Morris Minor Owners Club

The time is nearing to strip and rebuild one of our engines. Whilst it's apart I may change over to a 12G940 head, but am unsure of what would be an average price.

Obviously it depends on the condition. So lets say a head that will be good, reliable and very usable, but possibly not brand-spanking-new-perfect (or perhaps that's the only option???).

Many thanks for any thoughts.

Several unleaded ones on 'e' bay at the moment.
There is also a nice 12G295 head with bigger valves on there as well which is an option and would be an easier fit. Item No: 321324616203.

I used to pay £5/10 for them.... Sometimes pays to look 'outside the box' - so maybe 'Mini head' rather than '940'... I think the average price these days is ~ £50 - for a standard valve head. You don't really want 'unleaded' - since you will likely have to sink the exhaust valves into the head anyway. And obviously one near you, will save the £20 or so carriage.

As it appears that you are going to totally rebuild the engine and possibly fit a 940 head I consider it is better to pocket the block for the valves than sink the valves into the head. When you sink the valves into the head you are increasing the combustion chamber cc and you cannot then skim the head to bring the combustion chamber back to the standard 21.4 cc as you will be back where you started with the valves fouling the block. Pocketing the valves into the head is alright if you want to fit the head to an already running engine but it is not a perfect solution and does not get the best out of the head.

Phil - the head is 'so good' compared to a small bore head - that it is not too important to get the absolute best from it. Pocketing the block a) spoils the block for ever and b) exposes the top ring. The 21.4cc is actually a bit small anyway - so sinking the valves helps here, and is easy to do. And the head can always be 'rescued' if necessary in future by fitting 'unleaded' seat inserts. The point of the exercise is an easy/inexpensive power upgrade. Often older 'leaded' heads already show some exhaust valve recession - and don't need any valve sinking to get the necessary 320 thou clearance. The infamous Keith Calver ran an article (quite a while ago) where he claimed sinking the valves actually HELPED gas flow in the exhaust port - by shortening the path to the 90 degree bend. I'm not supporting that view - but I do believe any flow restriction is very minimal - certainly when compared to a small bore head!

bmc
I do not disagree with what you are saying but if I was going to totally rebuild the engine I would want to maximise the potential.
So long as you are careful and do not cut below where the top ring comes to you will be alright. Measure twice and machine once as the saying goes. I would consider a pocketed 1098cc block to have more value than a non pocketed one and more desirable to a prospective purchaser. As you are no doubt aware it is only the exhaust valves that need to be pocketed.
Recessing the valves into a 940 head may throw the valve/rocker gear out of kinter and if you have wear on the rocker tips the valve stem tip may ride outside of that wear pocket and cause rapid wear of the valve guide. In practice you should either reduce the length of the valve the equivalent of the pocketing or pack under the rocker shaft pedestals by the same amount. You may even need to change the valve push rods to get the relationships right. It is all a matter of measurement and judgement to get it right!

I'm getting confused, if the block is pocketed this must lower the compression ratio in the same way as sinking the valves. If the head is then skimmed to compensate then the block would need to have deeper pockets, and so it would go on and on. Am I right? or shall I go away and do more thinking?

Les
It could go on and on and you could end up with no head and no block.
You are right in that if you fit a pocket in the top of the block for the exhaust valve you do lower the compression ratio because by default the pocket becomes part of the compression chamber. That will be the same as recessing/pocketing the valves in the head. But having the pocket in the top of the block does allow you to do other things to the head i.e. modify the chamber (which can alter the compression ratio), fit a high lift cam etc, etc. Recessing the valves into the cylinder head to overcome the exhaust valve fouling the top of the block is the easy option but is not a good engineering solution because of the reasons I gave above and it does stop you from somewhat modifying the head to get the best out of it. But it is horses for courses and you pays your money and takes your chance! Perhaps I am being too fussy; it is just that I would not use the 'recess the valves into the head solution' to overcome the exhaust valve fouling problem.
So Les you are not loosing it

That's a relief I agree about the engineering point of view, several engines seem to have their pistons recessed as opposed to sinking the valves but realise the ease of the latter, and if done neatly and blended in, fair enough. I think the top piston ring will still be covered if the block is done but again it involves a strip down. Keep on being fussy !

I did the pocketing about 45 years ago...and because I was running a high lift cam, I had to go v close to the top ring. But even a 40 thou pocket will let the gases down to the ring much more than normal. It doesn't affect the CR as much - because you only need to cut a 'fingernail clipping' in the block - not a complete circle. And yes - it upsets the rocker geometry - but again we are not talking about a full race /every last bhp engine here - so it's not making a significant difference there. I did stone my rockers nice and smooth to take away wear , and I did re-align them nicely over the valve tips. But I stress again - it is an 'economy' mod that gives v good power increase for relatively little cost - or engine modification.