Every time we rebuild an engine, we want to know why it failed. Not just what broke, but what caused it. This one made us work for the answer.
The engine came in running heavy at idle, with no real knock, and it ran fine at high RPM. We pulled it apart expecting an obvious culprit, something like a broken ring. Usually a stripped engine tells you straight away what let go. This one didn’t.
Then we found it. The aluminium skirt of one piston had melted around the area where the gudgeon pin sits, the pin that connects the piston to the conrod. Something had got that piston hot enough to melt it, so we kept digging to work out what.
The answer was the piston squirter, the little jet that sprays oil up onto the underside of the piston to keep it cool. Inside it sits a small non-return valve that opens and closes with oil pressure. We checked all four, and two of them were sticky at the start of their travel before they freed up.
At idle there’s a bit less oil pressure, so that sticky valve probably wasn’t opening fully and wasn’t sending enough oil to cool the piston. At high RPM there was enough pressure to force it open properly. That lines up exactly with how it failed. The engine was left to idle and warm up for five or ten minutes before going to full RPM, and that idle period was cooking the piston.
Working out the why is the whole reason we strip these engines so carefully. Once we know what failed and what caused it, we can stop it happening to the next one, and we can pass what we learn on to other customers.
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