12g940 Calculations prior to fitting

Post by **Declan_Burns** » Wed Mar 18, 2009 11:34 am

I am still gathering bits and pieces but I have been doing some calculations prior to modifying/fitting the 12g940 head on my 1098cc engine and would like to know if I am getting it right. Sorry about the mm but I only have metric gauges.
I read on here somewhere that the cam lift is ¼” (6.35mm). I cannot find it in the manual.
The 12g940 is as I was told, fitted with standard rockers, which I believe have a ratio of 1.27:1. This would give a movement at the cam lobe of 8.0645mm.
Assuming a standard valve clearance of 12thou (0.3048mm) just for the sake of the calculation.
This would result in the valve traveling 8.0645-0.3048 = 7.7597mm from closed to open.
Assuming a compressed cylinder head gasket thickness of 0.8mm (I measured the new one - 1mm). I read in Des Hammills mini tuning book that there should be a clearance of 0.5mm to allow for valve bounce. This would mean the open valve can theoretically protrude 0.3mm below the base of the cylinder head so that when the head gasket is fitted, there will still be 0.5mm clearance to the block. This would imply that the valve when closed should sit at least 7.7597-0.3 = 7.297mm below the base of the head.

I measured the current distance from the valve tip to the base of the cylinder head (several times) with a dial gauge calipers and the results are

Valve 1: 7.5mm
Valve 4: 8.3mm
Valve 5: 7.9mm
Valve 8: 8.0mm
It looks good on paper but perhaps I should sink no. 1 valve by 0.5 mm.
I have not yet removed the valve springs to check the condition of the valve seats.
I also intend to measure the actual valve lift after setting the tappets on the 12g202 head. I will also check the actual12g940 clearance using Blu-Tack and cranking the engine prior to any test run.
Q Is the cam lift really ¼”?
Q Is this the right way to calculate this or have I missed out somewhere?
Q. Is the 0.5mm (20 thou) clearance to allow for valve bounce really enough?
I would appreciate any advice and/or tips before I proceed.
Declan
(DEC 1964H/formerly HMP 960B)

Post by **MarkyB** » Wed Mar 18, 2009 6:20 pm

0.5mm (20 thou) doesn't sound like enough to me on a road car.
What happens if you get a little carbon build up on things?
OK for a race engine that gets stripped down and cleaned regularly.
I'd put it all together with blue tack in the danger areas and turn the engine over by hand and see what the situation actually is, rather than rely on what a book says it should be.
YMMV I don't build tuned engines and this advice is worth what you paid for it

linearaudio · Post by **linearaudio** » Wed Mar 18, 2009 8:24 pm

Your calculations look good. My 1098 has actual valve lift of near as dammit 7.8mm on all valves. My 940 head has valve head to mating surface of the same amount, so I am using the headgasket thickness (reckoned to be about 0.85 mm compressed) as my clearance. 0.5mm does sound a little tight to me, though I doubt you need to worry over much with valve bounce unless you are really trying!!

Post by **PSL184** » Wed Mar 18, 2009 9:27 pm

Fit double valve springs and it's neigh on impossible to get the critters to bounce

Post by **bmcecosse** » Wed Mar 18, 2009 11:04 pm

No matter how much clearance I made on my 998 engine fitted with a Cooper S head - every time I lifted the head there was evidence that the valves had been touching. But it was a 'race' engine that got hammered to 9000 rpm !
Your calcs look good - although I agree that #1 looks a bit 'iffy'. Don't fit double springs unless you really want to use high revs - there really is no need to go bouncing the valves on a road car, and double springs just absorb more power and cause extra wear on the valve gear. Exhaust valves are better set at 15 thou clearance anyway. Remember you need a 1275 head gasket - and the rockers need to be re-aligned on the shaft so the rocker tips act vertically on the valve stems !
Edit - just checked - the cam lift on the AEA 630 cam (1098 engine) is indeed 0.250" If you have the 948 engine - it's rather less (probably 0.220" - but possibly only .200") , so even less to worry about there!

Post by **Declan_Burns** » Thu Mar 19, 2009 7:03 am

Thanks,
The head is fitted with double springs, can I simply remove the inner spring on each valve?-see photo.
I am not looking for a racing spec and am doing this for two reasons.
1) Increasing the power will hopefully help me overtaking tractors and lorries on the open roads. It's very difficult with a RHD, driving on the right, being stuck behind a lorry, and not always having a passanger to see if I can pull out safely!
2) The fun of getting to know the car better.
I recall reading what BMC has confirmed about clearance to the block and that was why I questioned the 0.5mm (20thou) as mentioned in Des Hammill's book. I think I'll go for 0.8mm from what linearaudio has stated.
I have realigned the rockers already since the photo was taken and it looks a lot better-but not quite finished-still a bit of filing to do. I am also watching ebay for pressed steel ones as advised.
Q. Can I use the rockers from the old 202 head?-I haven't had the chance to measure it up yet.
I already have the 1275 gasket+spare and the inlet valve seals ready to go on.
Q. There was also an older thread on packing under the pillars or washers at the top of the valves to compensate spring tension when valves are recessed. Any updates?
Q. Must I anticipate pinking problems due to the increased compression or is that also a bordrline issue?
I'll post a picture of the finished head before it goes on. I won't get much done this weekend as I have visitors from home arriving, so it will be a beer tour rather than a Moggy tour-but I can certainly live with that!
Thank you all for the assistance.

Declan
(DEC 1964H/formerly HMP 960B)

Post by **bmcecosse** » Thu Mar 19, 2009 11:50 am

If it's just the standard MG double springs - just leave them in. Unless you actively 'sink' the exhausts by 40 thou - there will be no need for packing washer under the springs. Yes - the pressed steel rockers from 202 head are ideal - but will need re-aligning since the exhaust valves on 940 head are relocated to allow the larger diameter valves to fit in the combustion chamber. there have been some v cheap pressed steel rockers on ebay recently! You certainly don't want any 'packing' under the rocker pillars - unless very exceptional circumstances (ie huge amount skimmed from head and/or block).The comp ratio will be nicely raised - pinking will be down to what fuel and ignition timing you use. You may well find using top quality fuel will be worth the extra cost - in increased performance and mpg (or kpl). Ignition timing will need to be set to 'just not quite pinking'. I can't remember - but I assume you have a better/larger carb and manifold lined up to go with this head ?

Post by **Declan_Burns** » Thu Mar 19, 2009 6:13 pm

Thanks Roy,
I think the springs are standard so I will leave them in.
And yes, this I have nearly ready to go on.
Still wondering about that black cylindrical knob on the side though.
Any idea if this needs adjusting or where I can get a manual on the HIF 38 to read up on it?
Thanks
Declan
(DEC 1964H/formerly HMP 960B)

Post by **bmcecosse** » Thu Mar 19, 2009 8:33 pm

It's just a breather from the float chamber! Don't worry about it. These carbs are easy to work with - the mixture adjust screw is just below that black knob - and slightly towards the inlet face of the carb, the idle screw adjustment will be obvious where the accelerator cable connects.

Post by **little_pete** » Wed May 01, 2013 8:38 pm

Sorry to bring up such an old thread,

Did you complete your conversion? how was it? any issues I should be aware of?

you seem to be going for exactly what I'm going for, I have a hif38 waiting to be cleaned rebuilt and my 12g940 just arrived, I paid about £55 for it, some may say that it is a lot but it has new valve guides in and oil seals and looks ready to go to me. my digital calipers are out of battery, but checking with the calipers and a steel rule it looks like all the exhaust valves are about 8mm recess. (they are a the smaller ones right?) The inlet valves are much less 7.2-7.6 will this be ok? (will check again with batteries) There is some pitting and scoring on the valves , I've tried to look for cracks between valves but can't see any. There's a little pitting on the surface of the head and a deepish scratch, is this going to cause problems? Taken a picture of the worst bits.

I read on calver's site that: 'It is prudent to plug the front water gallery transfer port holes and re-drill them using a 1275 head gasket as a template. This is pretty easy even with the engine in the car by tapping the holes then fitting suitably sized grub screws which are then drilled to allow water through in line with the 1275 gasket. Crude but effective and means the block face doesn't need re-surfacing afterwards. If this realignment isn't done, the head gasket is likely to leak water out and down the front of the engine.'

Has anyone done this/not done this, and has there been any issues? I plan to use the car some miles so want it to be reliable. But also on a budget ;-)

I've got a payen gasket for 1275, I've heard it's more reliable? is this suitable? I Also have the composite gasket from a standard gasket kit (again 1275)[frame]

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Post by **bmcecosse** » Wed May 01, 2013 9:35 pm

Take everything Calver tells you with a HUGE pinch of salt....... I have never heard of plugging/drilling - and I don't have any water leaks... 8mm is fine - the ex seats are slightly 'recessed' which is ideal for you. Do the best job you can grinding the valves in. When you ahve the exhaust valves out - stick a finger down the exhaust throat and feel how tight it is at the corner... Anything you can do there with a mounted point in a leccy drill will ease the escape of the hot gases. It doesn't have to be perfect (or 'polished') - but a few minutes work can easily make the passage there MUCH larger, which can only be good! You do know to file the top of the water pump to make sure the head can sit down with some clearance there. And to re-align the rockers......

Post by **little_pete** » Wed May 01, 2013 10:05 pm

The head was supposed to have been rebuilt the original owner had intended it for a 1000cc mini so I think he may have recessed the valves already

. It's all assembled with valve springs so I'd rather not take it apart again as I have no spring compressor or have ever done anything like this. I've read all the threads over and over, have never attempted a normal head swap so will be doing this very carefully. Is that scratch nothing to worry about then?

File down the top edge of the water pump to stop it fouling the front of the head. The heater valve maybe problematic. was considering plumbing something like this in: (more joints = more leaks)

http://www.ebay.co.uk/itm/261149822471? ... 1438.l2649

With a heater take off instead of a valve on the head.

My battery seems large (and bodywork pretty good, don't fancy attacking it with a lump hammer) I don't think I'll get away with the standard heater valve. I've got a new thermostat just while I'm there only a couple of £ might as well swap it and know it's good. (also will renew the pesky bypass hose) You just used the standard thermostat housing, was that off the 1098 or 1275? as apposed to the MGB one? will the standard rad hose fit?

First thing I want to get the standard set up running sweet enough so I know there are no other issues.

Post by **little_pete** » Wed May 01, 2013 10:21 pm

You mean around where the exhaust manifold should go? looking in it's pretty smooth I don't know what else could be done, can cleary see the exhaust valves from the manifold holes and, the corner is smooth not sharp. I'm finding it difficult to take a photograph. I'm keeping the standard exhaust with an alloy inlet and Hif38 I want it as quiet as possible, as quick as possible, as reliable as possible and as cheap as possible ;-) Oh I've got sintered rockers they look in reasonable condition, but haven't arrived yet.

Post by **bmcecosse** » Wed May 01, 2013 10:24 pm

Well -I would look at the valve seats.... I used an old bottom hose - the curved section - and it fits just fine as a top hose. You MUST realign the rockers - or have you got a set of the ghastly sintered rockers??

Post by **little_pete** » Wed May 01, 2013 10:39 pm

Ah have quite image search of porting I see what you mean. Will there be much real world improvement? Considering I'm using the standard exhaust and manifold and that I don't have the tools to do it (and the confidence). Yes, Ghastly sintered rockers easiest bolt on solution for the first timer?

How hard are the valves to press down? (this is how little experience I have) I hope they should be hard to press, because I can't budge mine, for some reason I was expecting to be able to press them down from above?

Post by **philthehill** » Thu May 02, 2013 6:53 am

Whilst it is good to calculate you cannot beat a piece of plasticine on top of the piston. Assemble the head with an old gasket, torque it down, turn the engine over, remove head and see what the clearance indent in the plasticine is. If the engine will not turn over do not force it, ask yourself why?

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