I hope I can get some help here…. Just rebuilt my K9b, fresh bottom and top end. Engine was running great all weekend long, right up until the piston smacked the head. Not sure if it stuck first, and then somehow that caused the piston to hit the head, or if it blew up due to the hit. Can anyone help by letting me know how to figure out why it hit?
It is a 53.93 piston, squish was .030
what do I check, what do I look for prior to installing new cylinder and piston?
thanks in advance for any help
Steen, thanks for the response. The piston definitely hit the head, you can see the imprint on the piston of the part of the head that sits down inside the cylinder bore.
Hi Steve. Please post a picture of your piston to give us a better chance at diagnose the problem.
As stated above squish is a little tight but in order for it to change check the lower rod bearing to see if it has up and down movement something had to change to allow piston contact. .043 is a good compromise on squish, usually when the piston contacts the head it will squish the ring in the groove. Check to see if it is pinched if it is you have had contact.
I’m down in Sebring, FL for a car event, and didn’t have the time to tear down my engine before leaving. As soon as I get back, I’ll post a picture (if I can figure out how to)
Glenn, thanks for the input, it definately hit the head, I can see the imprint on the piston of where it contacted the part of the head that recesses inside the bore.
I pulled the had nod cylinder so far. There are mangle shards of metal that appear to be thrust washer, that are located just under the reed cage. The bottom rod bearing definitely failed. The rod hit the fwd part of the opening between the case halves (not sure how big a deal that is as far as if the case is ok?). Can anyone tell me what could make the lower rod bearing fail after a fresh rebuild? Or what could cause the thrust washers to fail?
Does the rod look like it got really hot on the bottom end, blue discoloration of the metal? If so, it was too tight possibly not enough thrust washer clearance. That is suspect since you say you see pieces of thrust washer in strange places…
What you have, is a rod big end bearing failure. That causes all kinds of havoc including cutting up the thrust washers. Bearing failure can happen for Many different reasons, i.e. Detonation, lack of lubrication or poor installation. Etc etc…….
The bottom end of the rod didn’t seem like it was blue. But I’ll let you know for sure when I have the case halves split. It could just be that it’s blue only right around the bearing and I’m not seeing it. I’m leaning towards thinking that the crank was poorly rebuilt. I was trying to save money, as I’m on a budget that should not be in karting but I love it too much to stop. I had a “guy” from a motorcycle shop rebuild it. I’ll never ever do that again. I was just under the impression that if you have the tooling to separate the crank and rebuild it, it must be fairly universal in method. Please don’t crucify me for that choice, I’ve already paid the price.
Are you sure the piston hit the head, or is it detonation. There are way too many unknown factors to make an educated guess, but one thing that sticks out already is your squish, at 0.030″ you are well below what these engines are commonly set at.