Worse Case Scenario, Need New Lower End Questions w/ Carnage Pics

Finally figured out why my 99 YZ400F wouldn't start. The lower end is burned up. Luckily it was caught before the piston fully slammed into the valves, although you can see where the intake valves had started to barely kiss the thing (yikes). Lucky on that, it was just enough to show in the cutout, but not enough to leave a dent in the piston or valve. I also have some really pretty blues and oranges on my crank!

I'm considering finding a used lower in decent shape just to get going again. Will a 426 lower fit with a 400 jug and head? I'm trying to stay as cheap as possible, so rebuilding the current lower is probably too pricey. Everything else on the bike (to include clutch and transmission happily) is in good to really good shape, so there's some value in carrying it along, but I don't want to be too in the hole. I'm asking about the 426 lowers just to increase my chances of finding a decent used part.





The pretty orange and blue was put there by the factory on the day the crank was first built. It's not an indication of any kind of problem. Does the rod have excessive clearance?

The valves should be checked for damage and overall condition, and some attempt made to determine the reason for contact.

My guess is that it wouldn't start due to a bent intake valve (or 3) from the looks of your piston. I wouldn't even bother looking for a used bottom end. Your chances of ending up with worse problems than you already have are really good.

My guess would be that the intake cam simply jumped time or seized and slipped in the sprocket/and or jumped time as a result.

I am absolutely amazed u didnt notice an abnormal noise or a loss of power, WAY before it got this bad. When your 4 stroke is acting up or loseing power YOU NEED to tear it down before castastrophic failures like this 1 happen

to be honest I cant see anything wrong with the bottom end... I do see that your valves were contacting the piston but that doesnt mean your bottom end needs attention as its usually like greyracer said.... does the crank move easily? is there excessive slop in the wrist pin or the large end of the connecting rod?

Had it checked out by a Yamaha mechanic. The connecting rod bearings (crank side) are shot. I got schooled again about the colors on the crank. Makes sense, just surprising. It was actually a big "duh". The crank lobes don't touch anything...

The valves actually are good to go. I agree with ForsheeMS, another lower would be a gamble and a risk of throwing good money after bad. So now I'm parting it out.

For the few minutes I heard it run, it was popping like crazy. We worked the usual suspects, carb, valves, and header seal, then the thing didn't run after all that.

It's a lesson learned.



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