What causes rod bearing failure?


New Member
Local time
7:23 PM
Dec 2, 2023
So I split my engine after observing an atrocious knocking sound and discovered that probably 3/4 of the needle bearing connecting the rod to the crank was destroyed and sitting in the bottom of the crankcase. I'm not concerned with repairing this engine; a new one is already on the way. My question is if any of you who are a little more versed in these engines could tell me what likely caused it to spin(?) that bearing.

Some history on the engine, I've had it for almost 2 years and ride this bike multiple times per week. I cut the skirt of the piston to match the intake port at TDC and raised the exhaust a little bit about a year ago. I've ran the stock carb with the stock jet which I believe is on the rich side, and the bike does 4-stroke pretty significantly above 6500ish rpm. I've also been running it with no air filter because it wouldn't fit on the bike, which I understand is stupid, but knowing what I know I don't think that would have caused the crank bearing to spin. The only other thing done to it was a high compression head and I ran it on 93.

I was full throttle topping out (around 6k-7k rpm) for a good couple minutes before it started knocking, and I'm not opposed to the theory that I might have just been stupid and blew the motor that way. No matter the case, if any of you could point to something that I should avoid on the next engine I would appreciate it, thanks for taking the time to read & respond.
I've got one engine, a cheap Seeutek, that I put over 6,000 miles on over a 5 year span of time sitting on a shelf right now that still looks almost pristine on the inside
I don't doubt that they can last a long time when treated correctly. This one still ran surprisingly well for the amount of wear on the inside

Those ports weren't beveled before it was put into service.
Is that a mistake that I made in my porting? I don't doubt that it could have been done better; I've never ported a piston motor before this one, just rotaries :)

Looks like you had some fun with it for quite a long time. So all in all pretty good.
For the price I think 2 years is a bargain.
The bike is definitely a blast to ride and I agree that I got my full $60 worth out of it over the last 2 years lol

Ive got a good solution for your air filter. I use a stubby cnc manifold.
I'm not sure if that would work for my application because there isn't a lot of clearance vertically either. I have the carb rotated probably 70 degrees or so to run the throttle cable off to the side because the carb is contacting the frame at the top of the "engine compartment." The motor just barely fits the bike. It's hard to explain without just showing it, so once I get it put together with the new engine I'll get some pictures of it so you guys can see what I mean.