How to replace a piston on an engine

Approximately every 40 litres of mixture used, it’s necessary to replace the piston, the true “heart” of the engine. Follow the advice of the doc to find out how.

Tucked away at the heart of the engine, where temperatures are hellish and forces reach maximum power, the piston is one of the most stressed parts of the engine and, therefore, need to be kept an eye on more than most. Its replacement is an operation that all kart drivers will have to reckon with, sooner or later.

There are two main reasons for replacing a piston: they either break or wear.

The recommended period for replacing a piston is more or less every 40 litres of mixture used. Such is the case for amateur kart racers, while in other cases it is advisable to change the piston after each race. The rules apply to any kart engine, be it an old 100, an OK (formerly KF) or a KZ.

On this occasion, the Doc is using a direct drive 125 cc engine by Iame, but the operations are more or less always the same.

Step 1

Let’s start with an overhaul: firstly, use a 13 mm T-handle wrench to remove the cylinder head.
To do so you need to unscrew the five nuts that fasten it in place (some newly homologated engines use six nuts for added safety) and physically remove the cylinder head.
Then, use a 12 mm combination wrench to unscrew the four nuts that fasten the cylinder and slide it off the crankcase, making sure not to break the gasket.
At this point, the upper part of the connecting rod and the piston still fastened to it will be visible.

Step 2

To release the piston from the connecting rod you need to remove clips using a tool similar to a screwdriver, but simply has a pointed tip (neither cross-slot, nor flat head).

The pointed tip must be inserted into the groove on the edge of the piston and the operation needs to be performed from both sides.