Morris Minor Owners Club

Phil,

you've confused me here...Its a 12G940 I'm trying to fit, which is 21.4CC as standard, obviously less now they have skimmed it by 40 thou!

I am awaiting the CC of the head from the machine shop, I will post it as soon as they have called me. But it sounds like it is going to be too small.

Thanks,

Edd

Well a standard combustion chamber is 21.4cc on the 940 head. But with a 40 thou skim it is NO USE on a 1098. You could start grinding away at the combustion chamber (a LOT of work) - but that head as it comes flows more than enough air for a 1098 to develop all the power it can safely handle. The valves are so much bigger than the 202 head, and the flow passages so much better - there really is nothing worthwhile to be gained from modifying the head - remembering the strict 6000 rpm rev limit! I do however suggest doing something to ease the flow of the exhaust gases as they go round the corner in the exhaust throat. Stick a finger in there and feel how tight/small that corner is! It's well worth spending some time in there with a couple of mounted points - the corner can be easily gas flowed and made MUCH larger to let the gases escape more easily.

Phil - a standard 940 head on a 1098 puts the CR up around 10/10.5 depending on how far the valves have recessed. This is more than enough for a road going 1098 - so NO skimming should be done on a 940 head intended for a 1098 engine. It's part of the beauty of this simple conversion! And as I mention above -some find the exhaust valve clearance is enough without any sinking - so just a 'bolt on' job.

Edd
Sorry if I am not clear.
What I am saying is that to retain a similar compression ratio to the 1098cc H/C engine - add the volume of the current combustion chamber of the 12G940 head and the volume of the pocketing (the head gasket thickness volume can be assumed to be 3cc) and then skim the head to attain that 26.1cc.
Phil

Phil,

The 12G940 CC is smaller than the 12G202 CC though? Or am I a total idiot.....

12G202 26.1 CC
12G240 21.4 CC

So I need to make the combustion chambers on my 940 head BIGGER by almost 5 cc, subtracting the pockets. Have I understood the principle?

Edd

Edd
I have taken the word MINUS out of previous post and reworded the sentence as that is where I believe the mix up is occurring.
Your aim is to achieve a total combustion chamber volume - that is the head chamber volume, the pocketing volume, the head gasket volume of 26.1cc.
You may not be able to achieve that 26.1cc exactly but again that is what you should aim for.

Phil

Hi Phil,

You have made it perfectly clear, thank you. It appears to me that to achieve 26.1cc from the 940 is going to require a lot of material removed from the head, as I do not imagine the pockets are going to have much of a volume? From what I understood, you were suggesting to SKIM the head to achieve 26.1, but surely this would only serve to lower the cc, as what I need to do is increase the volume of the cc in the head?

Thanks for your patience in explaining things to me,

Edd

Edd
The intention would only be to skim the head if the total combustion chamber fell above 26.1 cc.
Phil

Phil,

OK that makes sense, I understand the principle. However the likelihood of the CC of the pockets plus the 21.4CC of the head (or whatever the CC of the head is after its skim) being more than 26.1CC must be miniscule?

That would require the pockets in the block to be around 4.7cc, which is the same as 4.7 millilitres, so to use non-engineering terms, I need to remove a teaspoon of material from my head's combustion chambers in order to achieve 26.1cc and maintain my compression ratio? This sounds a lot! A teaspoon-sized pocket is a lot bigger than I was expecting, they said to me it would be more like the edge of a thumbnail.

I am still confused as to why you originally suggested that there might be plenty of scope for head skimming.

(a teaspoon is 5ml, which I can picture a lot more easily)

Thanks,

Edd

EDIT - I've pulled out my Vizard, and have come up with the following calculations, please let me know if I am mistaken!

1098 overbored +040 - 1132cc (thanks Phil!)

1132 / 4 - 283cc

Lets say I want a compression ratio of 10.5/1, which I believe is as high as I want to go on a road car? I am fine to run the higher octane fuel if that will help, as I do not do many miles!

So 10.5 - 1 = 9.5

283 / 9.5 = 29.8

29.8 - 7.5 (the figure Vizard gives to allow for piston dish, head gasket and space above the piston) = 22.3cc. This is the CC volume I require in my 12G940 head for a compression ratio of 10.5/1.

So to my mind, 22.3cc is a lot more achieveable when the figure does not allow for the pockets cut into the block?

I await correction...

Many thanks everyone!

Edd

Edd
If I remember correctly the plenty of scope for skimming relates to your rusty head / possibly located in Manchester? which has a rusty gasket face? My comment is that even when rusty and or skimmed it can still be savable even if previously skimmed.
Possibly too many heads to keep track of I am afraid.
What has to be taken out of the block is determined by the cam lift and rocker geometry - it may be a bit more than the edge of a thumbnail - I am sure that the machine shop will do the job just right.
The top of the piston is always well below the top face of the block and the top piston ring well below that.
The pocketed block has been around for many years and I have never heard of any problems with its use other than where the engine builder has gone absolutely silly and sunk the pocket more than necessary.
I personally would pocket the block every time over sinking the valve.
Phil

For decent power you want to end up with smaller than 26.1 cc combustion chambers. A 940 with sunk valves runs very nicely without pinking and should come up with ~ 175/185 psi compression when tested - assuming good bores and pistons etc. This is partly what gives the performance boost over the 202 head - and mostly of course from the larger valves etc. So ideally -aim for something rather less than the 26.1 but slightly bigger than the 21.4 cc Certainly NO LESS than the 21.4 cc . The other problem with a skimmed head of course is that now the sinking or pocketing has to be even deeper - by that extra 40 thou in the case of the skimmed head. Save that head for a 1275...

Thanks all!

I now understand a little bit more about cylinder heads...

I am still waiting to hear back from machine shop with CC for the head, but its going to be too small to be practical. I'll head up to Manchester this week, collect my unskimmed head and give them that to recondition with as small a skim as possible, none if one is not required. If that is not possible, I'll sack the whole plan off and rebuild it back to standard!

Can the Cometic spacer head-gaskets be used to lower compression on these engines? They are expensive, but if designed for charger application I am assuming they would be OK on a standard engine.

Cheers,

Edd

NO! Sounds a terrible idea to me... Just don't skim the head.

Hello chaps,

The head has just come back CC'd as 21.5!

This doesn't make much sense to me. The head has been fully reconditioned with unleaded seats, but they definitely said it had 40 thou skimmed off the face. Is there anything that could have occurred to increase the combustion chamber size in the reconditioning process? I can't call them till tomorrow now.

Thanks,

Edd

First step - measure the head thickness - should be 2.750" if it has NOT been skimmed - any less = the amount they have skimmed off. Maybe they meant 4 thou - just to clean it? Maybe the new seats and valves are sticking up slightly more in the combustion chamber - so reducing the capacity. Now you need to measure the distance from the exhaust valve head surface - to the head surface. You are looking for 300 thou minimum. I guess if they have fitted 'unleaded' seats you will then be stuck with making the horrible pockets in the block....

Well I assumed that the unleaded seats might have reduced capacity, but the head is 0.1 CC larger than standard?? From what I can remember it was an unmodified head from a 1275 Midget.

I'll get the other measurements from the shop now. Hopefully if the gap between exhaust and face is OK, the pockets won't need to be too deep and I can still use this head. It will save me a couple of hundred to do so.

Edd

I wouldn't be worrying too much about 0.1cc - the measurement process is not that accurate. Just stick a steel rule on the side of the head and tell us the thickness!

The head is still at the machine shop so I am relying on them for measurements, its an hours drive away so a bit reluctant so go there just to measure, although the time they take to get back to me...too busy building Jag V12s for rich people.

As soon as I hear I will post the relevant details. I really want to crack on as all the rest of the parts are sat clean and shiny in the garage waiting to fit!

Thanks again,

Edd

In the meantime, any volunteers to chip on for one of these with me?

https://www.youtube.com/watch?v=mEsuTX6 ... BGnxjN2PLg

I'm in for a fiver