12g940 head, sinking in the valves

Post by **youngun** » Wed Jul 11, 2007 10:50 pm

Its been mentioned a few times that if you get a 12g940 (midget) head, then you have to sink in the valves? How is this done and can it be done at home?

Thanks,
YG

Post by **RogerRust** » Thu Jul 12, 2007 5:57 am

I got mine done at a local car engineering shop cost £60 all in. Not worth doing it it home in my opinon. Depending on what cam you are going to use you will have to have 40 thou or more cut back.

As per several threads check the lift on the cam 0.25 fo me as I used a 1098 cam. multiply by the rocke ratio 1.2 for memory. then measure the height between the face of the head to the valve face and cut back enough to make sure they clear.

The inlet valves arn't a problem only the exhaust. We had to sink mine by 50 thou, but the head had been skimmed.

If you are using a HS4 carb I'll let you know what needle we have fitted next week when I go back to the rolling road. I had a problem with the jet assembly and am waiting for spares to arrive.

By the way make sure you loosen the rockers right off before you fit the head then adjust them when its tightened down or the valves might hit the block face.

Post by **Alec** » Thu Jul 12, 2007 6:36 am

Hello YG,

that is not the only way to do it, the better way in my opinion is to cut pockets in the block. This is not that easy an option but you avoid shrouding the valves and also upsetting the rocker geometry.

Alec

Post by **bmcecosse** » Thu Jul 12, 2007 1:50 pm

The sunk valves trick is very easy to do - and a very cheap head with 'recessed' valves acn be used - mine cost £5. We used a simple pillar drill with a 45 degree countersink - done 'at work' in less than 10 minutes all in !! (£60 sounds a LOT just for this - or did it include skimming ?) 40 thou will be fine provided the head has not be skimmed. Only do the 4 exhaust valves - and then easy to 'un mask' them with a little grind stone in a leccy drill. You also must add 40 thou packers (= washer!) under the exhaust valve springs - to keep the pre-load correct. The rocker geometry can cope fine - but you do need to make sure the rockers are correctly aligned directly over the valves - to get this right may need some work with a file (on the pillars and/or rockers) and/or the fitment of some thin washers - but it's it's only an hour or so work and well worth doing. Pockets are also easy enough to do - can be done 'in situ' ina bout an hour with grind stone in leccy drill, but you MUST seal up everything really really well to keep the dust/swarf out of all the moving parts. And of course there is no going back - the block cannot be re-instated to roiginal condition. Obviously you also must use a 1275 head gasket - and you will probably have to file some metal off the top edge of the water pump to allow the head to seat down and still leave a little clearance.
The power improvement to the engine (assuming decent carb is fitted) is very significant!

Post by **youngun** » Thu Jul 12, 2007 3:11 pm

BM, you mentioned that i may be able to get away with not recessing the valves on my 948 as ill be running on a 948 cam??

Is this all covered in Vizards book?

As for carbs, i sourced a 1275 Mini HS4 carb with the cast inlet/exhaust manifold. Its a bit of a lump but an improvement on the original ally one which looks stupidly small!

Post by **bmcecosse** » Thu Jul 12, 2007 5:24 pm

IF you stick with 948 cam (but the 1098 cam is SO much better) you may be ok doing nothing. Only way to find out is trial assembly with some plasticine on the valve heads - and gently turn the engine over by hand. Then inspect the plasticine to see what clearance there is. Some say they have got away with it even using the 1098 cam - my measurements say this is not possible - others claim it's true.

Post by **youngun** » Fri Jul 13, 2007 3:38 pm

Ill have to see,

cheers everyone.

Post by **youngun** » Sat Jul 14, 2007 5:27 pm

Regarding the carb, manifold i have. (HS4 with a mini 1275gt cast inlet/exhaust manifold) what will i have to consider set up wise?

Post by **bmcecosse** » Sat Jul 14, 2007 7:53 pm

The manifold is not ideal - lot's of power lost there due to the exhaust section heating the inlet. Would be better to cut off the inlet and throw it away - use the exhaust part ok - and look for an MG Metro alloy inlet.