A friend ran his (maybe 3.5 hp) Briggs/Stratton push mower over an
unmovable (sawed off) metal standard. CLUNK!
Didn't do much to the blade, but the shaft looks bent. Will turn OK
for 180 degrees, -very- tight for the other 180.
Can such a shaft be straightened? If so, how?
While nothing is impossible, strightening a crankshaft bent enough to
create that "tight" result will cost your friend a lot more than buying
and installing a new engine or just purchasing a new mower.
Replacing the crankshaft with a new part (and new gaskets, keys and
seals where required) is a possibility, but only if your friend knows
what he's doing, has all the proper tools at hand,and doesn't place a
dollar value on his time.
A big chunk of Sched 80 iron tipe the right size to fit over the
shaft, with the blade removed, can quite easily straighten a B&S
crankshaft. Taked a couple of tries, usually. A short chunk and a
light sledge hammer can do the same job.
All manufacturers and most mechanics do not recommend it. However, it can be
and is done. Some engine shops have a tool/fixture for doing it and others
will not attempt it. I have straightened several, some while in the engine
and some removed, and have never had any problems with them. There is a risk
that the shaft might be already cracked or be cracked while straightening
and that is why it is not a recommended fix. A shaft bent severely enough to
bind would not be a good candidate for straightening.
"It takes a special tool/fixture and there are (obviously) risks involved"?
I had to ask: for all I knew, there might be a splined connector or some-such
between crank and blade-shaft, which could be easily replaced. No such luck,
I'm pretty sure the blade is connected more or less directly to the
crankshaft but I've not yet determined if the threaded fitting it attaches
to is removable. It looks like it should be. In any case you can often
find people giving away lawnmowers with those engines (or even better
engines) on Craig's List so as long as you can get one that doesn't have
another bent crankshaft you could probably make one good engine from the
The mowers I have owned had a hub on the motor shaft that is keyed and
the bolt goes into the shaft holding the blade and the hub.
I have hit plenty of stuff myself and simply replacing the blade
usually makes the shaking go away, at least to a tolerable level.
I hit something so hard with my current mower that I sheared off the
bow tie key that engages the blade.
That is why I know about the hub ;-)
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