built-up areas of weld metal to the worn part of the threads, then dressing back and recutting the thread where the threaded race bears. This gives a very satisfactory repair even if the thread area is only about half that it might have been originally, provided the locknut is properly snug.
So (finally the point...ahem...
) you can see that the quality of the fit between the threaded race and the steerer need not be that good in order for the headset to work. If the fit is a bit substandard, a bit of threadlock will often sort it out. It is however important that the locknut has decent threads on it, because it sees increased loads in service. Note also that if the head tube is very short, the rocking loads on the threaded race are increased considerably, and the fit etc needs to be better.
hth
cheers
It is an old Triumph town bike, but I think it was made by Raleigh in the 1970's possibly. The steerer is 1" dia and is threaded to 26 tpi. I have tried Ebay and everywhere I can think of but to no avail. So I was wondering if anyone has one they might sell me, I would be very grateful.
