TKM Insight – Illegally modified engines

stock-tkmTal-Ko have foiled an engine builder’s plan to sneak in. Grahame Butterworth tells the story.

If there’s one thing we pride ourselves on, it’s close attention to regulations and ensuring that as far as possible everyone is racing on a level playing field. It’s the only way that is fair.

And 99 per cent of the time the system works well, catching nothing more sinister than the occasional accidental under-weight or perhaps gummed up piston ring.

But very recently we have had two pistons come into our possession which show clear signs of being modified. One to the crown and the other to the skirt. In both cases modifications to gain an advantage on port timing.

These were true cheats. Even the piston size had been re-stamped to make it look OK. But actually it was not very well done and soon picked up by ourselves. We are on the trail of the culprits and will not hesitate to apply full litigation against them.

The class regulations and fiche make it very clear that the piston must be neither modified or machined. There is no grey area here and no excuse for doing anything to a piston other than removing carbon without damaging or modifying the surface. What has been done to these pistons is simply cheating – and that is something we will not tolerate.

So over the past week or two we have issued drawings which set out very clearly the way to identify the cheat pistons. And we have had the Scrutineers at all TKM events throughout the country to carry out careful checks on pistons so we can ensure a rapid closure to the potential problem. And of course a special effort at the TKM Festival at Kimbolton.

The check method which anyone can use is:

Remove the four nuts and two cap head bolts holding on the cylinder head which should then be removed.

Next lift off the cylinder barrel carefully holding the con-rod and piston steady to avoid damage to the piston assembly.

With the piston now fully exposed, wipe the top of the piston with a rag to remove any oil mixture.

First look at the top surface (piston crown) of the piston close to the edge next to the top ring. It should go from a shallow approx 10 degree angle into a small 45 degree bevel on the edge. If it is square edged then the crown has been machined making it illegal.

Run your finger (or straight edge) over the edge of the piston to the top piston ring. If the ring is level with or above the edge of the piston crown, it is illegal.

If the top ring is below the top of the piston crown edge then check the thickness of the top piston ring. It should be 2mm on 100cc Junior and 2.2mm on the bigger 115cc Extreme pistons. If it is thinner, then it has been modified and is illegal.

As a final test if you have a new piston then simply insert a gudgeon between the two and place on a flat surface. Both pistons should have the same height at top and bottom – though bear in mind the carbon on the top of the piston might make it seem slightly higher.

So if you think you might have an illegal engine then contact us now on We will keep your identity secure.