If you are trying to figure out what size your used cylinder is, these "cheat sheets" will help. These sizes are for new production Hepolite pistons made by JCC, and may not be the same size as NOS (new old stock) Hepolite, AE, or other brands of Triumph pistons.
Please keep in mind that a .020" overbore (.5mm) will only take off .010" (.25mm) PER SIDE. If there is deep corrosion or pitting in the bore keep this in mind as sometimes you will need to go two sizes up to clean up the damage.
The finished clearance may be more or less than specified (depending on engine builder's personal preference). Click the above photo or the photos below to open these sheets for viewing or printing.
These sizes are not guaranteed, but based off actual measurements of these pistons and not just a shop manual. One thing to note is that the 650 iron head pistons (low compression) are .002" larger than the 650 9:1 compression.
TRIUMPH 650 PISTONS (9:1 COMPRESSION) T120, TR6
| SIZE | PISTON SIZE | FINISHED BORE (.005" clearance) |
| STANDARD | 2.7900" | 2.7950" (71mm) |
| +.020" | 2.8100" | 2.8150" (71.5mm) |
| +.040" | 2.8300" | 2.8350" (72mm) |
| +.060" | 2.8500" | 2.8550" (72.5mm) |
| +.080" | 2.8700" | 2.8750" (73mm) |
TRIUMPH 650 IRON HEAD (7:1 COMP) 6T, T110
| SIZE | PISTON SIZE | FINISHED BORE (.005" clearance) |
| STANDARD | 2.7920" | 2.7970" (71.04mm) |
| +.020" | 2.8120" | 2.8170" (71.54mm) |
| +.040" | 2.8320" | 2.8370" (72.04mm) |
| +.060" | 2.8520" | 2.8570" (72.54mm) |
TRIUMPH UNIT 500 PISTONS (9:1 COMP) T100, TR5
| SIZE | PISTON SIZE | FINISHED BORE (.005" clearance) |
| STANDARD | 2.7120" | 2.7170" (69mm) |
| +.020" | 2.7320" | 2.7370" (69.5mm) |
| +.040" | 2.7520" | 2.7570" (70mm) |
| +.060" | 2.7720" | 2.7770" (70.5mm) |
TRIUMPH 750 CAST PISTONS (8.6:1 COMP) T140, TR7
| SIZE | PISTON SIZE | FINISHED BORE (.0045" clearance) |
| STANDARD | 2.9875" | 2.9920" (76mm) |
| +.020" | 3.0070" | 3.0115" (76.5mm) |
| +.040" | 3.0270" | 3.0315" (77mm) |
| +.060" | 3.0470" | 3.0515" (77.5mm) |
| +.080" | 3.0670" | 3.0715" (73mm) |
Hopefully this will help in determining if that old cylinder is any good or what size you will need to order.
Also if you order the wrong piston size you can always exchange them for free as long as they are not modified or the rings have been file fit.
Please see the Triumph piston and ring ordering page by clicking here.