12g940 head clearances !

[Miggins]

Hi all,

Sorry to resurrect this perennial question, but I've trawled though all the discussions and can't find the answer anywhere !

I'm wanting to breathe a little bit of extra life into my wife's 1098 Moggy convertible as it's struggling to keep up with the traffic at the moment and so it's not getting as much use as we'd like. I've got a very nice condition 12g940 head on my work bench and the exhaust valves luckily have the necessary 0.320" clearance between the valve face and block face - BUT it's fitted with the bigger 35mm inlet valves.

Looking through other threads those in the know say the small valve version is better, but is this just down to the performance characteristics of the smaller valve head or is it because the bigger inlets will foul the block ? The inlet valves on this particular head are not as recessed as the exhaust valves and so would have less clearance.

I don't really want to go down the route of pocketing the block as I'm not in a hurry to take the engine out and I don't like the idea of the top rings breaching the pocket.

Any help gratefully received !

[svenedin]

I don't know the answer to your question but I also have a 1098cc convertible. I don't find it struggles to keep up with traffic at all except on faster roads. Around town it is absolutely fine and I am often held up by modern cars. Are you sure your engine is in good order in other respects? Is the compression good? If the mixture and timing correct?

Stephen

[Miggins]

Hi Svenedin,
Thanks for the reply - I know the oil pressure is good. It pinks on hills despite retarding the ignition quite a lot. I've not checked the compression yet, but do have the gear to do it. On my list !! It's just that I've got this decent head, plus a nice HS4 carb, alloy inlet and also a Mini Cooper exhaust manifold, which I could modify - so if the head would work, I was going to go straight down that route.

[svenedin]

If it's pinking on hills there's something wrong. Obviously it suggests the ignition is too advanced. If you have a strobe it would be informative to find out what the timing is running at. Are you sure you retarded the ignition and didn't advance it more? The distributor rotor rotates anticlockwise so to retard the ignition the distributor must be rotated anticlockwise. If you have no strobe, retard until it stops pinking on hills. IF it has been pinking badly for a long time there is the possibility that the pistons have been damaged so a compression test would be very informative. As for the mixture, a check of the spark plug colour would be helpful and they might have been damaged by the pre-ignition. Pre-igntion is more likely if the fuel mixture is too weak.

Stephen

[Miggins]

Hi Svendin,
I've checked it with the strobe and tbh there's so much play in the timing gear that the marks on the pulley are way way out, so I tend to do it the old 'suck it and see' way. Yes, I definitely moved the dizzy the right way, I've been doing this stuff for years so have an inkling of what I'm up to. I've a CO analyser which I've had up the exhaust pipe and the readings look OK, it's certainly not running lean and the plugs are a healthy colour. I doesn't use any oil or blow smoke either ! Does drip a bit though !! Always starts first tug as well, second when not been run for a while.

When I do the head swap I was going to add new timing sprockets & chain at the same time.

I've put it off as I know once I take it apart, it will be a while going back together as I'm bound to find other bits that need sorting

[geoberni]

Hi Svenedin,
Thanks for the reply - I know the oil pressure is good. It pinks on hills despite retarding the ignition quite a lot. I've not checked the compression yet, but do have the gear to do it. On my list !! It's just that I've got this decent head, plus a nice HS4 carb, alloy inlet and also a Mini Cooper exhaust manifold, which I could modify - so if the head would work, I was going to go straight down that route.

Well Engines are certainly not 'my thing', but I do know enough to recognise that it the engine is Pinking, the Ignition isn't correct.
Wondering about Head alterations, when the Ign isn't correct, is a complete waste of time.
How about replacing the timing chain that you say has 'so much play in the timing gear that the marks on the pulley are way way out,'...

Seems to me that what you're really doing is looking for a reason, aka excuse, to use the 'decent head, plus a nice HS4 carb, alloy inlet and also a Mini Cooper exhaust manifold,' not sort out a problem with the car "struggling to keep up with the traffic" ....

[svenedin]

Hi Svenedin,
Thanks for the reply - I know the oil pressure is good. It pinks on hills despite retarding the ignition quite a lot. I've not checked the compression yet, but do have the gear to do it. On my list !! It's just that I've got this decent head, plus a nice HS4 carb, alloy inlet and also a Mini Cooper exhaust manifold, which I could modify - so if the head would work, I was going to go straight down that route.

Well Engines are certainly not 'my thing', but I do know enough to recognise that it the engine is Pinking, the Ignition isn't correct.
Wondering about Head alterations, when the Ign isn't correct, is a complete waste of time.
How about replacing the timing chain that you say has 'so much play in the timing gear that the marks on the pulley are way way out,'...

Seems to me that what you're really doing is looking for a reason, aka excuse, to use the 'decent head, plus a nice HS4 carb, alloy inlet and also a Mini Cooper exhaust manifold,' not sort out a problem with the car "struggling to keep up with the traffic" ....

I would agree. I would sort out the worn timing chain and sprockets (you could consider a Duplex chain kit), get the timing right and then see what the performance is like before going down the route of modifications and losing originality.

Stephen

[Miggins]

I agree with you both, but as I've already got the parts, I might as well do the lot ! The timing chain replacement is easy and not expensive to do.

However, I'm still no wiser as why to use the small valve 12G940 rather than the larger valve version.

[philthehill]

There is little benefit in fitting a 940 big valve head (or a standard 940 head) if you do not fit the additional parts i.e. larger carb and manifolds.
Bigger valves are not the be and end all of a performance upgrade. The inlet tract has to have some 'RAM' effect in that its size helps to speed the mixture to and through and past the valves. Too big and the 'RAM' effect is negated and you loose power.
The same happens with the exhaust tract, too big an exhaust pipe and you loose power.
Everything to do with tuning the 'A' Series involves more than just one item - everything is related and needed to get additional power.
As regards the duplex timing chain - the simplex system is a waste of time as it soon looses its tensioning ability even from new.
You cannot time the ignition correctly if the timing chain is worn out as the slack will allow the timing to wander.
Replace the timing chain with preferably a duplex set up and get the ignition timing set up correctly and then go from there.

[Miggins]

Thanks for your replies, but we seem to be assuming I'm a mechanical numpty here !

I'm simply trying to find out if the big inlet valves will touch the block of a 1098cc engine. The timing is a separate issue that can be dealt with when the car's off the road.

[les]

The method used to determine the clearance for the exhaust valves, should be able to do the same for the inlets, yes/no ?
What am I missing here ?

[chrischris59]

Thought about it and I think there will be no problem. I overlayed a 1098 gasket on a 1275 one picture attached.

[Miggins]

Thanks Chrischris59, brilliant. I'm only asking to be forewarned - no point in disturbing the old head etc if the new one is a non starter from the outset.

I'm just trying to save a bit of time and heartache. I'll go ahead with the timing gear replacement, and do the head with the blutac on all valves just to make sure and check the compression and dizzi bob-weights are all free and stick all the other goodies on there.

The thing is with these old cars, I'm already in my 60's and remember them affectionately as my grandma drove one, but the generation behind aren't interested as they are seen as irrelevant and slow - just look how Brit bikes are plummeting in value as the old boys that rode them hang their boots up. If I can make ours as original as possible, but still be able to drive up the A49 without every Audi and Tesla driver getting steamed up, then so much better the chances of it being used regularly and for some long time to come.

[chrischris59]

I was in the same position as you. But having fitted a Hif 38 carb and manifold and a proper air filter solution in my mind with a larger bore exhaust system first to make easy space in the garage, found the car went so well the head has sat on the bench for months. But having fitted a 3.9 diff I'm now looking at the head again.

[Miggins]

Hi ChrisChris59,
Which head version are you planning to fit ?

[chrischris59]

A 12g940 like you but it has the smaller inlet valves.

[liammonty]

So - remembering form what I've read (and I haven't fitted a 940 head to a small bore engine myself as I have a 12G295 head) - there IS a risk that the valves will touch the block, hence people pocketing the block, but it's marginal and in some cases people have ground the valve seats (as it sounds like has been done with your exhaust valves) and got away with getting the extra clearance that way. I would offer it all up and use some plastigauge between the head and the block to see if anything touches when you turn it over and take it from there if you want to avoid pocketing the block . I don't think that the inlets are an issue re clearance but the plastigauge will confirm that.

You'll need to consider the compression ratio too as I'm pretty sure the combustion chambers on the 940 are smaller than the standard 202.

Phil mentions the valve sizes but of course the 940 head has advantages beyond that as the combustion chamber has much less shrouding around the valves so has the potential to flow a lot better with a decent carb and manifolds

David Vizard's book, 'Tuning the A-Series' has a bit of info on it.

[Miggins]

Hi Liammonty,

Thanks for the reply, I suspect you're right that the exhaust valve seats have already been lowered, as they are about 40thou lower in the head than the inlets.

I've also taken 10thou off the valve faces to give an extra clearance. I think the exhaust side will be OK, but will take your advice and check both sides with Blutac. Given the comparison of gasket diameters in Chrischris59's photo I suspect it will be OK.