Morris Minor Owners Club

So I've got the 12g940 head, and the casting is very rough, so I wanted to clean it up a bit. In the past the worst bit for me to do has always been the "floor" of the chamber. Today my eye wandered to the Power Devil woodwork router, seldom used..... Somewhere sometime I had bought a set of cheap mounted stones, which happened to have 1/4" spindles.... worth a try! It may not be the "done thing" , but it doesn't half make quick work of the job!!<br><br>Obviously, cutter speed was set to limit for wheel and appropriate safety measures taken...

In fact - slightly rough finish is best for performance! And by taking material out you are reducing the compression ratio. If you do want to make a very worthwhile improvement to that head - open up the exhaust throats where they turn the corner at the valve guide and boss. If you stick a finger down there you will find there is a VERY tight corner with only a small gap. Some work with a small grind stone in a drill will open this up to double the size in a few minutes -but don't shorten the valve guide, although you can taper it like a pencil. You can also open up the inlet valve throats (and taper the guide) - but that is slow work and the improvements although real enough are not as obvious as that restricted exhaust flow.

Why not cut down the guides? My brother had his head proffesionally built up and given 'the works' and they cut down and blended in the valve guides.

Because they need to be full length to support the valves properly. I suggest anyone who cut down the guides didn't know what they were doing! You should absolutely NOT shorten the guides - just 'pencil' shape them.

Ah I see, well the engine in question wasn't an A-series and didn't have much of the valve guide protuding into the port to begin with. As for the engine builder, he is well known for producing quality race and road engines from a very old fasioned workshop. Perhaps it was something you can get away with on some engines?

Maybe - but not on A series engines - where the action of the rocker is always trying to push the valves sideways - they need all the support they can get. 'Race' engines are different case anyway - they can be stripped and rebuilt regularly - maybe after every race. You want an engine that can do decent mileage in a road car.

My object was to remove the steps between the "floor" and the machined area round the valves which I thought I had seen alluded to in an earlier post, rather than super polishing! I was considering a skim of the head anyway, and wondered what the optimum compression ratio is thought to be by our resident experts?

Eliminating the 'steps' is good - but as you remove metal so you reduce the CR. The beauty of a 12G940 head is that it needs NO skimming when used on a 1098 - and will give ~ 9.5/1 as it comes. As you skim - so you reduce the room for the exhaust valves before they whack the block! I can't remember - are you a 'sinker' or a 'pocketer' ?

As I have very hard seat inserts, I reckon pocketing with careful use of the router is my best route. So you don't think I should aim upwards of the "standard" 9.5? Just wondered while I still have the option. Another niggle makes me think I should be doing something soon as I have pinking which I can't shift and number 2 plug is running hotter than the others, so I think I may losing the head gasket on the standard head, this gives me a little more justification for swapping heads (thinking of how to explain the head swap to the wife in terms other than just wanting to go faster!!)

how to explain the head swap to the wife in terms other than just wanting to go faster!!

I'm going through the same process. Both the head swap and trying to think of a good explanation!

Are you going for a better manifold/carb/air filter combo as well?

Andrew

Using an HIF38 carb on MG metro manifold, MG air filter at moment as its there, better than standard metro, and quiet, (will investigate a free flowing element for it.) Once I've got the plot going, I will look towards the RC40 system as shown recently by Matt, but probably with a Marina cast manifold, again for quietness. Not sure that a fancy three branch is a great necessity for the 1098, though I feel someone will argue against me!

The larger exhaust manifold will be fine -must fit mine some time soon! Check the pockets with plasticine before final assembly.<br><br>

By "larger exhaust manifold" do you mean Marina, or the 3 branch fabricated type? If Marina, will the small Minor pipe mount onto it, if so I'll fit that with the head, then do the large bore exhaust later! (No thoughts about going over 9.5:1?)PS the photo has only just come up-thats a pretty deep looking pocket!

I meant the Marina single exit - which has larger 'ball end' and so will need a larger bore exhaust. I have some parts to make my new system - but just never got round to it. If your system is just normal bore then may as well stick with standard manifold - that's what I have had up to now - and for the forsee-able future! Yes - i've been struggling to get the picture up. Hard drive has just been wiped and re-installed - and it's lightning fast! But I lost all my pictures and so having to bring them back from photobucket. Now using 'Opera' on advice of the lad who fixed the computer - but I seem to have lost the ability to copy and paste!!
The picture is not of my block - I stole it (picture - not block!) from an ebay post some time back. Pockets will need to be 40 thou deep PLUS any amount you skim from the head. Comp ratio - well the later 998 Mini engines ran 10:1 as standard. I have run 12:1 on competition engines - but i suggest 10:1 should be enough for you. If you do the 'polishing' then maybe take 40 thou off the head - but now the pockets then need to be 80- thou deep!

At the moment I have 7.8mm valve lift from "best" exhaust lobe. My 940 exhaust valves have same 7.8mm from seated to flush with head face, so I was reckoning on the head gasket at ~0.9mm compressed as being sufficient clearance margin. So if I skim the head by any amount, then that would represent my pocket depth. I was thinking of striking for about 10:1, say about 0.8mm skim to allow a "wee" bit for my adventures with the head clean-up! Am I missing anything? By the way, should the modern unleaded fuel be leaning us towards using a lower cr to limit pinking, or am I wrong there?

Just out of interest who is the engine builder you refer to minorhickup?

That 7.8mm (312 thou in 'real' money) is about right - although it should be 318 thou - less the valve gap. If your head is deep enough to keep the valves below (or just level) with the head surface then you are one of the lucky ones! I've tried this on my heads and they are all less deep than the valve lift - but others certainly DO manage to get away with it. The 40 thou pockets (or valve sinking) are designed to give decent clearance to allow for valve float.
Unless you can get the head skimmed 'free' I wouldn't bother! It will make little difference - and it means you have to start messing about making pockets! It will also make the engine more liable to pink - of course you can use the better grade of fuel and/or add octane boosters to combat this. Worry more about the exhaust valve throats, and to lesser extent the inlet valve throats - that's where the gains can be made.

Many thanks (yet again) for your words of wisdom-I'll open the exhaust throats, not going to worry about the inlets as they're already huge by comparison to the 202 (though I always keep in mind that flow diagram of our Lord David which demonstrates only too bluntly what a disaster the valve area is compared to the rest of the flow system!) Soon going to have to bite the bullet and cut up that moggy manifold!!
Oh and sorry about lapsing into modern lingo- I've now reset my vernier to read in INCHES! Another aside-how come in this metric age my 10 year old daughter weighs 7 STONE and is 4' 9" tall? She and her friends have no idea of their dimensions in Kg and cm!

These manifolds are 10 a penny - just make sure you cut into the inlet section if you need to keep the exhaust. The exhaust valves are also huge compared to the 202 and 295 heads - that's the major advantage of the 940 head - much bigger valves and better flow in the ports.