Twin Cam Head refurbish.

There Comes A Time.

When the exhaust, and perhaps inlet manifold faces of the cylinder head require re-machining to restore, or to make like new again if you will.

The twin cam lotus head is no exception, most of the original cylinder heads are now more than fifty years old, and a large majority, have been reworked, ported, skimmed and otherwise modified.

During the next pull down, take the time to make a close inspection of the condition of the aluminium metal along the face of the exhaust ports, you wont have to look very hard to discover significant deterioration of the metal, but don’t despair, the gasket face can be made as good as new, like the day they came out of the Lotus Factory… but it does require some careful machining work.

First Of All, You Need Access To A Milling Machine.

This engineering operation will require a reasonable size milling machine, you wont manage with a small mill-drill machine.

The sturdy DIY made cylinder head holding fixture, which is securely bolted to the milling table, then the cylinder head has to be trued up longways and crossways (X & Y or which ever way you want to describe it). Note the slotted holes which will allow for adjustment raise or lower while truing the cylinder head with the milling spindle/cutter. Setting the holding fixture is a bit fiddly, but once the head is loosely mounted, the process all comes together. The holding fixture is quite ridged, and did not vibrate during machining, but the cuts were extremely light and a slow table feed to achieve a nice finish.

The Twin Cam Lotus cylinder head is an odd shape cyl head to accommodate in the vertical orientation. The thermostat housing, and the inlet manifold tracts, which are all cast into the body of the head, create extra things that have to be considered when mounting and setting up for the machining operation.

There is really only one way to set up the workpiece (Cylinder head). A indicator dial is the tool to use, but we are not looking for .0001″ accuracy, all we aimed for was to have .001″ variation along the full length of the manifold face, and .0005 across the flanges. If you work carefully, you’ll get it right. You might even get lucky and the face will be dead flat and true to the mill spindle, but the reality is that you possibly wont.

It may not look that bad, (Until close inspection) some might say, just add a bit of extra gasket goo to seal any gaps that may be there. Well, that may be fine, but if both the head gasket face and the extractor manifold face are pretty much perfect, the seal between the two will be a helluva lot better, rather than applying a thick layer of gasket goop, which is going to ooze and weep out when the manifold nuts are over tightened to compensate for the crappy state of the manifold faces.

Let the machining begin, once the head has been made true to the table and or milling spindle and cutter, the operation can begin. A very light cut is taken, but before a full machining sweep is done, make one last check at each end just to make sure that the manifold face is actually dead flat or true to the mill spindle, otherwise you will end of with a deep cut one end and a shallow cut the other end, and then the messy correction job will begin… don’t stuff it up.

The photo above illustrates the problems encountered during the machining operation, the tall (Wide) thermostat housing will not allow a normal facing end mill to reach down onto the manifold face, so a face end mill extension is required, and note how close the mill spindle comes to the housing, there is not much to come and go on.

We are now halfway through the first milling cut of .003″, and with a bit of luck, that might be all that needs to be removed in order to restore the exhaust gasket face to as new condition once again.

How about that, just one very light milling cut was all it took to restore to a brand new gasket face. I’m sure that you don’t need to be told that perfectly flat manifold faces promote excellent gasket face seal, with a minimum amount of gasket sealant required.

While The Cylinder Head Is On The Mill.

It makes sense to also inspect, and if required, take a very light machining cut from the top of the thermostat housing gasket face. DO NOT ENCROACH ON THE CAM COVER GASKET FACE AREA . The blue area indicates where the machining will be take place, right up to the scribed line.

Note the speckled bits of blue die that still remain in the corroded parts of the metal, this particular area can be a worrying spot where coolant leakage can occur, caused by a combination of pitted and corroded metal, which is why we decided to machine, and also a warped thermostat or possibly also corroded thermostat top housing. But again, if the gasket faces are re-machined to be “like new” again, the gasket and sealant should result in a perfect seal between the two parts.

And as a side note, the small amount of metal removed does not affect the fit of the thermostat into the machined recess of the housing.

The extended brass nuts are custom machined to fit, and they are longer than standard and fit in well with the custom exhaust headers or extractors.

There are few engines like the twin cam lotus that have the distinctive and seductive clucking sound of four pots breathing through weber carbs. I bet its makers had no idea that it would go onto become one of the most successful performance engines of all time.

Croucher Engineering – Sharing Knowledge and preventative maintenance tips for the lotus elan owner.