The stock 4AC pistons are cast aluminum units. The basic size specs:

Diameter – 81mm

Wrist Pin – 18mm

Compression Height – 30.5mm

Compression height is the distance from the center of the wrist pin bore to the top of the piston. The piston is dished, meaning that there is cavity in the top of piston. This cavity is approximately 10cc in volume.

Stock Piston weights:

Piston with no rings – 270.8grams

Piston with rings – 295.7 grams

Wrist pin – 88.3 grams

These pictures show a used stock piston next to a new Sealed Power aftermarket stock replacement piston.







There are several small differences in the pistons. The major thing that I see is the drain holes for the oil control ring. The skirts are not connected to the rest of the piston except next to the pin bosses on the Toyota piston. It looks quite scary to me. Sealed Power did not do this, they have three small holes for the oil to drain. It looks more reasonable to do this than the slice Toyota did. Sealed Power also added an oiling groove in the pin bore which Toyota did not have. Toyota had a hole drilled through the pin boss that Sealed Power does not have. The biggest issue with the Sealed Power part is that the compression height of the piston was reduced to 30.2mm which is about 0.012” less than the standard part. They assume that you have re-machined the top of the block and perhaps the head as well, so they make the change to keep the compression ratio from increasing, and to maintain the piston to head clearance. If the block and head are not machined, your compression ratio will decrease. I imagine that this is a standard thing that all the aftermarket stock replacement pistons will have. The Toyota pistons may not be like this, buy they cost more than going to a performance piston place and having stronger and possibly lighter custom pistons made to what ever spec you want.