Crankshaft spares & parts for Triumph 1937-62 Twin Models

The standard bore is as follows, 350cc 55mm bore with a stroke of 73.4mm. 498cc – 63mm bore with a stroke of 80mm, 649cc 71mm with a stroke of 82mm.

Unless requested not to, we will supply the nearest oversize piston available. Do not bore your barrel before obtaining a piston, as the size you require may not be available. In any case a barrel should always be bored to suit a piston. If when boring the barrel you find that a larger oversize than that which you have obtained is required we will exchange it. Do not despair if your cylinder has run out of space for a rebore, we have liners available for most models. A liner is a sleeve that is fitted inside the cylinder that reduces the bore back to standard size.

If you are not sure whether your piston, bore or rings are worn then your local automotive machine shop will normally check them for you without charge. Piston ring gaps are frequently quoted as a method of ascertaining bore or ring wear. When using this method, the ring gap should be measured near the top of the bore and compared with the gap obtained in the unworn section of the cylinder at the bottom. Divide the difference by Pi (3.142) to obtain the amount of wear on the cylinder. The figures that are given for ring gaps are the minimum in order to prevent the rings closing up when they expand, a larger gap, within reasonable limits, is not detrimental. Some of the re-manufactured rings do have a larger gap than would normally be expected. The piston clearances are listed below. All pistons are complete with rings, gudgeon pins and circlips. 1.0 mm is equivalent to .040″.

Compression ring gap 0.010″-0.014″
Scraper ring gap (498cc) – 0.007″-0.011″
Scraper ring gap (649cc) – 0.010″-0.014″

This and much more can be found in the workshop manual, which you can buy here.