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Discussion Starter #1
I just bought a 10546 yesterday. Real nice mower. Blown engine. You can see the damage here:

http://s1012.photobucket.com/albums/af246/melroseplant/Lawn-Boy 10546/

The rod and the piston are destroyed. I have another F long block (or short block--I guess they're the same with an F-engine). It's from a 7268 that I dismantled for parts. I never got it running due to lack of lack of good carb, along with other issues, like cracked castings for mounting the shroud. It turns over easily, and ran two years ago, according to the previous owner.

Here's the dilemma: Should I swap the whole 7268 longblock for this trashed engine, or should I split the case on the 7268 and mix-and-match parts?
 

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Well, you need that crank to keep the pulley/drive action working, so I'd only reuse the rod from the 7268. I'd opt to replace all bearings, seals and rings with possibly a new piston considering the condition of the other donor. A quick hone job, and you should be good to go. For less than $100 you'd have basically a new engine. How did this grenade on you?
 

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Discussion Starter #3
I don't really know, since it didn't blow up on me. The P/O said he was mowing with it last year, and it suddenly and violently stopped. Except for the piston and the rod, everything looks fabulous. I figure either a rod cap bolt failed, or the rod itself just snapped in half due to porosity perhaps????
 

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Good point as that and striking an hard object are ususally the only things that cause this kind of failure.
:ditto: echoman that's just what I was thinking if the crank isin't bent I'd say it was running WAY to fast!
 

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Discussion Starter #7
The previous owner bought it new over 10 years ago, and he says it was the best mower he ever had. He hates the Toro he replaced it with.

Here's an interesting side note: His dealer recommended switching to Opti-2 the year before. He says he's not sure, but I could see by his tone of voice and his facial expression that he pretty much blames the non-Lawn-Boy oil for this failure. I told him Opti was good stuff, and it was VERY unlikely that it was the cause. I'm not sure he believed me.

Given the man's long ownership of the mower, and he obviously took excellent care of it inside and out, how might it have been running too fast?
 

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Years back when the F engine was introduced, LB had a problem with the bottom of the throttle shaft wearing out, and that would release the butterfly, causing the engine to overspeed and blow up. It was a very common failure at the time, and almost cost LB their commercial cutter following.
 

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Judging from where the connecting rod broke, I don't think the cap bolts were the culprit, just looks like the conn rod broke at it weakest point. I say like echo says, new conn rod and bearings, cylinder hone and you are on your way. I would just make sure that you address the overspeed situation to make sure that doesn't happen again once you get it all back together.
 

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Take another look at the pics. That engine seized, notice the piston pin support missing. The rod didnt break until it was banging around in the cylinder. F engines use forged rods which are much stronger than cast duraforce engine rods.

On your 7268 short block. Did you put it together? If not, check it over well. Like Echo suggested, new rings, bearings and seals in whatever you use, and loctite the rod bolts.

When you reduce the amount of oil that the engine is designed to be run on, you risk the chance of catastrophic failure. 32:1 is 4 ounces of oil per gallon. In the older designs the amount of oil that is present sets the tolerances
 

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I question whether that is a forged rod. Look at the numbers on the piece of the cap, it is not possible to forge numbers in that direction. If strength was the issue, it would have a forged steel rod like a chain saw. There is no benefit to a forged rod in a low-speed engine where it is always loaded one way.
 

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Discussion Starter #13
Oh, there's no question about that--it's a cast rod. Maybe I'll never be sure about the cause of its failure. It's just curiosity.
 

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Discussion Starter #16
Update on the mower.

I spent quite a bit of time on the crank, and I think it's acceptable. The thrown rod put quite a few dings in things.

My donor engine is a mixed bag. For some reason, the crank looks dinked up, but that doesn't matter, since it hasn't got the hole for the setscrew for the self propel pulley anyway. I'm really good with a drill, but not THAT good. That would be a machine shop job.

The cylinder from the old 7268 STILL HAS THE CROSS-HATCH MARKS. I think maybe it's a replacement cylinder. The bearings on the 7268 crank are filled with grease-mud, the big-end needles are in excellent shape. The wrist pin bearings are awesome. The crank bearings from the 10546 are just as nice as the connecting rod bearings from the 7268. Wish these engines could talk.

So here's the plan: Get a new piston and rings, get new seals. Put it back together. I have enough beautiful parts from each mower, I think it will work just fine.
 
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