Welp, the day finally came where my engine went big boom. I have been tracking my engine hours pretty diligently, and ultimately try to take very good care of my engine. I ran two “unplanned” club races in the last couple weeks, and unfortunately this pushed me over the edge of the recommended service interval. The culprit?..lower rod bearing
As background, I track all of my engine hours based off of ON-TRACK time pulled from my downloaded Mychron data. So warmup time on the stand is not included in these numbers…just the way I do it, not saying it’s the right way. I had ran the entire season on the same rod kit (new at beginning of year), and operate the motor at “race pace” any time I’m on track. My jetting is usually aggressive, though never to the point of detonation or crazy high EGT numbers. I have always run VP MS98L, and Elf HTX 909, and usually at 5.3%. Recently I did some testing with VP C12 and slightly lower oil mix, but I don’t think that had any hand in the failure.
So based on the above tracking method, this bottom end had just shy of 13 hours of run time, and approximately 130L of fuel run through it. Factory recommendation is to replace after 120L of fuel, so I was playing with fire, and I was well aware of that. Couple that with some aggressive downshifting (16,800 max rpm ) during the race in which the engine failed, and this thing was bound to pop.
Luckily there was no other material damage to the engine, so overall pretty low drama. Now for the aftermath photos…enjoy lol
So what’s this gonna cost you, parts wise?
Not sure yet, and I’m not really going to touch in until well into the winter. We’ll be receiving our second kid into the world on Thursday, so racing stuff will be parked for a bit
The parts that failed were due to be replaced anyways. If the crankshaft itself needs to be replaced then that will add ~$500 to the bill…so hopefully it can be reused.
My brother had a bottom end failure of his R1 recently with what we suspect was hurt by a bad downshift. I recently found out that a company makes a slipper clutch for the TM, which I didn’t know was a thing previously. Definitely stands to reason that it could help save the engine a bit on downshifts, an extra cost though and don’t know where it stands as far as regulations go.
You’re beyond TBO, and just need to do the same work that the overhaul would’ve required anyway
Aw man that sucks.
If you combo a higher shift RPM with potentially worn crank bearings you can end up with the piston hitting the head like this, battering the rod and small end bearings in the process.
Have you been checking main (not rod) radial (up/down) bearing play periodically? A little axial (end float) is normal, but the radial should be pretty minimal.
Crank should be fine, head looks serviceable, judging from the piston skirt there’s probably little to no cylinder damage. Everything else (in the engines) though will need to be replaced
I had been checking, and all seemed good. You’ll notice the plastic retaining piece is damaged on the roller bearing, so I suspect that came after the needle bearing failed, but I could be wrong. The needles were gone completely, so it wasn’t like it welded itself together as with a bearing seizure. Maybe the main bearing was the cause after all
The cylinder is in good shape, luckily. A couple of very light scuffs, but no gouging. Hopefully the head can be cleaned up fairly easily. Not too worried about that though.
On track it felt/sounded like the bearing failed first, and then all the slop caused the rod to make contact with the head. I’ll post video here soon.
Congrats on the new addition to the family! Blessings
Best wishes to you and your spouse.
Congrats on the new addition to the family!!!
Having said that…
ouch! I think that may have had something to do with it
Yeah, it might have. I don’t precisely know what the spec overrev limit is for the KZ10 series, but the rev limiter hits near/at 14K on the ES, which is tuned conservatively from the factory.
16.8 at downshift is pretty steep
my x30 was up at 16,7 when limiter broke
Big Day! Thoughts are with you guys!
Thanks, Dom! We’ve got ourselves a new baby boy
20 new racers in bambino karts with giant helmets
Congrats on the kid Evan! Sucks to hear about the motor, but shit happens. We must have just missed each other this last weekend, as I was practicing saturday, but decided not to stay for the KRA race.