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Broken Excelsior Connecting Rod, a post-mortem examination

May 30, 2011

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:

IMG_7839 IMG_7876


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.


Front connecting rod is broken at the fork, and bent.  Rear rod is bent.  But while studying the pieces I figured out the cause of the failure.IMG_7886


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:

  1. 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.
  2. 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.


20 Comments leave one →
  1. Jim Abbott permalink
    May 30, 2011 10:21 am

    Would it have been better just to leave out the missing roller? There were two universals at each end of my racer drive shaft. On rebuilding I dropped one roller on the floor and quick like a bunny under the bench it went where it fell into a black hole. I just assembled it one roller short. Never had a bit of trouble. Ball bearings without keepers are always one ball short.
    Jim A., Tucson, AZ

    • May 30, 2011 10:37 am

      It might have been OK without one roller. But they are very cheap to buy, just a couple of dollars from the local bearing supply house. I’m sure that they guy who put those in there thought it would be ok. But that one roller was really short compared to its diameter, and it fit into that long slot in the cage… Short rollers in short slots are ok, but there must have been too much clearance for this one. I’m just kicking myself for not inspecting it better when I bought the box of partially assembled parts!

  2. Somer permalink
    May 30, 2011 10:25 am


  3. May 30, 2011 10:39 am


    OUCH! Interesating cause of the failure. I’ll be sure to not do that!

    I have 2 very nice cast iron pistons, what size do you need? I’ll have to measure them to see what I have.


  4. Anonymous permalink
    May 30, 2011 11:29 am

    Sorry to read of this Tragic event. Roller was buried deep. It could have happen to anyone.

  5. May 30, 2011 11:44 am

    Yes, OUCH. I am soooo tempted to start up my 1916 motor without going into the crank. What is the torque on the nuts holding it all together Iwonder? Need a fixture to get it true and what are the wear limits I wonder? I have couple of big end crank pins upstairs in a baggie. I’m going up to mike them now. Could cast iron welding fix the piston? Rods look pretty nasty, but what is the metalurgy of welding and straightening I wonder. Wish I was a Detroit metalurgist sometimes. I’ll ask Jerry Capa. I think he’s a mechanical engineer and has built a racing Excelsior thats very fast. Clearances he used are unknown to me. Thanks, Paul

    • May 30, 2011 12:27 pm

      Hi Paul. I didn’t measure the torque on the nuts, but remember that the nut on the timing side crank pin is left handed! So are both nuts on the both ends of the timing side main shaft. They look identical to the 3 on the drive side of the motor, but they have a small “L” stamped on them. Nicholson’s book gives torque settings, maybe 100 foot pounds from memory. The piston is pretty beat up and cracked on both bosses, so I don’t want to try to fix it. Cast iron is terribly brittle. I’ve heard that if you drop one of these, it will shatter. Let me know if you have a spare crank pin! Mine is pitted from the rollers. I’m going to try to grind it back to a smaller circle, but I’m not sure if I can get new rollers in a large enough diameter for that trick.

  6. Ken permalink
    May 30, 2011 2:12 pm

    Hello Pete , I feel for you , that’s a tragedy !!! hope you can get it running well soon !!! I’m doubtful about the little roller causing the problem , from your photos all the rollers look undamaged !! I had a cage break up on my 1918 Harley many years ago and it kept running , getting me home OK , only after inspecting the ‘odd noise’ did I find the roller cage broken and the rollers were flat and ‘U’ shaped to the curve of the big end pin , the rod was reusable but new big end needed as well as the rollers and cage , looks more like the rod broke and smashed into the big end breaking the lower rod bearings , I’m sure if the roller was the cause , that roller and others would be a very different shape !! BUT whatever the cause , hope you get it fixed soon and get some more rides this year !! Best wishes , Ken

  7. Lee Samuelson permalink
    May 30, 2011 10:27 pm

    Hail Pete!
    If you can stand doing a restification, go for aluminum pistons. We did that for a Harley Sport – fore-and-aft twin at Reynolds-Alberta Museum. I ended up with Indian Scout slugs from Egge.
    My druthers would have led me to one of the semi-custom piston makers (Venolia, Ross) which would have eliminated the grief of mating gudgeon to piston, a horror story for another day. Lotsa custom parts by yrstrly in that one.
    p.s. it runs… as accessioned it came attached to a Flxible sidecar.

  8. Lee Samuelson permalink
    May 30, 2011 10:48 pm

    Pete, me again
    More: gotta read evrything first. In the oilpatch we would send-out pins and shafts to be hard chromed and reground. Worked just fine for high buck machines making holes in the ground. This is not considered a difficult or onerously expensive procedure here. Max rebuild is considered 0.060″. It is also an excellent way to recover from 25 thou mistakes (mine).
    Glad you have one of Bernie Nicholsons books. Mine is the last edition which he signed and gave to me. Dykes Motor Manuals often had a section dedicated to specific motorcycles. I think I saw one with Excelsior… but I have a memory which can fabricate on cue. Any Dykes is worth buying. He bequeathed TONS of motor related material to the Detroit Public Library. He was also the inventor of the side cutter properly known as “Dykes”. He also produced a kit of components for making your own car way back in the early days.
    me go now, Lee

    • pete permalink
      May 31, 2011 2:41 pm


      Nicholson’s books are legend. I have two copies now, one is greasy in the workshop. The second one was a gift and it stays clean inside the house. Look for a book review on it soon.

      I’ll be getting modern aluminum pistons, but not right now. I want to get the bike back on the road for a few more tours this year. Then pistons in the winter…

      Any pics of that HD with the Flexi sidecar? that must have been a nice setup.

  9. Mike Weiss permalink
    May 31, 2011 5:59 am

    I know that “sinking feeling” quite well! At least the cases weren’t broken, could have been a lot worse…Mike

  10. May 31, 2011 6:22 am

    Any evidence of the failure on the flywheels? Did the bearing cage disintigrate against the cases? Thanks, Paul

    • May 31, 2011 8:25 am

      the rods hit the inside of the wheels and left some dents and scratches. but nothing major. same thing on the inside of the crankcase. The inner bearing cage is the one that disintegrated, the forked ones are fine. Further measurements showed me that the rollers were not the standard lengths. I think that the previous owner used what he could find, which was probably some HD rollers and a custom made cage.

  11. May 31, 2011 2:20 pm

    it just occurred to me, riding an old bike is kinda’ like pickin’ at a scab; it’ll never heal, and I think we must like it!

    old metal, used abused and generally bodged. But we run it until it breaks and thn go back for more.

    Maybe we shouldn’t be too hard on the PO’s as they were doing t best with the resources they had to hand.

    There was a comment I read recently from J.Decker basically about ‘keeping it true’. So maybe the next step is ‘and have the ability / skill ‘ ala P. Young to recover the situation.

    Feeling a little zenned out, keep up the good work, one of the last great m/c blogs.

    • May 31, 2011 2:37 pm

      thanks for the kind words. Yep, it is all sorts of fun to ride 95-100 year old bikes down the road. If I were a decent writer, I’d say something smart about the sense of achievement I get, and the fun of going 45mph on the backroads and hanging out with old friends and old bikes. And I don’t mind taking the time and effort to fix them when they break. I wish I had more money to spend though! This old bike has gone down a lot of roads, and seen a lot of people. The old guy that had it before me did what he could to keep it going out in the wild country of Tasmania. His work on the bike was done when I was still in short pants. I don’t mind if he missed one or two things. For now I’m the keeper of this bike, and I’ll do my best to put it on the road where it belongs, and share it with everybody.

  12. June 1, 2011 7:12 am

    Hey, thanks for the words about Nicholson’s Modern Motorcycle Mechanics. Good luck with the rebuild, hopefully the greasy MMM helps you out. And you did just fine with describing your Excelsior ownership philosophy!

  13. June 4, 2011 11:25 pm

    Creative destruction! Those pencil-thin rods are pretty weak…I prefer when a busted roller big-end just makes expensive loud noises… Ah well, back to the workshop for a while. Clearly, when you first started hearing noises, something was up, but I wonder what initial ‘event’ started the noise?
    Got a spare crank?
    By the way, I’m sending UNESCO your way to cite you for abuse of an ancient monument. :0

    • June 5, 2011 8:54 am

      ahh, but when we ride them as the maker intended, these things do happen sometimes. 😉 My further study of the parts leads to the center roller cage being home-made from inferior material. It broke up, but the rollers took the abuse. But the weaker of the two rods got jammed up…. I love a good mystery, and failure analysis is one of my favorite engineering activities in my day job. But it is harder to swallow when I have to pay the repair bills.

  14. July 21, 2011 8:31 am

    Straighten and weld???? I’m flushing my old crankcase soon with mineral spirits. Just found the catch tub. Thanks, Paul

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