Hi, am thinking of bolting a 1275 from an 1100S onto a
rod change gearbox. Do I need a later clutch housing
to match the gearbox ?
regards, Winston.

Yes. The rod change clutch housing has a gear change selector stop in it that your 1100S one doesn't. And, the idler gear bearing may be bigger in the rod change box, depending on how old it is.
Note these housings are not all totally interchangeable widgets, when the 2 dowels are engaged sometimes the idler gear or 1st motion shaft alignment is a problem. Check before fitting to motor..

_________________
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 might be worth sitting motor on box and turning to check the clearance between crank and gearbox web while you're at it.

Rod change boxes clear all 1275 cranks and rods fine.

_________________
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.