On 07/10/2017 15:45, Pet @ www.gymratz.co.uk ;¬) wrote:

The inspection chamber "top" is a double lipped frame packed with grease
into which the single lip of the heavy chamber lid nestles and is then
fixed down by 3 (originally 4 but one is sheared off) large brass screws.

Current steel lump is 600mm by about 740mm (whatever the imperial size
would have been)

Update - on further investigation it's the "clear open space" or such
that I should be measuring i.e. the hole, which comes into the realms of
450 x 600 so now I'm back on track...