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Motorcycle Discussions => British Bikes => Topic started by: barthelmurray on July 30, 2009, 09:14:01 PM

Title: smokin' 1971 650 bonnie
Post by: barthelmurray on July 30, 2009, 09:14:01 PM

Question: can loose bearing at the connecting rod big end cause ring gap to move..rotate..line up and cause oil to pass and cause smoke?    I installed new cylinder, pistons and rings and got smoke at one cylinder, tore it down and the only thing I saw was ring gaps on two compression rings were within an inch or so of each other, I'm certain they were spaced correctly, but I also see that the bearing at the bottom of the rod has more play than I would like but since I was not sure what was "too loose" I did not repair bearings yet. I am now in process of replacing those rod bearings but my manual does not give any info on what is "too loose". Thanks for any help. Phil.

Title: Re: smokin' 1971 650 bonnie
Post by: Goldie on July 31, 2009, 12:06:11 PM

Have you checked the valve guide clearences?

Title: Re: smokin' 1971 650 bonnie
Post by: L.A.B. on July 31, 2009, 01:51:46 PM

Quote

I installed new cylinder, pistons and rings and got smoke at one cylinder, tore it down and the only thing I saw was ring gaps on two compression rings were within an inch or so of each other, I'm certain they were spaced correctly,

Apparently piston rings can, and do, rotate.

After all, what is there to stop them if the rings are not pegged as two stroke piston rings usually are (do a web search of "piston ring rotation") which does seem to make the careful spacing of rings when rebuilding an engine a bit of a pointless excercise!



 

Quote

but I also see that the bearing at the bottom of the rod has more play than I would like but since I was not sure what was "too loose" I did not repair bearings yet. I am now in process of replacing those rod bearings but my manual does not give any info on what is "too loose".

Then I think you should get a better manual?

 If you can feel any up and down play at all in the big end bearings when they have oil in them then action needs to be taken to rectify that. If you mean conrod side play then there should be 0.012"-0.016" (0.305-0.406 mm) side play?

The bearing diametral clearance should be between 0.0005"-0.0020" (0.0127-0.0508 mm) and the big end bearing journal diameter should be between 1.6235"-1.6240" (41.237-41.25 mm).  

Title: Re: smokin' 1971 650 bonnie
Post by: barthelmurray on August 04, 2009, 09:02:38 PM

THANKS FOR THE FEEDBACK , I WAS HOPING TO GET BY WITHOUT GOING INTO THE BOTTOM OF THE MOTOR SINCE THIS WAS JUST GOING TO BE A LOW MILE FIX UP BUT SMOKE SUCKS..ANYTHING TO WATCH OUT FOR ON THE BEARING REBUILD PLEASE ADVISE, THANKS AGAIN, PHIL.