The conclusion of this discussion seems to be:

Cutter disk is held by a 3/8 UNF bolt (9/16AF) and a woodruff key to prevent
rotation. Bolt can very rusty and tends to be worn smooth by contact with
hard objects; would be better to replace but is unusual so would need to
order. Recommended to jam cutter disc with a mole grip which will jam
against a piece of wood against the mower skirt. Retaining bolts needs a lot
of torque to get off; is said to be high tensile so head will not shear off.
Recommend a six-sided (not usual 12) socket. If this fails then weld a bar
to the nut.
The hole in the centre of the disc is threaded so a 7/16 UNF bolt can be
screwed in against the crankshaft end and will force disc off. Need to put a
spacer in to prevent threads in crankshaft being damaged and because the
bolt is unlikely to be long enough to fully draw off the disk. If disk has
not been off for a long time then woodruff key is likely to be rusted into
the keyway and disk will be difficult to remove. Disk is thin so has only a
few threads and these are likely to strip. Some resort to WD40, blow gun and
big mallets. Some cut the disk off. Expect an hour of dealers time at £30.

When you get the disk off examine for cracks - if it disintegrated at speed
then not funny. If blade bolt holes are elongated then needs a new disk.

"Davy" wrote in message
om...
I am servicing a Hayter Hayterette mower and need to remove the large
metal
disc that the bladelets are attached to so that I can replace them. I
assume I undo the nut in the centre. But how to stop the disc/shaft
turning?
And does anyone know which way the disc rotates in use - I suspect that
the
bladelets are on the wrong way round!

cheers

Davy