Morris Minor Owners Club

Does anyone know what size valves a 12g295 head should have? I bought a 1098 head, or what i believed to be and it turned out to be a 12g295. However the exhaust valves look too small in their valve seats. Also if I was going to change them what sizes could I use? I know these heads had a larger combustion chamber, does anyone know how to measure the head to see if its been skimmed? I know the exact volume of the combustion chamber can be measured but I'm along way from home and my tools at college.

Any help greatly apreciated.

According to Vizzard...

Valve diameter for 12G 295
Inlet 1 7/32"
Zorst 1"

28.3cc combustion chamber volume

Thank you very much, You wouldn't happen to know the size of 1098 valves (12g202 i believe)?

Inlet 1.156" 29.37MM
Ex 1.000" 25.4 MM
According to Morris Workshop Manual.

Viz says 12G 202
In 1 5/32"
Ex 1"

Chamber vol 26.1cc

Pretty sure head thickness is 2.75'' this is from memory, see if someone agrees before acting!

The exhaust valves are the same size for all the 948/1098 heads incluing the famous 12g295. If the exhaust valves look 'too small' then it's likely the seats have recessed. You could fit the larger exhaust valves (1.156") from the 12G940 head and recut seats and open up the throats and open the chamber wall around the valve to the gasket line - note these valves are slightly longer and will need the matching longer springs and collets and caps from the 940 head. Or - you could have 'unleaded' seats fitted to the head - with a new set of standard later spec exhaust valves. However both moves are not going to be cheap - and the head - if not already skimmed -will NEED to be skimmed ~ 60 thou to give a decent compression ratio. This is the reason that the value of these heads has fallen recently - and most now fit the 940 head. It's a very much better head, has bigger inlet and exhaust valves, and doesn't need skimming. However - it does need the exhaust valves to be sunk into the head by 40 thou (1mm). This is very easy for any machine shop to do - takes about 10 minutes max! The 1275 head gasket must be used with this head - and to get the best from it a larger carb and good alloy inlet manifold are needed.
All heads start life at 2.750" thick - so if yours is any less then it has been skimmed.

Well Spag, if you divide 5 by 32 you will come to 0.156.
Which is what I said. So we are both right. Hurray.

I still reckon for a road engine a 12G940 is simply too "big" compared to a 12G206/295. The ports and valves are for a bigger cylinder capacity, so any real improvement must surely be up the rev range - Somewhere you probably don't want to spend too much time in, especially on the small bearing Minor/ 10CG MG crank.

Cost wise I've been quoted between £200 and £250 for an unleaded conversion and skim/ general cleanup (but no porting or other machining), which I do find just a touch steep. But then a good unleaded 12G940 isn't going to be peanuts, and given my doubts over overall suitability I'd stick with the 295. I have a spare 1275 Mini engine with unleaded head I could have already stuck that on, but instead I'm waiting on being able to afford to get my spare 295 overhauled and converted (the exhaust valves on that appear to have recessed a fair bit) instead due to the above.

I suppose I could go with my spare 998 Mini unleaded head, I'm not so sure the increase in CR is worth the drop in flow...

Even if the exhaust seats are worn, if you paid 12G202 money (5 or 10 quid autojumble prices) and it's not cracked between valves, it's a bargain

Indeed - at that sort of price it would be a bargain. The head of course could just be used with the valves ground in - the recessing will have a very marginal effect. The Cooper S engines used a head very similar to the later 940 - and they work well in 970 and 1071 versions ! My 1098 has a 940 head on it - and it pulls like train at all revs - much better than the well worked (and much skimmed) 295 I removed! 940 heads with recessed exhaust valves can be picked up for £20 - exactly what you want to drop them by 40 thou and use on a 1098. It's the best value engine mod by far.

I understand how the torque can suffer when using larger valves on any engine but I wouldn't be suprised if the 1098 did well with a 940 because having experienced the 295 on a 948 I can tell you that is a revalation! It has absolutely no shortcomings whatsoever.

I see what you mean BMCEcosse about the 12g940. However I already have the 12g295, it looks like the valve guides are new, also I'm trying to work out if its been converted to unleaded. The valve seats don't look factory. I don't actually have a moggy at the moment. But that doesn't mean I can't accumalate parts for the next one....

No harm getting the parts - and don't worry about 'unleaded' - not important unless doing large mileage. Pick up a cheapy 940 head - when you see the difference you will soon have the 295 up for sale!

bmcecosse wrote:when you see the difference you will soon have the 295 up for sale!

And if it's really been rebuilt and converted to unleaded, you may well find a buyer if the price is right too!

I still wonder if the 940 gives the noticeable results purely through a higher CR though, because I still have major reservations over the port sizes for a small bore road engine.

Well, the 940 def works well in a 998 environment, have had a couple of friends that built quite rapid Minis with them.

With a longish cam, they make quite a revvy engine, though not sure that's ideal for the longer stroke of the 1098.

Main concern is with valve/piston interferance (plastacine time in the dry build) and possible valve shrouding with the cylinder wall.

Neither build had any pocketing of the block or pistons, so was a cheap way to get a few more horses without too much effort.

Port size could be compensated a little by not going for the largest bore inlet manifold you can find so there should still be a reasonable gas speed at lower rpm on approach to the ports. I guess the MG metro alloy is a reasonable option - and probably comes free on a HIF44 anyway.

also I'm trying to work out if its been converted to unleaded. The valve seats don't look factory.

If its been converted it will have an insert in the seat but often the valve will need removing and the seat may need close inspection to see the insert but often once the valve been removed it can be seen.

I had a full recon and unleaded conversion done on a 12G295 head over the summer which cost me £220. The finished result is probably worth quite a bit, so I thought it was worth it. If you've accidentally managed to get an unleaded 12G295 for 12G202 money then you've done very well indeed!

If you've accidentally managed to get an unleaded 12G295 for 12G202 money then you've done very well indeed

I had a look at a Minor in a scrapyard to see what could be salvaged and by chance had a look at the head. I had to rub my eyes a couple of times when I saw the golden number 295 because in every other respect the car looked standard. Once off, the head looked very healthy and £20 later I was home grinning like a loon. The funny thing is that the day before another board member had looked at the car and missed the small print.
Strange bloke too; I've offered it him for £60 but he just snubbed me!