Hi all just a question can you fit a small bore head onto a 1275. If no. What are the differences between them.
just because I have a 1275 block and a few small bore heads lying around. Thought that they might be interchangeable.
Thanks Rob

Physically yes, it will fit.

But in practice it will not work. The main reasons being: the chambers are bigger (so less compression), and the valves are too small - So you have much less mixture getting into the cylinder and then a much smaller pressure being produced. Therefore much lower torque.

I brought a MG Metro head from the UK about 5-6 years ago... Cost me 40 quid plus shipping - Think it ended up being about $150 ish landed. Gave it a clean and relapped the valves and it was good to go. Not sure if those sort of prices are applicable now but maybe worth a look... The MG head also had the larger 'S' sized inlet valve and slightly better ports as standard.

The other problem is the head gasket. There are differences in water flow holes between 1275 and smallbore heads.
Usually when putting a 1275 head on a smallbore (it's more commonly done in UK but I did it too) you use the 1275 gasket. ie the gasket you use is what matches the head.
Unhappily, a smallbore gasket will protrude heaps into the 1275's big diameter cylinders and will not seal.

There are 1275 heads still about, put an ad in the wanted forum as I did last year.

_________________
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.

How do you distinguish a 1275 head from a 998?

Eiles

The thermostat bolts. 2 are at the front parallel to the front face of the head. Also there is a different angle for the heater takeoff and bigger gap between the heater takeoff and rocker cover. http://www.google.com.au/imgres?imgurl= ... CQQMygHMAc
Sent from my GT-S5830V using Tapatalk 2

Thanks heaps for that. Cleared it all up for me.

Thanks for info.

Most 1275 heads here (not counting the early Cooper S ones) have a casting number on top, either 12G940, or 12G1316 (smog head used on 1275LS and late 1275 Mokes).
The smog head works fine, just remove the 4 air injection pipes and plug the holes. Thread size is 7/16 UNF. A cheap way is use old seatbelt bolts then cut em off flush. These heads are sometimes found cheaper than the 940 heads.

If you find a 940 head with 12G1805 stamped next to the thermostat, it is a Mk2S head.

_________________
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.

So Doc

If you can't fit a small bore head on a 1275 block can you fit a 1275 head on a 998 block? If so, is there much modification required and is there a significant improvement in engine performance?

Eiles

You certainly can fit a 1275 head on a 998/1100. It has been done many times over.

I once built up a 120 thou over 998 and pocketed the block to fit a 12G940 head.

There are three key things to get right when fitting:
- Ensuring there is adequate clearance for the exhaust valves as they do protrude beyond the edge of the cylinders. Some have got away with a straight swap - A better way is to dummy it all up, measure clearances, and cut a relief pocket in the block. I aimed for 40 thou clearance when the valve was fully open and then a small profile around the valve and out to the head gasket. You need to pick all of your cam/valve train components first. And make sure your pocket doesn't intrude on the top ring (some pistons have the top ring very close to the top of the piston).
- And very closely interwoven is getting your compression ratio correct. A STD 1275 head was 21.4cc (ish) and a STD 998 head 24.5cc (ish) if memory serves me. So on a 998 you could, just by putting on a 1275 head, raise your compression ratio by as much as 1 full number which may not be a bad thing but it needs to be considered.
- And lastly, as Doc said above, you need to fit the head gasket that fits the head ie you need to use a 1275 head gasket.

If done correctly then you will see an increase in engine performance and usually for less than it would cost to get the same performance from a small bore head.

What he said ^^^.
I built a 1220 motor back in 1967 (1098 crank + 68mm Imp pistons)- with a 1275 head, works F/J 544 cam, & twin HS4 carbs it absolutely flew. Blew my mate's new 1275S away big time.
It might have lasted too, if I'd only used 7500 rpm, not 8500...!

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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.

Great info, thank you.

This is something I will look into. When I bought the Rust Bucket to help with missing bits for my resto clubby it came with a couple of spare heads. I'll have a good look and see what they are.

Sorry about hitch hiking on your post Norm

Had a look at the two heads this arvo but the photos I took came out rubbish so I'll take some better ones in the sunshine tomorrow. These are the numbers and letters I found on them:

12g202
SM
2
MOWOG


AF2
AS3
MOWOG
2A629

I don't think either of these are 1275's? The second one must be a small bore?

Eiles

Both are small bore but the 202 is better. Was used on Morris 1100, 1098 Mini and 997 Cooper.
The other one is 850/Mini Deluxe 998.

_________________
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.

In your opinion, would it be worthwhile putting the 202 head on my 998 block?

Cheers