Conrod...when's time to change?

[jasoncsc]

Hi guys,

My CMB 21 LS, after about 20 tanks, I noticed that the big end of the conrod have a little "freeplay" that when I turn the piston to TDC the crank could be turned a little bit without the piston moving due to the freeplay. I noticed that the copper bushing of the bigger end of the conrod is still round shape. Should I change the conrod now, or it could still be used?

Thanks!

 

[Kevin_R_Clark]

Hey I certainly am no pro, but going to give you my opinion anyhow. The loser the better as long as the bushing is still round. My limited experience tells me these engines will start vibrating when something is wrong. I have a couple of engines that I questioned the old timers in our club about that have the same symptons as yours, ran them all last year with no problems.

 

[[email protected]]

The shear number of 20 tanks sounds like a lot but actually that's about 4-5 days of playing at the pond. As for the slop if it's really lose I'd replace it now. Better to replace that then the entire P/L, rod, etc......

 

[TimD]

Measure the crankpin and then measure the rod bearing. While measuring - check them for roundness. List your findings here.

Personally I replace the wrist pin whenever I replace a rod too. You might also want to look at the piston where the wrist pin sits to make sure it has not worn in that area also.

Tim.

 

[izitbrokeyet?]

Are you certain the slop is in the "big end" ? All but the newest CMB LS are notorious for a bad connecting rod at the "small end" because it's not bushed. If you have an older CMB .21 con rod without a bushing on the small end I’d replace it because it will fail from fatigue.

Just checking,

KB

 

[jasoncsc]

My LS conrod is the latest batch with bushing on the small end. I finger checked the piston with the con rod with my fingers and see that its quite fit. The way I saw "freeplay" on the bigger end of the conrod is that after cleaning the engine I put a drop of lubricant on the big end of the rod and the crankpin on the crank, and when I turn the crank by hand by a little movement I could see that the surface of the lubricant drop got deformed and would be sucked into the small gap between the big end busing and the crank pin. The big end would slant at a very very small angle before it is moved, and without that drop of lubricant it would be very minor for bare eye to catch the slant.

BTW what 's the "acceptable" gap between the big end of the conrod bushing and the crankpin on the crank? I have a digital caliper so maybe I could measure both to come up with a figure to check.

Thanks for your input.

 

[Guest]

I think your rod is OK as all thrust on a two cycle rod is on the top side so the clearance is always on the bottom.

 

[Scott Schneider]

Conrod clearance for crankpin to rod is fine when between @ .003 to @ .008" and looser is better than tight on a bushed rod. That little of fuel use I would think your just fine. typically I get 2 seasons of RACING out of a .21 rod in my CMB and MACs. Very tough to wear out! Scott