Datsun Roadster Forum-311s.org

A 311 buddy asked us to elaborate on exactly where the cast roadster pistons crack. It is almost always the same place. A picture is worth a 1000 words, so here is about 5000:
Note those are 5 different pistons, from 2 sets, so it's not an anomaly. These cracks locations are specific to the OEM roadster 2L and 1600 design-and obviously any aftermarket piston that copies the design specifically. From an engineering perspective it's a fundamentally flawed design.

Ouch...

Detonation a possibility back in their day
It at least had 94 octane
Also decelerating on 20w50 cold

Agree, detonation would be the nail in the coffin, but it's really a design issue. All the R,H,U cast pistons crack. Lots of better engineered AL pistons never crack.

One of the (many) problems is the difference in thermal expansion between the steel pin reinforcement and the casting. The aluminum expands at a much higher rate and degree than the steel, so the joint is always under internal stress. Literally every time it goes through thermal cycles.

A good analogy is bonding fiberglass to steel body panels, a few thousand days in the sun, they will crack every time. It's just a matter of time and physics always prevails.