Rod Replacement, Need Piston?

[El_Schaf]

My B&S 17.5 HP (31C777)has a knock that is audible on low idle. It isnt really evident at full power, but if I idle it down it is quite apparent. My plan is to order a new rod/bearing and replace it.

My question:

If I have to remove the piston assembly, will the rings re-seat themselves if I just replace the rod? If not I would just by a new piston/rings as well and hone the cylinder (have never attempted that).

The reason I don't want to buy a new piston/rings (or just rings) is that I don't want to spend the money on a hone (or screw up the hone job) and it does not seem to have compression or burning issues.

Thanks!

 

[StevieC]

When you take the piston out take it out carefully and tape the rings to the piston with masking tape to hold them in the placement they came out in. (Plus/minus a milimeter is ok) and also make sure you put the side of the piston back in on the same side it came out.

If not it will reseat itself from my experience but it can take a while and burn oil in the process. Better just to do as I said above.

Dad and I have taken apart many small engines and reassembled them with little trouble.

If you replace the rod, replace the wrist pin and bearings that ride on the crankshaft.

If it were my engine and it wasn't consuming any or a ton of oil I would just put up with the rod-knock. Won't hurt anything if you don't run low on oil. We have a tractor that has no mowing deck on it and we use it for utility purposes and it knocks like crazy even with 20w50 in it and has been doing so for 10 years. No worries at all!

It's a 15hp B&S

 

[El_Schaf]

Oh Wow, Maybe I will. I just know that if it were to break free, I don't want to ruin the crankshaft (expensive). The rod comes with the bearings, but I don't think it comes with the wrist pin.

What sort of collateral damage will the rod letting go cause if it does?

 

[StevieC]

I have seen B&S engines that have a rod break that do no damage, and I have seen others take out pistons, crank-shafts, and blocks.

It just depends what RPM and where the piston is when it lets go.

FWIW: I have seen brand new engines that have no knocking or out of the ordinary sound let go and throw a rod through the block.

Just because it's noisy doesn't mean you are at higher risk for it to let go. Usually it's because the engine doesn't have oil in it and the parts get overheated that this happens.

I have never seen an engine with a noisy rod with proper care and oil level throw it through the block etc.

Just live with it, most likely you just have a bad bearing and it will do nothing more than be annoying.

EDIT: Do you have video capabilities that you could record a short clip so I can better diagnose it for you?

 

[El_Schaf]

Oh I believe I could get a video. The reason for the knock, I believe, is a combination of a former bad head gasket and neglect by my mother in law. I know it was run with VERY low oil for a period, because I checked it for her last year and observed the issue (at which point I decided that I had to maintain it for her). Later in the year, It wouldnt run, so I took it apart and found a blown head gasket and stuck valve. I fixed the head gasket and valve, got it running again for her, and then her boyfriend bought her a new john deere. She decided to give the mower to me, and now I'm trying to get it tip top as cheaply as possible. It doesn't seem to be using much oil at all with the gasket fixed.

I hope its just a bearing...

 

[StevieC]

As long as it wasn't run without oil and that never is the case in future you should be ok. It will just have an annoying reminder while running the engine.

With the video I will be better able to tell you how bad it is and whether you should replace it.

EDIT: My uncle recently brought us his B&S engine which he had run without oil. The rings were welded to the piston from the heat, the piston had a hole in it and was badly scored.

We replaced the piston, rings, rod, bearings, and wrist pin and put it all back together for under $100.

So it shouldn't be that bad if you had to do the same... But the fact that it's running and not consuming too much oil you might get away with just running a 15w50 in it and keeping an eye on the level.

Some MMO in the fuel wouldn't be a bad idea either.

 

[Papa Bear]

Originally Posted By: El_Schaf
My B&S 17.5 HP (31C777)has a knock that is audible on low idle. It isnt really evident at full power, but if I idle it down it is quite apparent. My plan is to order a new rod/bearing and replace it.

My question:

If I have to remove the piston assembly, will the rings re-seat themselves if I just replace the rod? If not I would just by a new piston/rings as well and hone the cylinder (have never attempted that).

The reason I don't want to buy a new piston/rings (or just rings) is that I don't want to spend the money on a hone (or screw up the hone job) and it does not seem to have compression or burning issues.

Thanks!


My brother was telling me just a little while ago about his 18 hp B&S knocking, he had a hard time finding the fault and it ended up being "outside" the engine. I will email him and see what the fault was again.

 

[Papa Bear]

My brother said it was a loose shield/spark arrestor on the outlet of the muffler.
He said it clanked and knocked like a square piston in a round cylinder.
After he tightened the shield the noise was gone and the old mower runs great again.

He said he hopes your repair is as simple. He too thought it was rod knock originally.

 

[Papa Bear]

My brother also said to check the screws on the carburetor throttle plate, his throttle plate came loose and fell right off making the engine want to go WOT when you started it.