I'm putting together a 4-syncro Cooper S gearbox for the Marcos I'm restoring. Unfortunately when it came to fitting up the mainshaft bearing retainer to check the bearing preload, the retainer wouldn't fit over the final drive pinion It appears that if you're going to fit a 3.1:1 crownwheel and pinion you've got to a enlarge the inside diameter of the mainshaft bearing retainer to clear the larger diameter of the physically larger final drive pinion.
I wonder if the later Rover Minis that ran the 3.1 ratio had a different bearing retainer, or whether all the rod-change boxes were built with extra clearance in mind?

Any ideas?

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT www.mm7056.wordpress.com

I concur.
I had to grind out the inside of my bearing retainer with a dremel.
Little bit at a time until I had clearance.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

I also concur
I have built myself a 4 syncro box with a 3.1 diff
I am probably not as subtle as Michael
I took to mine with the angle grinder
Don't own a dremell

The std issue (later A+) boxes have the lager Inner diam retainer , yes, factory item

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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.

Yep same problem with my 3.1. Got a friend to turn it bigger in a lathe

So many different ways of enlarging the retainer - I used a barrel type of flap wheel sander mounted in my pedestal drill, it took a while but leaves a nice finish.

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT www.mm7056.wordpress.com

I used a half round file

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did I tell you that I won a trophy?

I used a half round file

Sounds a bit slow...
I'm too impatient

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

.... so I get the retainer to fit over the pinion gear, sort out a shim for the correct bearing preload, find a couple of new lock tabs and torque up the 4 bolts - all good until one felt soft and pulled the threads out of the alloy housing - BUGGER
That means tomorrow's job is to strip it all down again and helicoil the housing. Luckily I have nothing better to do with my time

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT www.mm7056.wordpress.com

Don't helicoil it. More trouble than it's worth in that spot, the thread is very short.
You have stripped the hole, so it is now 5/16", = right size to tap it to 3/8" UNC. Then just drill out the retainer (and the layshaft lockplate, if that's the one that stripped (which is usual) with a 3/8" drill..
It won't strip again, the 3/8 thread is much stronger.

ps. if you pull the cluster and reverse gear out and are careful with some rags, you can do this job without stripping the rest of the box. As no drilling of the case is needed.

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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.

Thanks Doc, I'll follow your advice
By the way, it was the hole above the one that takes the locking plate that stripped out - at least it's probably the easiest one to get at .

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT www.mm7056.wordpress.com