Thanks for the response
I went with your suggestion and retained the original; and others who indicated that the felt seal is 'better' at sealing than the neoprene.
The decision was mainly based on no information regarding machining the cover.
I bought a new neoprene seal and normally I would say open the housing out so the seals installs as a tight fit. However the housing has a base that seats the felt seal. This is no more than 10mm from the front face of the gear casing.
Opening out the housing to this base would mean the neoprene seal sticks out by a fair margin.
The solution would be to remove the felt seal 'base' and seat to the rear of the housing.
Reading back thru that probably makes no sense (It doesn't to me!) however I'm hoping some of you will cut through the crap and realise what I'm saying!
Q. Can I remove/machine the base that the felt seal seals on so the neoprene seals sits properly in the housing?

I'm asking the question as the felt seal spring retainer is a pain in the a!*e and I'm fed up watching it spring out!!
TIA
Russ