The Stovebolt Site Forums The Shop Area Engine Shop Bent Rod

Background - I'm in the middle of a project on a single cylinder IC engine, Chinese made, that my boys ran without an air filter and ended up bending a rod. Long story short, I have found a new cylinder head, valvetrain, piston, rings. Everything for the rebuild except I cannot find a crankshaft and rod (single assembly). Because it is a single assembly, I cannot get a machine shop to pull everything apart and straighten the rod, it will cost more than the whole project is worth and they can't guarantee it. Question - I'm pretty sure with a torch, some patience, and a vice I can get it 98% straight and back together. It would be real easy to throw the whole thing in the trash can, but just doesn't seem like the right Bolter solution. What do you guys think?

Those integral crankshaft/rod setups will have needle bearings instead of the usual slipper-type insert bearings. More likely than not, the reason the rod bent was the needle bearing tried to seize up on the crank. Even if you do happen to get it straight, I'm betting the engine won't last more than a few minutes. I think I'd be more interested in adapting an engine to whatever the toy it powers that has a better repair parts source.
Jerry

Hi Allen, You should look for a motorcycle shop in your area that works on the old 2 stroke bikes and see if they rebuild their own cranks or who rebuilds them for them. Look for a long time Yamaha,Kawasaki,Suzuki, shop and talk to them. The single cylinder cranks take about 1 hour to press apart, and put back together and true. If the motor looks like a small Honda you may luck out and the Honda rod is the same. Good Luck, Have fun, Brian

Thanks Jerry. It was burning lot's of oil through the exhaust when it locked up. I was guessing that we scored the cylinder walls and piston finally welded itself to the walls, and that is what bent the rod. There isn't any play in the rod bearing. Is there any other way to test those needle bearings before getting too far?

Good Idea Brian. These engines are a Chinese copy of a Honda original. A zillion of them are made, but the factory that made our toy had a version with a slightly different crank that I cannot find. The factory burned to the ground, and they did not rebuild. I've actually got a new crank that I can rob a rod off of. I tried a car machine shop, didn't think about a cycle shop. I'll give it try.

My take on a roller bearing at the rod big end is this:

1) If it seized such that the rod bent, you'll know it. The rod won't freely rotate around the crank - It'll stick at worst and be "chunky" at best

2) If the piston took a hard enough hit to bend the rod, one of the races of the needle bearing is probably brinnelled and destined for failure anyway (crank is the inner race, rod is the outer race). Brinnelling occurs when a load through the bearing is high enough that the needles, or other rollers, actually indent the surface of the shaft or outer race. (If it's there, it's easy to see: marking on one or both of the races (usually the inner) at roller spacing. If it's really bad, you can feel it.) Once that occurs, it's only a matter of time before surface fatigue starts at the 'bumps' and progresses to bearing failure.

If it's a roller bearing rod, it's been hardened to function as a bearing raceway. That surface hardening may just be in the bore, but it could extend to the rod itself. If you heat the rod to straighten it, the case may temper back and lower the strength of the rod.

All of this is conjecture, but they are things to consider. If it is a journal bearing rod, like our 'bolts, none of this is relevant.