"Ian" wrote in message
...
On Sat, 26 May 2007 20:00:39 +0100, "Davy"
wrote:

Regarding which is the correct bolt to draw the disc off the crank;
Looking end on at the head with a flat horizontal there are three
lines one pointing to the top flat (12 o'clock), and the other 2 at
120 degree intervals (i.e pointing to about 4 and 8 o'clock). Does
this help identify it
yes, this is a 'unified screw' bolt; the bolts have three equally-spaced
radial markings, the nuts would have three circles. The bolts should also
have a figure at the 6 o'clock position which denotes the grade of steel and
hence the maximum tightening torque; if this is missing then the bolt is
low-grade steel. In which case the maximum torque that should be applied to
the bolt is below 65 ft/lb - more than this and you risk damaging the thread
or shearing off the head.
The closest match to your description is a 7/16 UNF bolt which has 20 tpi.
It will not fit a 7/16th BSW thread so my supposition about BSW being used
was wrong. But you state that your machine is metric so we would expect a
metric thread; a 7/16th diameter bolt equates to 11mm. The nearest common
metric sizes are M10 (17tpi) or M12 (14.5 tpi) - so no metric equivalent of
this size. Perhapsy Hayter changed the fastenings to metric by 89/90 but
not the components?

I look forward to hearing whether your old machine has the same arrangement
since it is much closer in age to my 1968 machine.

Many thanks, Davy