Thanks for info.
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| small bore head on a 1275 https://ausmini.com/forums/viewtopic.php?f=5&t=89502 |
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| Author: | norm74 [ Thu May 21, 2015 5:16 pm ] |
| Post subject: | small bore head on a 1275 |
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 |
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| Author: | ba_dm_tsh [ Thu May 21, 2015 6:15 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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. |
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| Author: | drmini in aust [ Thu May 21, 2015 6:50 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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. |
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| Author: | shnitzee [ Thu May 21, 2015 7:39 pm ] |
| Post subject: | Re: small bore head on a 1275 |
How do you distinguish a 1275 head from a 998? Eiles |
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| Author: | gtogreen1969 [ Thu May 21, 2015 7:44 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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 |
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| Author: | norm74 [ Thu May 21, 2015 7:59 pm ] |
| Post subject: | Re: small bore head on a 1275 |
Thanks heaps for that. Cleared it all up for me. |
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| Author: | shnitzee [ Thu May 21, 2015 8:01 pm ] |
| Post subject: | Re: small bore head on a 1275 |
Thanks for info.
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| Author: | drmini in aust [ Thu May 21, 2015 8:19 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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. |
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| Author: | shnitzee [ Thu May 21, 2015 10:52 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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 |
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| Author: | ba_dm_tsh [ Fri May 22, 2015 12:42 am ] |
| Post subject: | Re: small bore head on a 1275 |
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. |
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| Author: | drmini in aust [ Fri May 22, 2015 8:42 am ] |
| Post subject: | Re: small bore head on a 1275 |
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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| Author: | shnitzee [ Fri May 22, 2015 3:12 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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
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| Author: | shnitzee [ Fri May 22, 2015 9:04 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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 |
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| Author: | drmini in aust [ Fri May 22, 2015 10:33 pm ] |
| Post subject: | Re: small bore head on a 1275 |
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. |
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| Author: | shnitzee [ Fri May 22, 2015 11:00 pm ] |
| Post subject: | Re: small bore head on a 1275 |
Thanks doc. In your opinion, would it be worthwhile putting the 202 head on my 998 block? Cheers |
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