my last broken engine....(probably not least)

[filh]

Well , of course , you'll hear Knocking if engine is revving low , but I'm not sure that you can hear it at high revs. And it seems that knocking sensors are not very reliable (only expensive sensors 500€ like).

That engine was rebuild , but can't say the mileage it was used after rebuild .
No I didn't measure rings inside cylinder, and it's too late now . Block is out as 1st cylinder is scratched .

Yes , only the 1st piston is damaged .

What I find strange is that , even with new rebuilded engine , I got broken pistons (each time between rings). And I'm sure of the work done by the machinist . And it was always new pistons (brand MAHLE).
The last broken one is Toyota genuine .50

I know it would be better to have forged pistons , but I only have small cams ( +/- 252°) and adjustable cams gears .

Regards
Phil