12G295 Head

[chris_bates]

The rebuild of my 1098 engine continues and after the problems with the aftermarket rear seal, I have started to tackle the head. It is a 12G295 head with double valve springs as you would expect but there some anomalies from what I would expect to see. I don't claim to be an expert on engine rebuilding but as I work through this engine, there are more and more things that don't seem quite right and before I go ahead rebuilding as per the manual, I would welcome any observations.

First there is a lot of oily carbon on the valves and particularly on the inside the exhaust ports. This would indicate rich running but once the surface carbon is removed, there is also a very hard ceramic type deposit which is light brownish and very difficult to remove.

The second thing is that there are no "oil caps" on the exhaust valve stems (see photo).

Thirdly, I found the rocker locking plates underneath the rocker pedestals instead of on top being used as a sort of shim!

So to the questions: First, should I replace the exhaust valves given that I have managed to bring them back to the state shown on the photos. They will of course need grinding into the seats. Second, should I make sure all the valves have oil caps on them? Third, am I missing a trick regarding the rocker locking plates being used as shims?

Many thanks in advance!

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[bmcecosse]

The oily residue suggests an oil-burning engine.. The 'rear seal is a hopeless thing - forget it. You should NOT have caps on the exhaust guides, although I would fit the normal O rings under the collets on the exhausts. The shims under the pedestals are there because the head will have been heavily skimmed to get a decent comp ratio (measure the thickness - any less than 2.750" is the amount it has been skimmed) and the shims are there to help correct the rocker geometry. Put them back! Make SURE though that the one on the oil supply hole (between 1 & 2 valves) has a hole in it to allow the oil through!!

[chris_bates]

Thanks for your thoughts Roy. I'm learning all the time. The problem with inheriting a non standard setup is that there is no workshop manual to refer to so I'm really grateful to all the experts who post here.

The shims are 0.053 inches so is that the amount the head has been skimmed by or is it 2.75 ins minus my 0.053? What is the likely CR to be with this amount of skimming?

I have kept all the valves, springs, pushrods etc. in order as they came off the engine and that goes for the shims as well but I will double check the oil hole.

I had a go at checking the diameters of each cylinder with my micrometer to see if there was any wear and the discrepancy was no more than a couple of thou. Does that mean that if it is burning oil there is a problem with the rings? One of the reasons I removed the engine in the first place was because it was a bit smoky - not really bad like with a broken ring but certainly not a clean exhaust. (the other main reason was that rear oil seal) !! There was certainly no lack of power though.

One final point which may be relevant to the oil burning issue is that the carb is an HS4 and was fitted with a AAS needle. (I have now switched to an AAA needle as you suggested for the engine that is currently in the car and that is a good improvement).

Thanks as always for your help!

Chris

[bmcecosse]

The burning oil and 'rear seal' problem are probably both down to pressure in the crankcase from knackered rings and /or bores. I suggest you pull the pistons for inspection. If the bores are not obviously worn (no ridge at the top) then glaze bust -and fit new rings - IF the pistons are ok. Measure the head to find out how much has been taken off. It 'should' be 80/100 thou, maybe even 120 if whoever did it took care with the oil passage.