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| 12g202 head on a 1275 https://www.ausmini.com/forums/viewtopic.php?f=2&t=32067 |
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| Author: | mini-dunger [ Thu May 24, 2007 10:02 pm ] |
| Post subject: | 12g202 head on a 1275 |
Hi guys I have heard that a 12g202 is often better than a 1275 type head as far as portings concerned. would an 12g202 fit on an 1275 block ? Cheers |
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| Author: | drmini in aust [ Thu May 24, 2007 10:37 pm ] |
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No a (relatively standard) 202 one is not better than a 1275 head. And it won't fit on a 1275, the gasket that suits it won't seal on a 1275 block. Some people (like me) DO fit 1275 heads (with 1275 gasket) onto smallbores instead of 202 ones, yes that's possible (if you watch a few things), but a different kettle of fish to your question. 
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| Author: | Asphalt [ Thu May 24, 2007 11:36 pm ] |
| Post subject: | |
It`s better for porting, that´s true. As far as smalblock (850, 998, 1098) engines are concerned 
12g202/12g295 allow smal 1275 valves! But the maximum is 33/29 (some got 34/28). But: With an 12g940 (1275cc head) you easily can go for even bigger valves on a smalblock. But only if you modify the block too, to avoid contact between valves and block (Not in every case, but in most)! Modified 202/295 most propably doesn`t need mods like this but they`re pretty expensive in comparison to an standart 1275cc head. An other advantage of the 202/295 is the mor voluminous combustionchamber. It`s easiyer to get needed compression by just leveling the head. In detail: 12g202 = 26,1cc 12g295 = 28,2cc 12g940 = 21,4cc Modifying the block for valve clearance adds more cc, helps reduce compression. Kind regards, Jan |
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| Author: | mini-dunger [ Fri May 25, 2007 7:27 am ] |
| Post subject: | thanks guys |
thanks Doc and Jan |
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| Author: | drmini in aust [ Fri May 25, 2007 9:24 am ] |
| Post subject: | |
My 1132 Moke is getting a slightly modded 12G1316 (smog 1275) head, because- a. I have one, and I've done it before. b. I can't be arsed porting out a 202 head so it flows good enough. Yes the block will be notched for valve clearance, after I rebore it. 1275 heads are also less likely to blow head gaskets, they are more rigid with a thicker deck.
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| Author: | Tadhg [ Fri May 25, 2007 11:29 am ] |
| Post subject: | |
To my understanding, and the Doc and others can correct me if I'm wrong, but a 1275 head on a small bore is a good thing, because of the smaller combustion chamber and bigger valves. Yes, the block needs pockets to allow for the valves (which weren't designed to fit in the smaller bore), but that disadvantage is minimized compared to a small bore head by the smaller standard combustion chamber, and is easily overcome by the better airflow allowed by the bigger valves. That's why Vizard recommends a 12G940 on a small bore over a 12G202 or 12G295.
Given that a 1275 head is better on a small bore than a small bore head, it would follow that a 1275 head is better on a big bore than a small bore head. Why? Bigger valves and smaller combustion chambers, as shown by Jan. And the valves (generally, depending on the head) aren't as shrouded as a stock 202. Bear in mind, when you port a 12G202, its combustion chamber increases, so you end up around the same combustion chamber size as the 12G295 head. After all, porting a 202 is done to make it more like a 295, if not better, and the final combustion chamber shape is similar. The 12G202 after porting or the 12G295 might flow better, but, at the end of the day, the higher compression given by the wider, flatter combustion chamber of the 1275 head will give more power, and the bigger valves help balance out any flow advantage a worked small bore head might have. The best engine results will generally come from a combination of big valves and a small but open combustion chamber. In fact, ideally, that's the way all engines should be made. Economy can still be attained by using smaller carbies/throttle bodies/injectors, etc. 
This sort of thing - changing to a smaller head - is sometimes done on other engines - 350 heads onto a 400 in V8's for example, as a cheap way of upping the compression. This gives more torque, particularly at lower to middle revs. Compression is our friend - more compression, more torque. That's why turbo's and superchargers are a good thing - they up the pressure in the cylinders even further, which is why people sometimes reduce the compression before they put a blower on. The lower the compression, the more boost you can run (but the worse the car is to drive off boost). The only negative with more compression (or boost) is you have to use higher quality petrol - good thing it's a Mini not a V8!  Putting the 350 heads onto a 400 isn't all roses, though, as it costs them at the top end, because, just like here, the 400 heads will have bigger valves. Top end power is all about gas flow. Compression is still important, but if you can't get enough gas in, you won't use all that compressive capacity. On a Mini engine, your highest/best compression ratio come from a 1275 head, as well as the biggest valves, so there's not really any point going to a small bore head.
Hope that makes sense. Sorry if it's all a level below you... 
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| Author: | drmini in aust [ Fri May 25, 2007 11:41 am ] |
| Post subject: | |
The rule of thumb here is, always use the gasket that belongs to the head... Problem with trying to put any smallbore head on a bigbore block- the gasket is way too small in the bore, and bore spacing wrong too. And, a 1275 gasket won't seal all the water holes in the smallbore head properly. As far as using a 1275 head on a smallbore- Graham Russell doesn't like doing it, but Keith Calver does. He sinks the exhaust valves into the head a bit to avoid pocketing the block, and then carefully checks valve lift. |
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| Author: | peterw [ Tue May 29, 2007 1:06 pm ] |
| Post subject: | |
One of my minis had a 1275 head on an 1100 block and it ran great. The block was relieved for the valves. Because the water holes don't line up exactly the coolant ate away at the gasket and eventually got to the compression sealing ring which subsequently corroded causing the engine to lose compression. The block face is corroded where the sealing ring goes so the only cure is to deck the block which means a rebuild. It did last 15 years but it's a shame that I will have to rebuild a perfectly good working engine. I think I could have avoided it by changing the head gasket every 5 years or so and being more diligent changing the coolant. I would do it again though, those heads breathe really well. Pete |
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