So is there some sort of lip that the rod change seals sits against to stop it going through into the gearbox?

If I push another 47 seals in there am I going to fill my diff up...?

there is normally enough room for 2 seals, sometimes you can even fit 3 seals in there

but it will stop at the interspool lock,,, it can`t go further than that

i have awesome gearshift seal "Bushes" for sale tho if you`d like to not have any leaks from there

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

It's already got a Rover-type alloy bush in there. I'd swap it with one of yours but it's a PITA to get the old one out!

I put a new seal in yesterday, and found I could fit two seals in, so I wanted to make sure I hadn't just pushed the alloy bush into the diff

Nope as Matt says the interlock spool will stop it.
In the old days it was easy to just bung another seal in when it leaked. 2 fit fine and 3 sometimes.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.