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?

No signs of markings inside the bore, the markes you can see in the photo are oil streaks.

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.

Thanks again grumpy, I'll get onto to the test and report back when its done.

Ok test completed, and the results are as you explained, it suddenly has a lot more compression after I put in the oil.

That is a relief: it means there is definitely leakage past the rings, and the piston has to come out - no risk of unnecessary dismantling. That requires removing the engine from the mower. The most difficult part of that is freeing the drive shaft, by sliding the clutch along the shaft toward the chain-case on the left side of the mower. The right (engine) side of the clutch is usually held by a captive clamping bolt, which just needs to be loosened, and can't be removed anyway because it's captive. The retention of the left (chain-case) side of the clutch is obvious and less tricky, though at this moment I don't recall what kind of screw/bolt it is. You'll need to disconnect the speed control cable at the engine end. The engine is held down by four bolts, which are easily visible.

Sometimes the clutch is rusted onto the shaft and it's a bit tricky to get it to slide along. Soaking the shaft/clutch hub where one slides into the other with solvent is the first step in that process. If the shaft is rusty it will need to stand overnight to let the solvent take effect. Various solvents will work - Penetrene or WD40 for example.

I think I can handle all that, I'll get the new rings during the week as well as a gasket kit. I'll keep you updated on my progress although I may not make a start until next weekend.

Thanks Peter. The B&S overhaul manual has all the information you need, but you can get any help you want here, and of course we'd like to know the details all the way through - it is not only an interesting case, but your fully original, thirty year old mower is so pretty and new-looking that it is photogenic.

Thank you for all your help so far, its been very much appreciated.

It hasn't been a one-way deal Peter - if you keep us informed, ask any questions you wish, and post photos as you sort out your mower, you'll be making an important contribution to the Outdoorking archives. That can be useful to a lot of people for a long time to come. We've only been involved in half of one step in the process so far, and would certainly welcome being part of the whole thing, including your set-up adjustments on the mower itself.

Hi grumpy, well said, and a good outcome thus far.
As this is a continuation of an old thread and Peter has decided to rebuild the Briggs, this topic is now complete, so I will close and lock it.
Peter, for your next post re your engine, can you please start a new thread in "Questions on Briggs and Stratton Engines".
If a question arises re the mower chassis itself, please post in "Questions on Lawn Mower Frames" Scott Bonnar cylinder mowers.