"Ian" wrote in message
...
On Sat, 19 May 2007 19:00:28 +0100, "Davy"
wrote:
9) . Furthermore this hole is threaded to take a bolt to draw the
disc off the crank. I'm not very up on bolt threads but I THINK it was
described as 1/4" when I got it. It is about 7/16ths diameter and
pitch is about 19 per inch. The head has three radii on it which I am
sure indicates something! Over to someone else on this one.


thanks for that Ian, I don't think many of us realised that the blade disc
has a built in facility for its removal.

Regarding which is the correct bolt to draw the disc off the crank; garden
tools and mowers tended to have British Std Whitworth (BSW) threads. A 1/4"
BSW has 20 threads per inch and a obviously screw diameter of 1/4"; but you
say it has a diameter of 7/16ths - or maybe you meant across the flats? A
7/16th dia BSW is 14tpi so it can't be that. However a 7/16th dia BSF
thread is 18 tpi - maybe?
Not sure what you mean by it having three radii on it; but UNC/F bolts have
three circles in a row - I wonder if this is what you meant?. A 1/4" UNC
bolts has is 20tpi and 7/16ths across the flats - if you did mean 'across
flats' then this is a good match to what you have. But a UNC thread is
wrong for a British lawnmower. So my guess is that the Hayter Hayterette
requires a 1/4" BSW bolt; but that someone gave you a 1/4" UNC bolt which
will screw in to a 1/4" BSW thread; but a UNC thread is cut at 60 deg whilst
BSW is cut at 55 deg; so the mismatch will give a weak interaction because
the stresses are not transmitted through the flanks of the thread.

But none of this is making sense to me because I think I understand you to
be saying that your withdrawal bolt screws into a thread in the centre hole
of the disc? - and that "When the retaining bolt is out, the thread in the
crank is actually smaller than the hole in the disc through which the
retaining bolt passes" The retaining bolt which goes into the crank has a
diameter of about 5/16ths - larger not smaller than 1/4".

I think my knickers are now sufficiently twisted to hand the subject back to
you!

cheers

Davy