Such a violent blow is involved that it sometimes bends the crankshaft.
This can be fairly common with these blades. Sometimes the crank can be straightened, sometimes it can't.
I agree with all of grumpy's points regarding bar blades here!
FWIW, the first major local manufacturer to introduce bar blades to the Oz market was Masport, at about the same time they brought the Iron Horse 2-stroke engine on to the market, in the 70's.
A compulsory [but reasonably priced] special service tool, that their dealers had to buy at the time, was a 'crankshaft straightener'. It was basically a steel channel 'bolt-to-bench' base; with a guide rail and two loose vee blocks, a small screw press mounted above it, and a 'long lever' runout indicator.
Worked quite well, so long as the shaft wasn't too badly bent to get it out of the engine!
There are 'in-engine' tools to do this job available, these days -
http://www.milfordpower.com/R1709-Mighty-Midget-Crankshaft-Straightener-p/r1709.htmFor sharpening these blades, do as grumpy says, especially the balancing bit. If using an angle grinder, a flap disc will give you less problems with heat buildup than a grinding disc, though it won't remove metal as quickly.
