Hi rc46, thanks for the reply. No they didn't take the box out, they've just taken the rubber seal/spring out from the outside and replaced it with a new one, the bearings behind the seal haven't been changed. I thought the seal SHOULD be pulled out and replaced from outside the box once the sprocket was removed. It would seem wierd to have to pull out the transmission simply to replace that seal... But you'd probably know better than me. And are you saying that damage to the SHAFT itself is the likely problem? Not over-pressure?