Hello All,
I'm a mechanical engineer by profession, specializing in CNC applications, but I'm mainly involved with E Type restoration (S3 in particular) nowadays. I've contemplated the following issue for some time now and can provide a reasonable argument for both.

I've rebuilt many 6 and 12 cylinder Jag engines and accordingly, I've handled a lot of pistons. Having handled so many pistons, its not hard to notice that the surface finish on the Piston Skirt varies from various manufacturers. Some are Elliptically Ground, whilst others are Elliptically Turned. The obvious difference is that the surface finish produced by grinding is much finer than the turned component and I'm unsure which is better.

The arguments for each goes as follows:

1. There is infinitesimal difference between the crest and root of the surface finish of the ground piston. Accordingly a greater wear surface is presented by the piston sleeve and therefore, wear to the root of the surface finish will result in very small change to the major diameter of the piston. However, lubrication would be by way of a oil film between the Piston and the Cylinder Bore, with some oil being retained initially by the cross hatched honed surface of the cylinder when relatively young.

2. The surface finish of the turned skirt presents as one continuous helical groove created by the Tool Nose Radius of the turning tool insert. This finish can be easily observed and detected by running one's finger nail along the surface of the piston skirt. In this case, there is an appreciable difference between the crest and root of the surface finish compared to that of the ground piston. I've not measured the surface finish with a surface finish comparator; accordingly, I can't specify what the finish actually is. The disadvantage, as I see it, of such a finish, is that the bearing area of the skirt is reduced, as the crests of the surface finish will initially provide the bearing surface. In this case, there may be more rapid wear until the root of the surface finish (smaller diameter) is reached through wear. The counter to this is that the root component of the surface finish would retain lubrication better than the ground finished piston.

Any opinions on the above?

Regards,

Brent