This pin, also known as Gudgeon Pin – Piston Pin – Wrist Pin, provides the articulated connection between the conrod and the piston. Note that this pin sees very high forces. Often hollow to reduce weight.
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.
The gudgeon pin adds to the mass of the piston and so it is normally a hollow tube as this is the optimum for mass versus strength. Some designs thin down the wall of the tube on the outer sections to remove even more mass.
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.
Removing a Gudgeon Pin
If the gudgeon pin is excessively warn or has been too hot and is gunged up it can be difficult to remove. Firstly you need to remove any fixings, either circlips, pads or clamps.
You might be lucky and using a round wooden dowel you can push it out. If this doesn’t work then rest the piston on a wooden offcut and carefully tap the pin out using a wooden dowel or rounded square of wood.
More severe and you might want to warm the piston up, try to avoid directly heating the gudgeon pin as you want the hole in the piston to increase in diameter and not the pin.
There are a number of retaining methods:
- full floating
- locking mechanism – C type clip or Locked small-end conrod to gudgeon pin
- floating with end pads to protect cylinder walls
- Interference fit
The fully floating gudgeon pin is an option with 4 stroke engines, but can be difficult with 2 strokes and you will need to take care when designing the port locations.
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.
The use of soft pads is quite common in commercial model glow plug engines.
The other way is to lock the small-end of the conrod to the gudgeon pin.
The downside is you add more mass to the small end with the bolt fixing, you weaken the small end to make the clamping structure and introduce a failure mode whereby the nut and bolt can work loose and then damage the engine.
In small sub 50cc engines this is very difficult to do because of the size of the parts.
Engine Design – a number of short articles discussing aspects of engine design and in particular model engine design.