Davy wrote:
"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



As I understand it finally, the way it all hangs together is this.

The crank is untapered and fitted with a woodruff key in a slot. The
boss of the disk is slotted for this key and by an large fits over te
crankshaft.

The crankshaft is end drilled for a retaining bolt, but this is an
enormously loose fit in the disc, as this is fully located by the boss
and the key, and the bolt merely stops it falling off. Removal of the
bolt reveals that the end part of the disk is threaded and a bolt can be
screwed into the disk, to bear on the crank end and push the disc off.

So I might tale the disc bolt out and see what is there.

Now I am fairly sure that around 1990 or so, Hayterettes did NOT have
this arrangement..mine is 1995 but I never took the disc of myself yet.
It got serviced once about 6 years ago at the service shop. It had never
had even an oil change up till then. We USED to change the blades by
angle grinding the heads off the bolts that hold them in, that part of
the head that hadn't already been ground off by mowing lumps of bricks
and discarded spanners and the like.

Sine the bolts can't generally be removed any other way, there is no
need to remove the disc as well. Just grind off the heads, and fit new
blades and bolts. Flat side down and bevelled side up.


Only if the disk is damaged need it be removed. Or if you really feel
like draining the oil. Though I THOUGHT mine had a plug in the sump side
where it could all be tipped out of.