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What is the Best Way to Determine Preferred Piston Pin Offset?

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This was lightly mentioned in the Piston webinar (#139). The pin offset can affect loads between the piston and cylinder wall, the geometry of the rod vs. the piston and crank, and the volume of the cylinder at the top of the stroke. All of this would vary based on stroke, compression height, possibly piston diameter, combustion chamber shape, timing, and burn rate of the fuel.

Is there a set of equations and theories behind calculating the preferred offset for different sets of engine parameters? A lot of the literature talks about offset crank position rather than offset pins. As I understand it the goal is to have the rod perpendicular to the piston around peak pressures while also increasing the forces tangential to the crank at that point.

However, pressures are high at different levels over a range of crank degrees, probably varying with engine speed and fuel. There are benefits of reduced friction and improved geometry, but which has a greater influence on power output.

It would intuitively seem the effect is small, but it is so widely used with so many papers on it, it must have some significance.

Is there a standard set of calculations used to determine the pin offset?



I've never gone through and calculated an offset personally. As you've mentioned, there are likely to be numerous factors that go into this calculation. Most of the aftermarket piston manufacturers offer pistons with offset pins which means you don't need to overthink things too much here. The offset pin is usually aimed at reducing noise and potentially wear for long term durability.

From what I'm reading it also reduces ring friction reducing friction losses and the resulting heat that would be generated and put the connecting at a more favorable angle during peak cylinder pressures. The amount of the offsets seem to be small, the effects may be significant.

From what I've seen off-the-shelf pistons duplicate OEM offset (which is hard to find!) and compression heights, presumably to work with OEM cranks and connecting rods.

When the engine is knock limited (e.g. any pump gas turbo) or strength limited (e.g. as you've discussed on the FT86 FA20 and numerous others, reducing friction should be free power. Sort of like cams that have different lobes for different cylinders because the intake runners are different lengths.

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