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I have a Troy-Bilt mower with a 17.5 HP B&S engine that broke a rod. I replaced all the internal engine parts and installed the engine back on the mower. I had difficulty getting the engine to run. Sometimes it ran for a few seconds, sometimes it would backfire through the carburetor. There was no consistency.

I found an identical mower on Craigslist with a locked rear end and good engine for a good price. I brought it in anticipation of just using the engine.

I decided to first compare the two side by side to see it I could find the problem. What I found was the key on the bad engine was sheared so the flywheel would not always be in the same position which explains my exact symptoms.

I replaced the key on the bad engine and it ran for about five seconds before the collar on the flywheel cracked. Since I had nothing to lose, I drilled 1/4" holes through the collar into the shaft and inserted roll pins. Again the engine ran for about five seconds and the roll pins broke. I even thought about welding the flywheel to the shaft.

I removed the flywheel from the good engine just to see how it would fit on the bad engine and did not see and differences. So I put it back on the good engine and used a new key because the old one looked worn. The new key was a little snug so I hammered it in. I installed the top bolt and started the engine. To my surprise, that flywheel cracked also.

Now I am at a total loss and need help.

It looks like the crankshaft keyway is worn which may allow the the flywheel to rock back and forth slightly can crack a flywheel. the other possiblities are overtorqued flywheel bolt, flywheel hit with a hammer when being removed, loose flywheel bolt allowing crankshaft to rock back and forth slightly (least likely, would normally shear on startup or shutdown).

First .... WELCOME to LMF....

There are a couple guys here that work on lots of engines, they may have came across this type problem.
The only thing that I can think that may break that cast is something on the key way being too tight and interfering with the fit, to the point it would crack it... I guess too tight with the bolt on top could swedge it into cracking, the shaft is tapered....
Perhaps too loose could allow it to rock back and forth but I would think the key way would shear before the casting would break....
I'm left with just a guess of too tight and the bolt just pushed the casting down the taper till it broke out... :smile:KennyV

I would vee that out and braze it, and for good measure fabricate a band out of a piece of pipe or something similar and braze it around the repair for good measure. The brass weld should be quite a bit more malleable than the brittle cast and it should hold ok.

What did you use for a new key? B&S uses a aluminum key made just for that engine. Looks like you used the wrong one. That would be the only thing that would do the damage you have done. Unless you intalled the key wrong? I have installed a lot of B&S fltwheels and have never seen any thing like this. I had a repair job that the owner installed a steel key once. The flywheel and crankshaft were both junk.

The key I used was a 3/16" square x 1" that I got at Lowes. I'm not sure what material it was made from.

I you bought a piece of stock from lowes it was most likely steel, wrong thing to use, need a aluminum key,

One other thing, is the washer on the bolt holding the flywhell down seperate from the bolt? if so i almost looks like it is on upside down, meaning it is concave and it should be flipped as it won't tighten properly. too late in the day for me to remember is it is seperate or one piece.

yama0918 said:

The key I used was a 3/16" square x 1" that I got at Lowes. I'm not sure what material it was made from.

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Well that was sure your problem. The key is made by B&S and you have to use the right part number key. It is made to fit B&S only. Part number 222698S less then 2 dollars. ANYTHING ELSE IS JUST GOING TO DO DAMAGE AS YOU CAN SEE Now you will have to buy a flywheel to get your timing right. Don't try to repair the old one.

reynoldston is Right...
You Must use the correct key... It is intended to shear (made of soft aluminum) can Not use any other type key...

The key stock you used was not intended to shear... I didn't realize you substituted a different type key...

You should Not try to repair the flywheels ... replace them. :smile:KennyV

So glad this all came up I was about to do the same thing.

Sent from my iPhone using LMF

Steel key will do it. I still think that even with the steel key it wouldn't break until it either hit something or was not properly tightened down.

I understand that a steal key will not shear if the blades hit something. However, on my good engine, all I did was remove the flywheel and then reinstall it using the wrong key. I did not drive it or engage the blades. All I did was start it and a few seconds later the collar broke. The key was not protruding above the collar. The top bolt does not have a washer and was not over tightened.

yama0918 said:

I understand that a steal key will not shear if the blades hit something. However, on my good engine, all I did was remove the flywheel and then reinstall it using the wrong key. I did not drive it or engage the blades. All I did was start it and a few seconds later the collar broke. The key was not protruding above the collar. The top bolt does not have a washer and was not over tightened.

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Your money and mower do what you want. With a steel key you will not only damage the flywheel you will also damage the crankshaft. The best lesson would be do it and find out for yourself what I am talking about. B&S designed that key for a reason. With the right B&S key you should tighten the flywheel down real well.

I bought two used flywheels and got the correct B&S keys. After reading additional forums, I realized what the problem was. The main problem was that I did not tighten the flywheel bolt enough which would cause the flywheel to slip. If I had the B&S key, it would have just sheared. Because I used a steel key, it cracked the flywheel. I was mistakenly under the impression that it was the key that keep the flywheel in alignment with the shaft. But it is the torque of the bolt that pulls the flywheel against the tapered shaft that holds the flywheel in place.

After installing the replacement flywheel and key on one mower, I torqued the bolt to 55 foot pounds. The mower started right up and ran correctly. I then did the same to the mower with the engine I just rebuilt and that one started and ran also. I stopped and started that engine twice and on the third time the engine would not start. The engine was locked up.

I removed the engine and started taking it apart. I removed some parts so that all I had left was the crankshaft and piston and it was still locked. I removed the bolts from the bottom of the rod and now the crank would rotate and the piston would move. The cylinder sleeve is smooth as can be. So it appears that something was just binding. I reassembled the engine but when I start to tighten the bottom case, I cannot turn the flywheel anymore.

Any ideas???

Are you checking the oil clearances on your rod and main bearings? It should have at the lease .003+