I think i have read somewhere that an 1100 and 1275 gear box can go on any other sized engine but you can't do the other as in fit an 850 or 998 gear box on an 1100 or 1275 engine?
Is this so ?

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76 1275 Leyland S

There are so many gearcases...!

4 synchro remote (Cooper S or Mini K)-
will fit any motor from 850 to 1275. So YES you are right, here.

Ditto the rod change boxes, both A and A+ types.

With the earlier 3 synchro ones it gets a bit muddy:
Early 850 and Cooper cases-
will take up to a 1098 crank, perhaps needing a burr ground off here or there.
1275 will not fit without major surgery- I did it once, but never again.

3 synchro Cooper 1275S box 12G333 has 1275, will take all others.

Morris 1100 boxes- the 1100S box has a 1275 stock but I'm not sure if the Morris 1100 casing is the same...

My recommendation is to forget the 3 synchro boxes for performance road use, these days- 1st or 2nd gear is easily busted with a 1275, parts are hard to get, and the boxes are not as strong as the 4 synchro ones.

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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.

arnt 998 and 1098 box's the same?

NO- the 1098 (in Minis) came out in Mini-K in 1969, and got the 4 synchro box. Early Clubbie got this combo too.

998 Deluxe was 3 synchro, 998 Clubbie got the UK rodchange box in mid 70s., after Oz 1098s finished.

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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.