The BSA A7-A10 Forum

Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: roadrocket on 13.07. 2011 20:55

Title: (Too) wobbly con rod?
Post by: roadrocket on 13.07. 2011 20:55

Hello all

I have dismantled the old A10 engine because I feared a line near the crankcase mouth to be a crack. It turned out to be a superficial scratch... Better safe than sorry they say. But now I'm there, I decided to check on things in general, and the only thing amiss seems to be a wobbly big end on the timing side. It has no radial play, but will wiggle when wiggled. The drive side big end moves smoothly a bit along the journal, but only in a straight line, whereas the timing side one can be waved a bit when stationary. I would imagine that this is not good? I have not taken the rod off yet, but what do you folks think; will I be likely to get away with just replacing the shells?

Regards, Otto in Denmark

Title: Re: (Too) wobbly con rod?
Post by: muskrat on 13.07. 2011 21:54

G'day Otto,
                A little rock (wobble) is normal. If you hold it lightly to one side of the journal and rock it I would say any more than 5mm at the little end will need new shells. The crank pin will need to be measured, 1.461" for sml journal, 1.688" for lge journal. Re-grind sizes are 0.010", 0.020", 0.030", 0.040". If the pin has worn any more than 1-2 thou it will need grinding. So for a std sml journal measures 1.459" it is past it's use by date.
Cheers

Title: Re: (Too) wobbly con rod?
Post by: roadrocket on 15.07. 2011 10:47

Thanks  Muskrat

I'm safe I think. The whole bottom end is actually holding up quite well after the rebuild it recieved nearly ten years ago.

Otto in Denmark

Title: Re: (Too) wobbly con rod?
Post by: RichardL on 15.07. 2011 18:22

Had the same problem a few years back and posted some videos, starting here: http://www.youtube.com/watch?v=5IYligiWUac .  Also, we had a lot of discussion about it, here: http://www.a7a10.net/forum/index.php/topic,473.msg2511.html#msg2511 . (Most of page 2 of the discussion is about the little oil hole in the drive-side rod.) The problem was due to the machine shop deciding to hone inside the rod big ends without, first, machining the landings. The machine shop didn't want a flood of bad press from me, so they replaced my original rods with a billet forged set. The middle video in the series is after I tried to file down the landings, but I decided that was not a good fix, as it meant egg-shaped big ends. The question of how much the small ends should move with correct bearing clearances is interesting and mathematically calculable but, for me, it would be easier to draw it in CAD, which I might do if I get time.

Richard L.

Title: Re: (Too) wobbly con rod?
Post by: A10Boy on 18.07. 2011 10:23

In my view, while its apart you might as well remove the big end caps, check the crankpins and replace the shells if the crankpins are in reasonable tollerance, like less than  about 1. thou out of round.. Any more than this, the crank will need a grind - if that is necessary,  you might consider replacing the TS main bearing bush.....

Cheers