9 replies to this topic

[Grilled]

As I'm researching for my build I was wondering why is it that the 12G295 head is so popular? Could the same results just be achieved with work on a standard 1275 head?

[Rob Gavin]

what engine are you fitting it to?  the 295 is a small bore head (albeit a good one).  If its going on a 998, the block needs to be modified to take a 1275 head

Edited by Rob Gavin, 15 November 2013 - 02:40 PM.

[miniman24]

Are you talking about achieving the same results by bolting a 1275 head onto a 998? If so, generally the 998 block needs pocketing or the valves sinking into the 1275 head. I think you can bolt the 1275 head straight on if the rest of the engine is standard, but the valves could touch the pistons if you hoon it! The 12G295 was deigned for the 998, albeit with different pistons, but with the correct skim the same compression ratio can be acheived. The combustion chambers are similar design between the 1275 head and 12g295, so I imagine the results would be similar with the same compression ratio.

Edited by miniman24, 15 November 2013 - 02:41 PM.

[Grilled]

If I were to be fitting one it would be to a 1275-1330 block, it may be I have the head number mixed up and I mean a different one though (think I've an MG one in mind?)

I originally intended to have the original head worked to a stage 3 or 4 but I am wondering if there was an alternative head that would see better results so really I am trying to find out how to get best results the first time around so it need not be done twice.

[Rob Gavin]

the 12g295 isn't suitable for a 1275.  If your looking for a good factory head, the normally aspirated MG Metro head would be the one to go for - its a 12g940 however most of the later heads are the same cast number; its the valve sizes that your more interested in

[Grilled]

Ah thank you that is what I meant! If it is the valves that I'm interested I'll check out the casting on my current head then can figure what I want to do from there. Thanks for straightening that our for me.

[Cooperman]

As above. The 21G295 is the head which was fitted to the 998 Cooper from about 1964 until final production in 1969. It is known as the 'small-bore open chamber head' and it will only fit onto a 998 or the earlier 997 Cooper blocks or an 850, all of which are known as 'small-bore' blocks.

 

All 1275 engines have a 12G940 casting, although the early Cooper 'S' had different castings which were prone to cracking. The 12G940 is a generic head and the castings do differ from model to model and are finished differently with various valve sizes.

 

The MG Metro head has 35.6 mm inlet valves and 29.5 mm exhaust so it is ideal. It can be gas-flowed to give excellent results, but don't pay too much attention to all the 'Stage this, that or the other' terminology as they are really just sales terms.

 

The head is the key to improved performance with the A-Series and it pays to get a well done one and to make sure the compression ratio is around 10:1 or slightly higher.

[Ethel]

A 295 has smaller inlet tracts that'd make it more drivable on a small bore, where you'd need more revs to get the gas velocity up.

 

If we're only talking 940's for large bores, only the pre unleaded MG Metty's had the bigger valves, they all use the same 12g940 casting so can be worked for the same valves. If anything the later castings, derived from the Turbo MG, with the  raised profile under the rocker & stat gaskets are superior.

[Grilled]

Thank you cooperman for your clearing that up as well. So as it would seem for best performance results the MG head gas flowed is the one, unless there is anything else that the engineer can do whilst he has it?

Everyone here is really helpful thanks, I appreciate all your knowledge and input!

[Turbo Phil]

It doesn't matter if you start with the bigger valved MG Metro 940 head or the smaller standard 1275 head as far as whats obtainable from the head when modified. It just means some of the work is already done for you on the inlets if you use the larger valved MG head as a basis.
All 940's will give the same results when worked to the same spec.

Phil.