Replating Service for Motorcycle Trials Cylinders

Your barrel will need to be taken off and stripped of any head studs, gaskets (and power valves, reed valves if yours has them). This is required as the barrel is dipped in strong chemicals to prepare the barrel for the new plating.

The pictures show a cylinder requiring a replate, you can see the piston and rings have worn through the nicasil plating to the aluminium, just above the exhaust port. The Nicasil plating tends to be nice and shiny, the aluminium tends to be more of a matt finish. You can also see that all the original cross hatching has been worn away another indication that the cylinder is ready for replating. The same wear sometimes occurs first at the top of the cylinder where the top ring sits at TDC.

The cross hatching seen on a new bore is created by a process called Plateau honing, after replating the bore is diamond honed back to the original size, this is done firstly with a very coarse honing tool, this leaves relatively deep scratches (peaks and troughs) in the cylinder and a rough finish. A finishing hone is then used to create the plateau surface, this takes the tops off all peaks creating a smooth surface finish that still has relatively deep troughs in it, these valleys or scratches form a criss cross hatched pattern in the cylinder and they help to hold the oil in place to lubricate the piston and rings.

Nicasil plating is also porous, these tiny holes also help to retain the lubricating oil. Because of these tiny holes it is recommended to only use diamond hones, as these are very expensive and the nicasil plating is very thin (0.1mm) it's not recommend to re-hone the cylinder before replacing the piston or rings, instead simply clean the bore with WD-40 and a clean lint free cloth. If there is any roughness that you feel needs removing, some hone manufactures recommend a aluminium oxide abrasive as the broken off abrasive particles are big enough not to get stuck in the tiny holes. Do not use a Silicon Carbide abrasive, these will leave bits stuck in the nicasil and lead to premature wear of the piston and rings. Again make sure you clean the bore with WD-40 and a clean lint free cloth (use a light coloured cloth so you can see the dirt), repeat till the cloth no longer picks up any dirt and it comes out clean.