Wednesday, February 28, 2007

Mystery Revealed

I am sorely tempted to save this title for a post concerning theology, but it fits very well here also. The mystery that I am going to address is a damage that most will not have ever encountered, but if you have run in to it before, this will be helpful. I have been working on Harleys professionally for over 25 years, and only come across it a few times. The cause of the damage remained a mystery until quite a few years after first encountering it.

The mystery of which I speak is the unexplained "melted" piston pin boss. The initial symptom will suggest a wrist pin keeper that has come out. The cylinder will be destroyed from the wrist pin coming out and gouging the cylinder wall. Once disassembled though, things look a little less straight forward.

Instead of the wrist pin bore in the piston being intact for the most part, with perhaps some damage to the keeper groove, the spot where the keeper groove would have been looks for all the world like it was melted. This may be on only one end of the wrist pin bore, but more likely will be on both.

There will be no sign of heat discoloration, but it will appear as if the ends of the wrist pin bore in the pistons has melted away. This "melting" does not extend to the top of the piston, which rules out the melting originating in the combustion chamber. The first time I saw it, the word "erosion" immediately came to mind. At the time (mid 1980's), I was employed in a Harley Davidson dealership and had the luxury of calling the factory for technical help. Since the piston was from a recently "out of warranty" evolution, it was important to find an answer to what the cause was.

The tech rep at the factory who I talked with did not sound surprised by my description of the problem, but he dodged my question as to what had caused it. His solution was to replace both the piston and cylinder. Since the rep authorized extending the customer's warranty coverage to pay for this repair, both the customer and the dealership were satisfied. I, on the other hand, felt quite unsatisfied with this "solution" since I really didn't know if the origin of the problem had been addressed.


As a side note, the "non answer" from the Harley tech rep, reminds me much of the way the factory writes their service manuals. They will often state that "such and such" procedure MUST be done in "such and such" fashion. I for one find it much easier to remember the correct way to do something if I understand why it needs to be in a certain fashion. Most often the factory service manuals will give dire warnings about performing a task in a certain manner with no reasoning offered to support their "command". Sometimes the reasoning behind their procedure is sound, other times it is of no consequence and only added because they assume all Harley technicians are idiots!


Sorry for that rant, now back to the mystery. It was not until quite a few years later that I ran into this mystery again. The second time, the victim was a Shovelhead, and it was on a generic aftermarket piston. By this time I was working for one of the larger "unauthorized" Harley shops in this area, and so did not have access to the factory tech reps. This time I called a couple of the industry sources for pistons to see if anyone could tell me what this weird looking destruction was caused by. No one seemed to know, although Ron Dicky from Axtell came the closest when he said that it wasn't anything inherent in the piston that had caused the problem.

Since the original engine that had this problem at the Harley dealership had not failed again, I felt a little better this time about merely replacing the cylinder and piston, but the cause still remained unknown.


Again a few years passed, but then, mostly by chance, I found my answer! I was getting ready to final assemble a top end on a Shovelhead motor, when I happened to look through the end of a brand new, and just washed wrist pin. There to my surprise was a "gob" of metal filings. Have you ever seen how the metal filing that come off of a bench grinder can sometimes stick together because they were so hot when they landed that they kind of "fused" together? That is what was on the inside of this wrist pin! Just washing the pin in the parts washer and then rinsing with water had not budged the filings. I had to give the mass a tap with a screwdriver to knock them loose. There may have been about a 1/2 teaspoon in all, and once loose from the inside of the wrist pin the "gob" easily broke apart! Now it all made sense! The filings had obviously found their way into the wrist pin bore during the manufacturing process, only to come loose once the engine was running. Just add oil to the filings to help them slosh around and it is no wonder that the results looked so much like soil erosion!


Since that time I have heard of one more case of this same type of damage. I had a local "shade tree Harley mechanic" call me a few years back describing an identical problem on a bike he was working on for someone, and asking if I might know what caused it. I offered the story of what I had seen, and my conclusion, which he promptly rejected. (But then, of the half dozen times he has called me over the years with questions, he has always rejected my input, which really makes me wonder why he ever bothers to ask. When I have a problem I don't have a solution for, I don't call someone whose opinions I don't respect. There, done with that one last rant!)


I will let you be the judge as to whether or not you think I have solved the mystery of the melting piston pin boss or not, but as for me, I try to visually inspect the bore on every wrist pin that I install!

2 comments:

Anonymous said...

The answer to your piston mistery is the keeper came out and rattled up and down in the hole many times.Think about how many times the piston changes direction in just a minute of time.It blasted the aluminum away as it disinigrated itself leaving a smooth blasted away surface.This is almost always caused by the lower con. rod worn badly.Usually a grove worn in the crankpin.This forces the rod to push on the pin at an angle forcing the keeper out.It almost always is on the rear piston.I have seen this scenario many times.

St.Lee said...

Anonymous, I have seen the situation you are describing many times myself over the years. However, what I am talking about in this post is something different. By chance, a couple weeks back, someone came in with a piston and cylinder as I described. One thing that I somehow neglected to mention (probably because I was describing the situation from memory, and I am getting old)is that if caught in time, the wrist pin retaining clips will still be in place! That was the case with the most recent example. I will take pictures and write a new post about it as soon as I find the time.