The picture to the left shows a piston and conrod with a drill bit taking the place of the gudgeon pin.
The drill bit is 2mm diameter and the final gudgeon pin was made from solid steel bar, I should have drilled a 1mm hole through the centre just to remove some of the weight.
This particular piston and conrod is from a small 5 cylinder rotary engine which is why this conrod is different from a conventional conrod as it is the master.
For model engines the gudgeon pin is made from a hardened ground steel. I normally use silver steel as this is easy to machine and then harden.
For model engines the lubrication is normally achieved from splash lubrication of the big-end in the sump, some lubrication comes from the cylinder wall into the ends of the gudgeon pin, but generally lubrication is minimal.
There are a number of retaining methods:
- full floating
- locking mechanism – C type clip
- floating with end pads to protect cylinder walls
- Interference fit
The circlips work well on the slightly larger model engines, 20cc per cylinder and upwards. Below this size the circlips can get very fiddly.
Maching the groove for the circlips can be quite difficult.
In this example shown on the right the gudgeon pin is held in place using a spring wire clip, the end of which can be seen clearly bent round.
This piston is from a high performance 20cc 4-stroke engine that revs to a maximum of around 11,000rpm.
A well used glow engine with the top edge of the piston above the ring showing a layer of burnt oil.
The soft floating end pads can be made from bronze, brass or PTFE.
The soft end pads is one of the easiest solutions for small engines, you still need to be careful with regards to port locations on 2-stroke engine designs to ensure that the gudgeon pin cannot slide out and lock the engine.