Gudgeon pin offset - Query

[daxman]

Hi All

Recently bought a replacement piston for Suzuki A100 - the vendor says it is marked back to front (arrow pointing wrongly).
Comparing it to the original however reveals that the gudgeon pin offset is towards the rear (transfer side) in both cases,
which appears to contradict some other conventional wisdom - Do Suzuki know better ??

Cheers

JS

[markush]

Hi!

To A100 I can not say anything. But I can tell, that in the GT250/380 the piston pin is shifted 1mm towards inlet side.
When the engine is viewed from the right side, the inlet is left (rear), and the crankshaft rotates clockwise.

[daxman]

Thanks - that appears to be the same as I am seeing. On 2strokeworld though the consensus appears to be that
the offset is towards the exhaust port - as this is the side that takes most thrust (on a clockwise engine).

Curious eh ?

JS

[markush]

Hi!
I think this can not be generalized, it depends on the design of the motor. Timing, long stroke, short stroke ....
It's all about making the change in the contact side of the piston without hard hitting.
Let's look at an engine with clockwise rotation, the cylinder above the crankshaft below, left inlet, outlet right:
With the pin offset towards the inlet (left) the piston moves from the right (front) to the left (rear) cylinder wall before reaching TDC and before the maximum combustion pressure occurs.
With a pin offset towards the outlet the piston changes the contact surface after ignition while the combustion gases expand. Thus, the change does not occur too hard, so occurs later, the pin offset must be greater than a pin offset to the inlet would be. In addition, the contact surface of the outlet side is smaller, precisely because of the exhaust passage. Therefore, it is better if the greatest pressure occurs when the piston is applied to the inlet side.

But there are many engines without these niceties without piston pin offset, which also function properly.

[Alan H]

The original idea of an offset gudgeon pin (or offset crankshaft) was to stagger the port timing as the ports would open and close at different times of crank rotation depending if the piston was rising or falling in the bore.

[tz375]

The idea behind pin offset is to reduce noise.

The other idea is that offsetting the pin or the cylinder can also reduce friction and make more power but that needs offset in teh opposite direction.

Offset also changes dwell time around TDC which can also help power but increase peak piston velocity. Tricky stuff indeed and I have no idea which way round Suzuki wanted the offset, but I'd go with the piston marking.

[old racer]

The important thing to remember here is the position of the rings,
You don't want the ends of the rings traveling over the ports,
As they are likely to trap and break.

[pjmcburney]

Stock RG500 pistons have a small gudgeon offset because, I assume, the crank pins at TDC do not line up with the the bores, so the offset takes this into account and allows equal thrust to the front and rear of the bores as the pistons move up and down.


When I had my engine rebuilt, I used (I think) RG150 pistons which have no offset and, so far, I've not had any issues with noise or vibration or wear.

I don't think 1mm either way would make a great deal of difference, although you'd want to be putting an offset piston in the right way because the offset is gonna be significantly out if it's in backwards.

[daxman]

Crikey ! Think I'll just try it the same way round as the last one...

Thanks for all the answers..

Cheers

JS