Part 3/3 by Ramon Wilson

Part one here Part two part three

To tackle the pistons a previously made lap for a larger bore was skimmed to suit.

The liner was lapped in three stages - cleaning lap and liner between each in a jar of cellulose thinner. After the initial lapping to remove tooling marks - 320 grit carborundum paste.

The interim using 600 grit.

And finally a 'polish' with 1000 grit.

The pistons were turned from Meehanite the inner profile turned and milled to lighten as much as possible. Held in the usual fixture they were lapped using another 'recycled' brass lap from a previous build using 320 grit until the piston would just enter the cylinder bore at the base. A quick lap with 600 to smooth the finish and the slow process of lapping the piston to the liner by hand could begin.

The same process as used on a previous engine.

The piston is mounted on the T-bar with a brass pin and a minimal amount of 1000 grit paste is applied with plenty of paraffin and oil lube. (95% to 5 %)

The piston is entered into the bore with a slow twisting motion gradually abrading both surfaces to a perfect fit. This is a slow process and one which can't be rushed - patience is key. Despite having done it many times now there are still moments where I think it will never wear in but it always does. Once nearing the top of the stroke it's important to constantly take the piston out and clean it then go back in with just plenty of lube allowing the residue in the liner to take that final few microns off

The gudgeon (wrist) pins were made from steel dowels. Case hardened to provide a good wearing surface they have a soft core which can be drilled to accept the brass wear pads at each end. A fresh dowel requires the end cutting off to get past the hard surface for drilling...

Then the pin can then be cut adding a small amount for trimming back to finished length with a carbide tool.

And that's it - with the rotors made from Tufnol it's time for a final clean of parts and assembly.

Second engine.

And here they are with the original 3.46 version as a comparison