Holy Piston!

[Rokoneer]

Looking for opinions on why this may have happened. The story is such: bought this bike, heard it run before buying it, but DID NOT ride it due to repulsive weather. Finally got registration issues worked out and went for my first ride. About two miles in to that initial ride the bike sputtered and seemed to be running out of gas. It eventually quit running, but started up again immediately and appeared to run fine. Off I go and another mile away the same symptoms, sputtering and losing power. This time it dies and will not turn over when attempted to re-start, which ended up being due to a hunk of piston being wedged between the exhaust valve and valve seat.

Detonation? Pre-ignition? Foreign object made it's way in to the cylinder? Opinions please...................

[James.A]

MAN....THAT SUCKS ! ! !
I think you might have gotten hosed.
Model/year?
Repair or part out?

[username]

this is a bum deal for you. how long did you hear it run before you purchased it? just a few minutes?

have you sent that photo over to the seller? is it possible that they are just a dopey person and unwittingly sold you a bike that was hosed, but they didnt know how bad?

[James.A]

The failure your motor suffered would happen over a relatively short period of time, but the underlying cause was almost certainly present for quite some time. The main question is what did the seller know and when did he know it? I experienced a similar failure on my first bike, a used, 1967 BSA. In that motor, one piston was eroded on the dome from pre-detonation. It looked like it had been attacked with an ice pick on the face. That bike experienced a kind of runing seizure. When the motor cooled and I re-started it, it had a deep knock. When I took it apart, it had 2 different pistons in it. One flat and one domed. The domed piston's face was eroded and it was moments away from a failure just like yours.

[Rokoneer]

The motor is a 75/5 with R65 heads on it. The block has 45k miles on it and the heads were put on two years ago. The first person I sent pictures to was the seller, he called me immediately and repeated to me not having any problems with the machine in the two years that he had owned it. The seller is 500 miles away from me, but we spent a couple hours together when I went to look at the bike and he did not seem like a guy who would have misrepresented the machine. I'm not sure that there would have been any obvious way for him to predict a piston meltdown anyhow. The bike was started and run five different times with the longest run duration being around 5 minutes. The motor had plenty of time to warm up and sounded excellent, with no discernable odd noises, vibrations, or emissions. I don't believe that I was intentionally sold a lemon, and you'd have to see me and the seller to understand why I don't think that's the case. I'm really not too upset that it happened at this point, but I want to figure out WHY it happened so it doesn't happen again.

Another photo of my new 'momento':

[James.A]

Well, your'e the best guy to know if you were hosed or not. If you think not, I'm with you on that. If your motor was running with pre-detonation long enough to eat a piston, the other piston ought to be ready to fail also. Did you inspect it? Did you find all the parts of your broken piston? Did you drop the oil pan to look for 'em? Any obvious damage to the head?, the cylinder? It's hard to imagine how a foreign object could get into the combution chamber if the intake was correctly assembled with a filter in place. The motor in my BSA was destroyed because one cylinder was running much higher compression than the other. Probably not the situation with your bike. The R75/5 owners manual specifies a minimum octane rating of 97 as calculated in the 1970's. I can tell you for a fact that the motor would experience pre-detonation if run on modern 87 octane unleaded gasoline, and it might not produce an audible spark knock. There are good tech articles on octane on the IBMWR web site and on airheads.org. If you want to e-mail me, we can exchange phone numbers and discuss your problem.

[Rokoneer]

The only checking I got to do so far on the opposite piston is the old 'flashliight through the sparkplug hole' method and there is no noticable problems with that very basic scan. I will pull that cylinder head off sometime to have a look and get a more thorough look to see if there are issues in there as well.

As far as broken piston parts go, all I have found is a LOT of slag (as in melted piston) all over the cylinder head, valves, and in both the inlet as well as exhaust tracts. Have not found any chunks, but have yet to drop the pan and see what is sitting in there. I have to believe that there are some good size pieces in the block somewhere and I'll find them eventually.

Both the cylinder and the cylinder head are in good shape. Both valves are bent and will need to be replaced. I will hit up the valve seats, but they should clean up and be fine. I also fainted when I heard the replacement costs for the valves and the piston (new) from Bob's. Any hot tips on getting better pricing on either new, non OEM, or used parts?

I have to agree with your thoughts about something getting in the cylinder because it certainly looked to me like everything was sealed up pretty well and there is a K&N is the airbox. From what I've found in searching it would really appear to be a pre-ignition problem, although there are those that are convinced it is a detonation issue........

I appreciate the feedback and opinions very much.........

[James.A]

Definitely a case of a super heated combustion chamber. The result is the same whether the premature ignition is caused by poor fuel or advanced timing or a combination of both. I order parts from Re-Pscycle in Lithopolis Ohio. They often have used parts at very reasonable prices. IBMWR.org(Internet BMW Riders)is a good place to save money on parts as well. They have a classified section that is a great resource for used parts for sale by individuals like you and me. Good luck. Let us know how it turns out.

[dlearl476]

R65 heads on a R75 makes me wonder if there wasn't a valve clearance or spark plug clearance issue. Were both plugs the same?