You can buy a crankshaft straightening tool from a lot of places already. It was listed on here a while back.

that shaking means it ain't straight and therefore is out of balance. i would not want to be near it when it lets go as it will likely be one hell of an explosion.

I have used a hydraulic H press with the shaft on a pair of v-blocks. This is the way they are straightened when they are taken out of the heat treat furnace When they are made.
spinnerken

spinerken: I think I know what an H press looks loke. Can you use that with the crank installed in the engine. I am going to goole H press and see what come up. As far as buying a crank straightening tool to expensive.

If you have to take the crank out of the engine you have voided the economy of straightening the crank rather than buying a new mower or engine. If you have to go to those lengths, you may as well do an overhaul while you're in there. I don't know of any current lawn mower engine that's worth doing an overhaul on unless you can do it all yourself. If I bent a crank, I would either replace the engine or mower, depending on which mower.

Ditto on what saxman said.

C'mon guys. He's just trying to do some backyard engineering. Saxman....Voided the economy. Whatever...I say go for it. That's how American ingenuity got started years ago. It's how we crossed the hedgerow country with our tanks in WWII.
davidandkasie , It won't fly apart and it won't blow up.

My first lawn mower, a 20$ used, suffered from a bent crankshaft. I used a hammer to straighten it. It lasted a few more yrs until something else killed it

Well so far no ideas or tools we can use other then a hammer and maybe a 10 foot pipe and simply reef on the pipe till we have a so so straight shaft.

This what I found on the Web. Yet another hammer method

I have straightened them for 30 yr.and without taking it apart. You do have to take engine off. I have a peice of railroad track about 8 inches long. Lay the track on its side. If you can visulise how that looks you have a real good solid narrow heavy peice of iron sticking up. Take the engine and lay the crankshaft on this as close to the engine base as you can. Turn the crankshaft untill the end is at its highest point. Take a heavy ball peen hammer and give it a good whack. Keep turning the high place up and whack. You can use anything and put next to the end of the crank as you turn to see how much runout you have. I've done this many times with simple make shift tools to mesure with. You can get fancy and use a dial indicater, but it isn't nessessary.It doesn't have to be perfect to run smooth.Usually 2 or 3 good whacks takes care or it after you do a few and get practice on how hard of a whack moves it how far. I've done many and none have ever broke. I have streightened them that was so bad it was binding and couldn't ever turn crank a revolution. That peice of railroad track works but you can use anything that isn't very wide to support the crank.

"Ditto on what saxman said."

+1 for sure.