Reconditioning the Lotus 912 cylinder head

Having removed the head from the motor in the car, it seemed to me that only the exhaust valves were damaged, the inlets being in first class condition. I should therefore only need to replace half the fittings in the head. I do of course also have the head that came with the spare motor, and although this is in good condition it has done 12k more miles and I dont think has had any new parts. It should therefore be cheaper to recondition the head from the car rather than that which came with the spare motor.- provided that a) I am right in my reasoning, and b) the head hasn't already been skimmed to its limit.

I wasn't going to do this job myself but this is a record of what was done.

Firstly I wanted to check that the head had not yet been fully skimmed so I measured its depth from the pads between cylinders 2 and 3 and also between valves 1&2 and 7&8. This value should be 82.04 mm +/- 0.254 mm. Given a maximum skim permissible of 20 thou (0.5 mm) this means that the minimum depth of head (before it's scrap) must be 82.04 - 0.254 - 0.5 which comes to 81.286 mm taking into account max manufacturing variation and max skim.

Problem: my value was only just over 80 mm!!! I was getting values around 80.2 - 80.35 mm between these three determinations.

This would suggest that the head is already too skimmed to be worth any more effort, but on checking the measurement pads between  two heads it did seem that those on this head had been deliberately ground down and now sat below the level of the cam carrier housing mating flange. This would obviously reduce the reading and if this had been done by hand might well account for variation between the three pads as I had observed.

Pad between cylinders 2 and 3. Pad seems to have been ground down; there is no flat surface to get a micrometer anvil on and the pad itself is recessed well below the gasket face for the cam carrier house.

The other head- note height of measurement pad. Grooved machined area for accurate measurement

I have no idea why this might have been done, but it does mean that perhaps I shouldn't condemn this head too soon. I will experiment with other ways of measuring the head depth. Luckily I do have a second head where I can compare measurements made in different ways. The depth of this spare head could be measured using the pad above to derive a depth of 82.18 mm. (... as an aside only Lotus would position the measurement pad opposite an opening in the gasket face making perpendicular measurements almost impossible!)

Die fitted into circular recess, can then use a micrometer to determine the head's depth.

The cam house mating surface is inclined and so lacks a clear reference point from which to measure. The surface of the head inside is flat and there is a handy machined circular  depression, but sadly the geometry of  the head prevents micrometer access to the head surface between the valves. So, as an approach to an alternative means of measuring the head, I  found that I could insert a spare threading die into the round recess between these springs as above. This has a machined surface and raises the low point so that a micrometer can now be used. Its then possible to measure the depth of the head from the top of the die to the gasket face of the head.  Measuring both heads this way shows that the measured depth to this point of the spare head (ie with the undamaged pads) is 72.28 mm meaning that the inserted die to pad height is 9.9 mm. Measuring the head with the ground down pads in the same way gives a value of  72.18 mm. The heads are thus  82.18 mm and 82.08 mm and both are still within manufacturing specs so if necessary both heads could be skimmed.


I have since also removed the valves from the second head, all are unbent although there is clear wear in the guides and/or valve stems as some sideways movement is detectable. I do not need to restore this head so I will simply clean and regrind all valves and seats before storing the head against future need.

I ordered valve guides and valves from Kemp High Performance Engines and arranged for them to be delivered straight to the engineering shop. Now its a case of waiting... but at least I can get on with the cam housing and camshafts.