13 replies to this topic

[paulrockliffe]

I had it in my head that if a cheap MG Metro head came up I should have it as it would be a decent upgrade on my standard head.

I've just been looking and spotted that the SPI head I have has the same valve sizes as the MG Metro, so perhaps it's not worth swapping?

What do we think? Thanks

[Dusky]

Either the SPI head was converted before,OR you have a metro head instead of MG. 35,6mm should be the size of the inlet valves of the MG head

[Turbo Phil]

Only the early MG head was fitted with the bigger (35.6mm) inlet valve. The later unleaded heads were fitted with the smaller 33.3mm valve.

Phil.

[paulrockliffe]

Sorry, I should have been clearer, when I said I noticed I meant I read this table here:

http://www.retromini...page=page&id=17

I think my reading of it was wrong due to the way the table formatted on my device. So the head would be a decent upgrade by the looks of it?

[wile e coyote]

Phil's correct here - it's the crazy fun of having a multiple possibilities under "12G940", although your existing head will be a 12G940B. No "big valve" '940's were UL heads as produced and there are conflicting  opinions as to whether they can be converted - and those that have have reportedly been prone to cracking between valve seats as there's not much "meat" left.....the insert coming  loose as a result has been known to happen - with inevitable effect.

 

The MG metro big valve (if you can find one in good order and not suffering badly with seat recession) is a nice upgrade - that improved breathing does have a noticeable, if marginal effect  (2-3hp from memory which is fairly minimal really - engine feels a little more responsive though) but unless converted you'd be taking the retrograde step of having to use a fuel additive.

 

For my money I'd keep existing head  and buy a decent set of 1.3 ratio roller rockers (lift on standard rockers variable but average between about 1:2.1 to 2.4 )- achieves IMHO more or less the same result...and can - with care (and luck) be installed without a HG change...

 

Obviously if you need to take your head off and can be without your car for a little while a decent expert engineering shop could fettle your existing head ....

[paulrockliffe]

Thank, yes mine is the 940B head. I can see what Phil said is right with that table formatted correctly.

Hadn't considered rockers, could be a good option. Mind you anything other than a straight head or rocker swap will have to wait and it's not really a priority at the moment. I should probably lift the engine out and do the cam at the same time, so it's probably a job for another day..

[wile e coyote]

Cam is the best £ for £ upgrade - but a real pain to do on a 1275 - I have ... once... managed to swap one in situ.(cheapo nasty sprung "pick up" tools used to secure the followers ..but a bit nerve wracking and not the most recommended of methods!!!

[skoughi]

Sorry for jumping in here, I have acquired a 1275 engine with the 940B head which may or may not end up being a long term build project to replace my 1330 which has a big valve MG head fitted (which has inserts). You said Wile e coyote that the slightly larger valves only have a few bhp gain over the smaller ones. So would it be just as beneficial power wise compared to the MG head to keep the valve size the same on these heads, to avoid the risk with the inserts, and spend money on getting valve angles, seat angles and porting done correctly? What I'm trying to ask is if the same modifications are done to these two different heads will there be a noticeable difference in engine performance if they were bolted onto the same short motor?

[Turbo Phil]

I wouldn't worry about fitting inserts with the 35.6mm inlet. You'll find every single aftermarket supplier sells heads with a 36-29 valve combination and inserts, it's very rarely a problem. From the flowbench work I've done the big valve head doesn't flow a huge amount more than its smaller valved counterpart, but with a few very simple mods, and without even touching the ports, some impressive gains can be seen.

Phil.

[Spider]

I've fitted loads of insets to the bigger valve heads, not had a hint of an issue with any of them.

[skoughi]

Thank you guys for the info. My wonderment was if the extra costs in fitting the slightly larger inlet valves were worth it, but thinking again about it a decent head refurb job would probably involve buying new valves anyway and if not then the cost of four inlet valves isn't that much, and the valve seats would be getting re-cut as well so i guess it wouldn't cost that much more compared to leaving the smaller inlets as is. Thanks again!  

[Turbo Phil]

There's a bit more involved than just recutting the seats if you want to get the most benefit from the larger valve. I keep meaning to do a bit of a writeup on it as this topic seems to get covered on a weekly basis.

 

Phil.

[skoughi]

Regarding a write up Phil please do. Always wanting to learn some new stuff from guys who actually know what's what. 

[panky]

That would be brilliant, I've got a head to play with for my Minor