Seeking any second opinions again please.
Have had the block drilled and tapped to take 5/16 bolts for block to g/box.
Should I make the holes in the 'box 5/16" or 8mm?

5/16" = 0.313" = 0.79375mm

So a little amount of movement at 8mm or virtually none at 5/16"
Does it matter??

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'98 Mpi Sportspack + 7 port alloy head

Mine were done to 5/16, dont know if it matters to be honest.

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1965 Riley Elf
1974 Ex Army Moke
1997 Rover Cooper
2002 JCW Cooper
2003 JCW Cooper S

I cannot see how it can matter unless the bolt holes are not square and the bolts are tight. You will find that your 5/16 bolts will all ways be undersize they always are.

Do a dummy assembly of box to block if you are concerned

Original bolt holes were at least 1/64" bigger than the 1/4" bolts.
I would drill them 8.5mm for 5/16" bolts.

My 2c..
This folklore of putting 5/16" bolts in to replace the 1/4" ones arose back when all the blocks were thin flange. I reckon it's a waste of time doing it with thick flange blocks, particularly for street use.
A 1380 thin flanged (1100S block) stroker I built a while back (with stock 1/4" bolts) still has NO leaks there.
Gasket sealants have improved in the last 30 years.
Don't believe everything you read in Vizard's book.

[edit] My car's Mk2S box was drilled out when I got it (had a thin flange S motor on it with 5/16" bolts). It now has a 1360 (1300auto block) on it, but with 1/4" bolts in. It doesn't leak there, either.

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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.

It shouldn't really matter if there is a bit of play there because you should have 2 dowels to locate the gearbox and block.

If you drilled it 5/16 the finished hole will be bigger than 8mm anyway.

Use a decent sealant. I use loctite 518. Its pricey yes at $50 a tube but works brilliantly. I use it on all oil joints when I build engines. Never had a leak yet..

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The best part of fixing your mini is the test drive

Minis..

1976 van daily drive
1975 van
1968 deluxe in the shed
1967 deluxe high strung 1310
1959 austin healey sprite

Thanks for all the details, much appreciated. Taken it all onboard
Will start with a 5/16 bit and go from there.

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'98 Mpi Sportspack + 7 port alloy head