Broken Excelsior Connecting Rod, a post-mortem examination
While riding on the Atascadero Tour, I heard a quiet rattle in the motor when the exhaust cutout was closed. I stopped and checked the valve clearances, they were ok. The rattle wasn’t very loud, but it worried me, so I kept the revs down and decided to open up the motor when I got home for further inspection. After I dropped in my new Bosch magneto armature from Lane Plottner (look for another magneto article soon!), I went around the block to check that it was firing ok and that’s when the motor made an expensive noise.
At the roadside, there were no new holes in the motor, and no parts on the ground, so I pushed home and started disassembly:
Before I took the motor out, I turned it over and heard little bits of metal fall into the bottom of the crankcase, and I was pretty sure that it was the bottom end.
The first broken thing I found was the wrist pin on the front piston. Not a big deal, a new pin is pretty cheap/simple/quick to fix, but it took out the bottom half of the boss in the 95 year old cast iron piston. Dang it. That can’t be fixed. So now I’m looking for an Excelsior cast iron piston, if you can help, please email me via the “Comment” button at the bottom of this article.
There were metal slivers all over the interior of the motor. Now to figure out where they came from.
Here is the culprit. On the left is a standard cage and rollers for the rear connecting rod (not the forked one). It is a steel cage and 12 long rollers. The rest of the photo shows the cage and rollers from the broken Excelsior. The cage is broken into a lot of small pieces. And 11 rollers. But the twelfth roller isn’t a long one, it is two small ones. The previous keeper of my bike rebuilt the bottom end 30+ years ago, but he never rode it. He must have been missing one roller, and he dropped in these two short ones to fill the gap. Each one is a different length, probably they were cut down from other rollers he had.
It appears that the short little roller turned sideways, which stressed the roller cage. The cage broke up, which quickly distributed cage parts all around the bearing, which led to more cage breakage. Surprisingly, that rod was almost ok, it was the weaker forked rod that broke up. Probably due to some bits of cage getting trapped on the side of the forked big end.
An aside: Another mode of failure for X forked rods is that if they have too much end play on the crank pin, the rod can come into contact with the nuts for the mainshaft. There is very little room for the forked big end to pass by during each revolution. Mine had the proper .013” clearance on the end play with the shims set ok. And there were no contact marks on the mainshaft nuts. If you have an X, this is something to check.
I learned a couple of lessons here:
- Don’t assume that just because the big end in your bike has no radial play or excessive end play, that everything is fine. I checked this one when I assembled the motor, but didn’t split the wheels for a visual inspection.
- Never use two short rollers instead of one long one. If I had seen those little rollers, I would have fixed it for $2 from my local Torrington bearing supplier.