Forum bmwbike.com

I know that one must balance pistons before replacing new ones, but does this also apply to connecting rods.....or is there an acceptable tollerance between the small and big end. I need to change one of the rods on my R69S and the repacement one is 9 grams heavier on the big end and 7 grams lighter on the small end....should I worry?

If you are going to the trouble to balance pistons then balanced rods wouldn't be a bad idea. If both rods are measurable, just start with the small end and match them first. After they are even, go with the overall weight and take it off the large end, or cap.
WARNING-I only need to let you know I mainly do vw engines- and will balance the complete top end like this when my R90/6 needs it. Smooth running.

Yes Pierre, you should worry. Any imbalance of reciprocating masses will magnify with speed, therefore make sure both rods are as near equal weight as possible.

Pierre,

You really need to have 2 new matching wieght connecting rods on your crank, otherwise the engine will vibrate badly & you will end up stripping the engine again. Bite the bullet & get another rod to match the one you have just purchased.

Pete

The rods are supposed to be balanced as a rod set, which includes the rod, crank pin and bearing.

Rod sets come in balanced pairs, but there is a tolerance or variation allowed. I don't remember the number very well, but I think it is 2 grams.

I ran into this in having my R69US rebuilt. My new rod sets weighed exactly the same. But then it was discovered that one of the new rod sets had an R60 crank pin which was about 1.5 mm shorter and would not completely fill the length of the web hole, and so I did not want to use it. When I was then given an R69S crank pin, the rod sets were weighed with 2 grams difference. I was told this was OK, so I accepted it, and the completed engine has felt OK for 6000 miles.

I do not know if the weight of each end of the rod alone, or the complete rod set, is important. I don't think the engine can feel such a difference.

I think only the net weight of each rod set is what is compared. In that case, if 2 grams is the allowable difference in net weight, than your new rod set might be OK to use.