OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Original Engine - Rebuild or Replace?

There are two ways we can go now: just assume the rings are shot, and take out the piston; or put the cylinder head back on temporarily and test the ring leakage. You don't need to put the air cowl back on, we won't be starting the engine.

The test consists of pulling the starter cord and memorising what it felt like, then squirting some engine oil in through the spark plug hole, putting the spark plug back in, and pulling the starter cord again. If it suddenly has a lot more compression after you squirt in the oil, it was leaking air past the rings when you did it without the oil. If it leaks, the oil will find its way into the sump before long, so you can repeat the with-and-without oil tests as often as you want. Don't put a huge amount of oil in, or you'll simply fill the combustion chamber with oil, which raises the compression ratio and makes it feel as if it was leaking when you didn't put in the oil.

Because your symptoms are a bit odd, I recommend putting the head back on and doing the oil test. It can ruin your day if you take the piston out and discover the rings are better than new.

I haven't had exactly that happen to me, but I've done something similar but worse. I once laboriously put the engine back in an old PA Vauxhall after replacing the rings - a horrible job, the in-line 6 cylinder engine had to go in tilted so the crankshaft was vertical, without the gearbox attached, then you had to tilt it back to horizontal and lower three spacers in between one of the engine mounts and the chassis rail and pick them up with engine mount bolts, one at a time, in zero space, unable to see what you are doing. The oil pan (sump) couldn't be removed without taking out the engine. I'd done all this, fitted the gearbox, and was just starting on the wiring when my father walked up to me with something in his hand and said "What's this?" He was holding the main oil feed line from the oil pump, which goes inside the sump - I'd missed putting it back on before I fitted the sump. I had to take the engine out again. Not a good day.