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I have my 17hp Briggs 31C707 all torn down after running it with a knock for a season. The knocking started recently after the head gasket was replaced last June and I have been running it since then. Its not used for mowing however it has seen heavy abuse from the snowblower attachment moving wet snow. It has never been low on oil and has been running on 5w-30 all winter as the manual suggests. Not sure where the knocking is coming from but it sounds like it could be the counterweight slapping around. What should I be looking for to locate the knock? I noticed the piston skirt and cylinder wall has a fair amount of light scoring; in addition the sump cover has scoring where I believe the counterweight rides. Is this scoring something to be concerned about?

In pic 3 it looks like you have some stress cracks around that pin...... The other pics show scoring on the wall of the cylinder and the piston....... I really can't make out the counter weight marks as you said on the sump though....

A loose piston wrist pin or wear in the crank journal and connecting rod surfaces can cause that......

You can put a micrometer on the crank and then check the rod assembly for specs and that will tell the story.....

Briggs has the specs for all that on their website or I can give them to you....

Something is wrong if you have a knock for sure........ That's what I have to offer Mon Ami according to the info you put in so far.............

Let us know Mon Ami ~!~!

Counter weight failures out number piston, con rod and crank shaft failures 20:1.
So check that bolt very carefully and also pay attention to the short rod on the counterweight.
That appears to be too much scuffing on the side of the piston & those groves look to be too deep, the sort of thing you see from dust being sucked into the engine, or running with very dirty oil.

The scratching and marks look like dirt intrusion introduced from the lower end of the engine, not being sucked in through the intake. Which could also explain the scratching around the counterweight slide. I would be interested in how much out of round the cylinder is. Possibly piston slap. Make sure the piston wrist pin isn't sticky causing a piston slap.

I'm almost positive that's a crack in pic 1.

If that is a crack, I can't imagine the block lasting much longer. If that's caused by the counter weight slapping, even fixing that problem, you'll still have to fix the crack. Maybe welding it will work? What say you Bert?

If you mean in the bottom right corner of No 1 that looks like a casting flash and the counterweight should never get that close to the bore in any case.
And yes Rivets looks like the scoring has gone bottom to top like some crud stuck in the oil scraper.
In any case I can't see that bore being any use with all those score marks.

This engine was running fine prior to it being apart.. Based on the replies, is it even worth assembling as it seems it won't last much longer in the current condition? Is boring out the cylinder an option?

I don't have a micrometer so am not able to check the specs for the crank at the moment.

"So check that bolt very carefully and also pay attention to the short rod on the counterweight." Not exactly sure what is meant by this..

The wrist pin is not sticky; the piston moves normally.

Picture 1 doesn't appear to be a crack however I may have found some actual cracks:

RISC said:

This engine was running fine prior to it being apart.. Based on the replies, is it even worth assembling as it seems it won't last much longer in the current condition? Is boring out the cylinder an option?

I don't have a micrometer so am not able to check the specs for the crank at the moment.

"So check that bolt very carefully and also pay attention to the short rod on the counterweight." Not exactly sure what is meant by this..

The wrist pin is not sticky; the piston moves normally.

Picture 1 doesn't appear to be a crack however I may have found some actual cracks:

Click to expand...

Looks like casting flash. Factory didn't do a very good job of cleaning up the block prior to assembly.

If you would examine the inside of the cylinder, you would be able to tell if it was cracked or casting flash. I too believe casting flash. You said the problem started after replacing the head gasket. I would be interested to know if the knock was at all engine speeds or just high or low. If valves were not set right, you could have a problem there.

UPDATE: Decided to scrap the briggs... Replacing it with the stock Kawasaki FC420V for my JD LX176... This will be its third motor!

RISC I would save that Briggs for parts if you have room somewhere.... Parts are valuable for those.....

Plus Tard Mon Ami ~!~!