2000 CR125 Banging! Help!!!

[zawetzel13]

I have a 2000 CR125. I just rebuilt the top end with new piston and rings. I was breaking it in and had about 6hrs on it when all of a sudden it started banging like crazy. I shut if off imediately and took the motor apart. I got the whole top end apart and everything looks good. no scrapes in the cylinder, the piston and ring looks good, no metal shavings anywhere. I put it back together and started it up. it still has a very loud banging noise. it does run, but sounds just terrible. I then tore it apart to look at the engine again and still nothing looks wrong with it. One of the reeds has a chip out of the corner but not even big enough for air to go through. could that be causing the loud banging or would it be something in the bottom end? note: banging is constant whether in gear or not, oil level good. no visible signs of wear or anything in top end, powervalve works good.

Anyone who has had this situation or any suggestions please comment.
thanks,
Zane

 

[76GMC1500]

It's most likely a crankshaft main bearing or connecting rod bearing.

 

[zawetzel13]

Hey thanks GMC fan. I found out it is the crankshaft bearing. now that I know that i can start tearing the motor apart :(

 

[76GMC1500]

Better start tracking down a cheap crank. The banging may have knocked the crank out of true, built up construction, and it will most likely be cheapest and easiest to throw a used crank in there. Get it measured before you throw it out, though. I give you a 75-25 chance it's straight. A good machine shop with the proper fixtures could true it quickly and easily for cheap. But, without fixtures it is a time comsuming process and time costs money.

 

[zawetzel13]

come to find out it is the rod bearing that is bad. I did order a new crankshaft. I looked all over the internet and found a used one for $150. new ones are $200, and I found a brand new weisco crankshaft on **** for only $130 + shipping. about $145 total so i went that route.
thanks for the help though.
zane