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I have decided to do my 1100S up at this stage. Havent done before so will be loking for some help.
EG: Can you put a 1275 head on a 1100s? If so what are pros and cons?
What else can I do to up the performance?
Any advice much appreciated

An 1100S motor is a 1275, it is just a 1275 that came out of a Morris 1100S (which was the 1275 version of a Morris 1100). Therefore the head is the same 12G940 casting (in most instances).
A good cam and some head work, up the compression and I recommend a balance for smoothness and away you go!

thanks for the reply Monaco.
Sorry I have a 1973 Mini S 1100 not 1100S.
Any info or advice still would be appreciated

I guess most of you are going to hate me because i just got a complete 1100S motor/rod change box for $300 which was driving fine until the car was crashed. The head was already off so i just had a quick look inside and a bit of a clean and started putting it back together so i can get it fired up on the floor. Turns over nicely by hand so i'll just chuck some oil in, hook it up to a battery and see if it sucks fuel and goes bang inside.

It will do nicely in my old road mini that i stole the motor out of 2 years ago

Yes you can put an 1100S (1275) head on a 1098, however to do it properly the top of the block should be notched to clear the exhaust valves. They stick over the edge of the bore- if you have a sporty cam, or hi-lift rockers, or the head has been decked much the valves can hit the block and get bent.
One way around this for road use (not race cams) is sink the valves into the head a bit.
You can get good power (with the right cam, carb etc), but torque will suffer compared to a modified 1098 head.

[edit] I did this back in the 60s- the 1100 bored out to 1220 with a works 544 cam was a real screamer, whilst the brand new balanced and shot peened crank lasted...

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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.