I am putting together an order for pistons for a Gardner Denver ACM compressor, 3.5 x 3.5.

These will be forged aluminum replacements for the original cast iron. That shouldn't be an issue. Stock bore stock rings.

These were used on 25 ton GE switchers and maybe others. Also used on shop compressors, service trucks, boats, who knows what else.

On our engine, I searched high and low, and it appears that no comparable compressor is available that will fit in the compartment. (our compressor is in the bottom of the electrical locker, in the cab, driven from a pulley on the end of the main generator).

Anyone who has one of these compressors, this is your chance to get a set of pistons for it. Last price is at least $1600 per piston from the factory, IF they can be had.

Price should be about $175 a piston.

PM me if you need a set.

PS, I can pass along sources for the rest of the parts if needed, hopefully to include sleeving the cylinders if needed.

Forgive the possibly dumb question that you may have already considered.

But going from cast iron to forged aluminum would reduce the weight and therefore change the reciprocating mass affecting the balance of the crank. Or maybe the difference is not significant enough to worry about at the slow speeds of a compressor.

I don't know the ramblings of a machinist turned mechanical designer without a degree in anything other then hard knocks from failed projects.

I would also highly suspect that rings and clearances would have to be CAREFULLY determined for the new application, because the coefficient of expansion in a device known to reach high working temperature has to be higher for the new aluminum pistons. This might be as simple as a little additional circumferential clearance in some of the ring lands and attention to the surface finish there -- but it better be done BEFORE the production run is made and any longevity issues start to arise.

If I were doing this, I'd core the piston to install the necessary weight to match 'stock', if there is indeed a concern with balance (particularly affecting long-term bearing clearances, perhaps?)

Another potentially dumb question: couldn't one of the companies doing hard-material sleeving for diesel engines provide for boring and sleeving compressor cylinders with material that would 'never need attention'? Perhaps compatible with some kind of oilless ring material?

These pistons are being made by a company that specializes in making pistons for many applications. They do a LOT of business replacing older cast iron automotive pistons. I am sure they are aware of the differences in performance of the two materials.

You can't switch them to oilless because you still have the crankcase bearings to contend with. Oilless compressors have sealed bearings. I don't think you'll find many oilless compressors in heavy duty applications like this, not in reciprocating compressors.

The ACM compressor completely lacks counterbalancing. Switching to lighter pistons can only help with any vibrations in that case. It's bolted to a 25 ton steel plate, so I doubt vibrations are much of an issue anyways.

Being aluminum, they will wear the cylinder walls much less over time. Very few of us is going to put the hours on these they would of got in the day. Many of these probably idled for months at a time, especially when the batteries got weak. The big boys do it, why not us?? I bet ours would of been in better shape if they would of paid attention to the oil quality and to the air filter.

From what I know, this is the only option other then a used unit, if you want to stay basically original.

I hope you are basing the conversion on experience. The thermal growth due to operation and/or adverse conditions result in total destruction rather than scuffing with cast which can mostly heal. I have undone material conversions and taken them back to gray iron either with cast or continuous cast. Gray cast iron is the ultimate natural lubricity forgiving running surface. Another option to consider is to braze or thermal spray "Navy Brass" on the pistons.