Cylinder head spec comparison - facts needed!
Discussion
Not sure even BLMC/Austin-Rover/Rover know for sure!
I used to think that the 1990+ Mini Coopers had 33.5 mm inlets, but the Haynes Manual quotes 35.5 for the Mini and the Metro GT. Inlets are 30mm for both, but I may be wrong, or Haynes may be wrong.
Anyway, for an unleaded head the best sizes are 35.5 mm Inlet and 30 mm exhaust. Get the valve seats cut, then blend the throats in before finally grinding in the valve seats.
I've seen all sorts of valve sizes in a 12G940. It's whatever fits best. Make sure you check the combustion chamber size is correct for the CR you want before finally assembling the head and fitting it or you could be very disappointed. I once stripped down a Cooper 'S', which was claimed to be down on power, and found the C.R to be about 8.6:1. No wonder it was gutless.
I used to think that the 1990+ Mini Coopers had 33.5 mm inlets, but the Haynes Manual quotes 35.5 for the Mini and the Metro GT. Inlets are 30mm for both, but I may be wrong, or Haynes may be wrong.
Anyway, for an unleaded head the best sizes are 35.5 mm Inlet and 30 mm exhaust. Get the valve seats cut, then blend the throats in before finally grinding in the valve seats.
I've seen all sorts of valve sizes in a 12G940. It's whatever fits best. Make sure you check the combustion chamber size is correct for the CR you want before finally assembling the head and fitting it or you could be very disappointed. I once stripped down a Cooper 'S', which was claimed to be down on power, and found the C.R to be about 8.6:1. No wonder it was gutless.
Cooperman said:
Not sure even BLMC/Austin-Rover/Rover know for sure!
I used to think that the 1990+ Mini Coopers had 33.5 mm inlets, but the Haynes Manual quotes 35.5 for the Mini and the Metro GT. Inlets are 30mm for both, but I may be wrong, or Haynes may be wrong.
Anyway, for an unleaded head the best sizes are 35.5 mm Inlet and 30 mm exhaust. Get the valve seats cut, then blend the throats in before finally grinding in the valve seats.
I've seen all sorts of valve sizes in a 12G940. It's whatever fits best. Make sure you check the combustion chamber size is correct for the CR you want before finally assembling the head and fitting it or you could be very disappointed. I once stripped down a Cooper 'S', which was claimed to be down on power, and found the C.R to be about 8.6:1. No wonder it was gutless.
so for my 1275 with a 286 kent cam and the usual RC40 + stage one kit, it's 35.5 & 30mm that I should go for? I'm afraid I have no idea at all what my CR is or should be, I need another cylinder head because I'm getting annoyed at all the smoke on startup due to (I think) buggered valve seals dropping oil on to the pistons which then blow it all out the back
and I know that my current head is an MG metro one (higher than standard compression?) so I need to measure the valve sizes on the one I've found to see if it's the same 
on a side note, how the smeg does anyone manage to tighten up the inlet manifold! I must have tried 25 different spanners and none of them can get a purchase on the studs to tighten them! with the result that it constantly blows. even the mini specialists I've taken it to never manage to keep it tight for more than a few days then it starts blowing again
minimax said:
on a side note, how the smeg does anyone manage to tighten up the inlet manifold! I must have tried 25 different spanners and none of them can get a purchase on the studs to tighten them! with the result that it constantly blows. even the mini specialists I've taken it to never manage to keep it tight for more than a few days then it starts blowing again
How do you mean? The clearance to get a grip on the nuts is certainly minimal...
miniman said:
Also, have you filed down the inlet manifold "feet" (for want of a better description!)?? The alloy ones are often a different thickness to the exhuast manifold and as such they never seal properly unless you file them down to the correct size.
you know, that might well be it...! I have noticed they're of differing thicknesses
I shall investigate. on the point of tightening it, I just cannot see how it can be done successfully! I simply can't get the spanner in between the inlet manifold pipes to tighten it all up. i'm going to have to ask the specialist chap tomorrow
minimax said:I use a socket with a short extension bar on it.
on the point of tightening it, I just cannot see how it can be done successfully! I simply can't get the spanner in between the inlet manifold pipes to tighten it all up. i'm going to have to ask the specialist chap tomorrow
minimax said:
also, just what are the differences with the two heads? and why is the MG one better than the standard one?
I'm almost certain that pre '89 MG Metros had larger inlet valves (35.5mm rings a bell, might be wrong though). After '89 the lead free heads were fitted and I think all 1275's shared the same (smaller) valve sizes.
Rumour has it that the Maestro and Montego 1275 lumps used the MG Metro sized valves to get a bit more power to lug themselves around with. Certainly the quote power output was higher than the bog standard 1275 Metro.
Mr2Mike said:
also, just what are the differences with the two heads? and why is the MG one better than the standard one?
I'm almost certain that pre '89 MG Metros had larger inlet valves (35.5mm rings a bell, might be wrong though). After '89 the lead free heads were fitted and I think all 1275's shared the same (smaller) valve sizes.
Rumour has it that the Maestro and Montego 1275 lumps used the MG Metro sized valves to get a bit more power to lug themselves around with. Certainly the quote power output was higher than the bog standard 1275 Metro.
Next time I'm down the scrapper, I will have a few off and find out. I think they are bog standard metro ones. I heard the unleaded (12HF01, etc) engines are sharing the smaller valves. I will go and find out.
Unleaded Metro heads, even the GTA ones have smaller inlet valves than the Leaded MG heads. The MG has 35 x 29.7 and the standard one has 33.5 x 29.7 IIRC.
Get an MG Metro head converted to unleaded.
Also anybody know what sort of difference 37 x 30 will make in comparison to 35.5 x 31.5
Get an MG Metro head converted to unleaded.
Also anybody know what sort of difference 37 x 30 will make in comparison to 35.5 x 31.5
WildfireS3 said:
Unleaded Metro heads, even the GTA ones have smaller inlet valves than the Leaded MG heads. The MG has 35 x 29.7 and the standard one has 33.5 x 29.7 IIRC.
Get an MG Metro head converted to unleaded.
Also anybody know what sort of difference 37 x 30 will make in comparison to 35.5 x 31.5
excellent chris, thanks
I was going to ring you and ask you this question but I thought i'd give you a breather seeing as I ring you about everything to do with my mini!
For the really big inlets you have to run leaded fuel.
The 35.5 mm inlets with 30 mm exhausts are fine with the 286 cam and, ideally, 1.5:1 rockers.
Tip of the day: Do away with the manifold studs and use 5/16" UNF bolts. It's then easier to do them up, easier to remove the head and manifolds and very easy to change the head gasket. All the rally cars use bolts (well, all mine do, anyway!).
If you need a 12G940 head, Minimax, I've got a whole load and you are welcome to one for, say, £25. You just get the valve seats machined for the bigger valves as necessary.
With regard to chamber volume as standard, it's not really important so long as you do the C.R. calcs and then measure and machine the head to suit.
The 35.5 mm inlets with 30 mm exhausts are fine with the 286 cam and, ideally, 1.5:1 rockers.
Tip of the day: Do away with the manifold studs and use 5/16" UNF bolts. It's then easier to do them up, easier to remove the head and manifolds and very easy to change the head gasket. All the rally cars use bolts (well, all mine do, anyway!).
If you need a 12G940 head, Minimax, I've got a whole load and you are welcome to one for, say, £25. You just get the valve seats machined for the bigger valves as necessary.
With regard to chamber volume as standard, it's not really important so long as you do the C.R. calcs and then measure and machine the head to suit.
Kinda vaguely on topic...
I have a 1275 Metro engine, and an old Speedwell alloy A-series head tucked away in the garage...
1) How can identify whether 'MG' spec head on the Metro engine?
And
2) The Speedwell head is currently bare of valves/rocker/etc - would it be possible (for knowledgable gent here) to estimate the valve sizes from measurements of the remaining seats?
3) Just how rare/valuable are alloy heads?
Thanks, Phil
I have a 1275 Metro engine, and an old Speedwell alloy A-series head tucked away in the garage...
1) How can identify whether 'MG' spec head on the Metro engine?
And
2) The Speedwell head is currently bare of valves/rocker/etc - would it be possible (for knowledgable gent here) to estimate the valve sizes from measurements of the remaining seats?
3) Just how rare/valuable are alloy heads?
Thanks, Phil
Cooperman said:
If anyone is interested I may have a 12G295 (Cooper 998) head for sale shortly. You can email me on this.
I finally caved in on Monday and took my half-built engine up to Somerford to have the 266 cam timed in properly and the 12G295 head, oil pump, clutch and all the other stuff I haven't got time to do taken care of by someone who knows what they're doing. Looking forward to getting it back ASAP!
That's a nice configuration, a 266 with a 295 head. It makes the 98 unit so much nicer to drive, especially if you go up a bit on valve sizes and just open out the throats in the head a bit, plus skimming a bit off the head face to getr a bit more C.R. A lot of gain for a little outlay.
The gear ratios are what lets the later Minis down then, the old 998 Cooper ratios were much better.
The gear ratios are what lets the later Minis down then, the old 998 Cooper ratios were much better.
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