Pockets for 940 heads

[philthehill]

As there has been recently some discussion regarding pocketing of blocks for fitting 940 heads to small bore blocks
I am posting this link for comment especially the size and configuration of the pocket machined into the top of the block.

http://www.ebay.co.uk/itm/Classic-Mini- ... SwZVlXqO~5

Phil

[bmcecosse]

Which is exactly why it is better to 'sink' the exhaust valve into the head. I had to make huge block pockets when I had a genuine S head on my 998, witha 649 cam -the top rings were only just below the depth of the pockets.

[don58van]

Is it really necessary to go as big with the pocketing of the block--I mean laterally, not depth?
It seems to me that the example Phil has shown us has had metal unnecessarily removed beyond the area where a valve could collide.

Don

[bmcecosse]

Possibly - it's a matter of carefully measuring/marking and of course depends on the size of exhaust valve fitted. In my case - the head was a genuine S, so had the larger exhaust valves than a 940 head.

[edd_barker]

Those look huge! Mine are the shape of a valve just poking out a bit. Those have had the shape blended to form a curve. Maybe to remove material for CR purposes?

Desperate to get that engine finished just so busy at the moment.

[philthehill]

Bmc
Thank you for your comment regarding pockets.
I have been looking at a 12G185 head (I am aware of the problems with the 12G185 heads) and at first glance and taking preliminary measurements it appears able to fit onto a 950 block without pocketing or recessing the valves.
The combustion chamber whilst no larger in capacity is much deeper for which it may have or require shorter valves stems.
The head does not have such an open combustion chamber as has the 940 head - more like a grown up 950 Minor head.
All comments gratefully received.
Phil

[bmcecosse]

You sure? There's no way the exhaust valves on my S head would go down the 998 bores (which are +65 on a 948 bore) - and the combustion chambers were full size easily as big as a 940 head - and of course the head had been skimmed. I believe S valves are longer than 940 valves.

[philthehill]

There are differences between the 940 head and the 12G185 head that may allow the 12G185 to fit on the 950cc block especially if used with a standard camshaft.
The main differences are that the 940 head combustion chamber is 7mm deep and the 185 head combustion chamber is 11.7mm deep. Both heads have a combustion chamber of 21.4cc.
The inlet/exhaust valves centres are 1.3mm closer on the 185 head compared to the 940 head - which is possibly why the 185 head was prone to cracking between in that area.
I have not removed any valves yet as other tasks are pressing but the valve spring seat to valve stem tip distance appears to be the same.
The valve guides are supposedly different on the 185 head but until I get a valve out I do not know what the guide differences are .
Below is a 12G185 head that was sold on 'e' bay a couple of weeks back. Note the shrouded combustion chamber which is very much like the 950 Minor head.[frame][/frame]
I did not purchase this head which sold for approx. £30.
Here is a comparison photo of the 940 and 12G185 heads. 940 at the top.

Phil[frame][/frame]