With the release of the 2018 STI RA, I was curious what changes were made to the internals of the engine. Subaru had indicated that new pistons were installed, but didn’t specific much else in terms of internals.

I ordered a 2018 RA engine block about 2 months ago, and at that time there were no spare engine blocks in the US. It was eventually ordered from Japan and arrived at the end of last week.

On first glance, the block looks almost identical to the existing EJ257 motor. The cases is the newest 705 case, the crankshaft has the nitride black finish, and the piston tops look very familiar.

I disassembled the engine and first took a look at the pistons, since that is something documented as being ‘improved’. The pistons are very similar, and both appear to be machined cast parts.

The most obvious difference is the construction of the bottom of the piston. The older EJ257 piston does not fully enclose the pin boss, while the new construction has significantly more material around both of the pin sides of the piston.

From the side, you can see the difference in material around the pin area. Construction and dimensions of the ring and ring area looks the same, with the exception that the upper ring area is fully coated on some of the older ones.

While not easy to see in these pictures, on the bottom side the depression right at the center of the piston is less, and the material is thicker. I would estimate the piston is between 1 and 2mm thicker going from the top to the bottom surface.

Other dimensions appear to be the same. Compression height, ring position, etc are all the same.

I did a quick CC measurement and they appear to be pretty close. I’ll do a more complete measurement in the next few days.

The crankshaft appears identical to the 2016/2017 crankshaft, with no special oil modifications.

The rod also looks to be the standard previous year EJ257 rod. You can see the slight difference in design between the original EJ207 rod, the early EJ257 (2004), later EJ257 (2008), and the 2018 STI/RA rod.

One interesting difference in the piston is the presence of a ‘Hitachi’ logo in the casting. This did not appear in previous pistons (which has a small Subaru mark in that location).

I will measure all of the factory bearing clearances to see if there is any difference there.

Thanks to Clark Turner for nudging me to look into this!