125-200cc Putting motor back together in a hurry= melting down motor in a hurry

well I forgot to put the inner O ring in the head after taking the motor all the way down to change the kick start spring, which lead to a melted down motor 2 miles down the trail. So just all understand each other the head actually melted down the cylinder wall. So its time to do a rebuild and that means More power I'm getting the race head from husky for my WR300 (or I'm going to try) and I its time for a new pipe, but from there I'm not really sure where to go. Any woods/ extreme enduro guys got any ideas or advise?

make sure you install the o-ring next time

Where abouts are you at? The 300 makes pretty good power stock. For technical woods riding around Missouri, Arkansas, and Oklahoma the suspension would be the biggest bang for your buck. Adding power, and smoothing things out at the same time, the 38mm Lectron is amazing. It adds a lot of range to the RPMs and is overall just a pleasure. For the pipe, really the stocker is my favorite. Good low end with it, and the pipe fixes well when smashed and with some BBQ paint, it looks new again. The Pro Circuit pipe is ok. Nothing spectacular, but it does give a nice boost in mid and high. The DEP pipe is a beast, but only if you want to ride it hard on the pipe. Since you asked about extreme enduros, the stocker would be my choice with a Leo Vince silencer.

I'm actually surprised it started without the o-ring in place. I'd assume it would waterlock with coolant. Or at least leak enough to spoil the combustion.

Wouldn't it have smoked like a bastard?

That sucks.

Oh yeah I'll never forget the o ring again. Yep it smoked that morning but it was one of the damp rainy foggy mornings where your mouth looked like it was running 15 :1. And when we left the truck that morning we were riding like we all had something to prove. The 300 has plenty of power stock no question about that. But I'm looking for more snap on the low end for Rock ledges and pivot turns.

after my attempt at humor, there were two good questions posted, how would it have started as the coolant would have leaked into the top end, and it must have really smoked
my thinking is there is a possibility that the o-ring was not there, but,,,, not likely, if it sealed well enough without the o-ring to get started and you consumed all your coolant in 1 mile and melted on the second mile there would have been serious power loss at least on the first as coolant is not flammable so it would act as a quenching problem, also if the head was hot enough to melt maybe the o-ring was consumed due to it's proximity of the combustion chamber, when you finally stopped was there a lot of steam involved? the reason I ask is it might be something else, is it possible you had a water pump failure, possibly due to being in a hurry you broke something there
regardless you will find it, let us know, oh by the way would like to see a pic of the melted head

just thought of another coolant scenario, did you look to see if it ended up in the gearbox

Well after I put the motor back together I started in my garage with no coolent which in guessing allow the head to seat in. Then once at Chadwick I added coolent. The bike did smoke a lot but I thought it was due to the weather. There was a huge loss of power right before she laid down. The spark plug was glowing white when I pulled it out on the trail. We pulled the head off trail side and no inner o ring. The crank case oil is a nice coffee cream brown. I'm taking back all the way Down again and taking my time with parts diagram and a highlighter

As far as the o ring melting.... That may of happened being that I couldn't find the o ring in my garage.

coffee cream brown? does that mean you found the coolant in there?
the loss of power is often a seizure, do you have piston marks that reflect this

Yes there was coolent crank case oil. I'll have to post pictures of the head and cylinder. But you guys have me thinking now ill defiantly check the water pump anything else you guys can think of

if there was coolant in the gear oil you have a water pump seal issue or a damaged part there

I'm going to go over the motor with a fine tooth comb this go around. I have no complaints out of the bike though I've road that thing hard for 2 years and other that normal maintenance I haven't had to spend a dime or turn a wrench on it. still need advise on the engine mods though. The big plan for the year is to quantify for the Tennessee Knock out race on Sunday. I've got my suspension set up about perfect (for me), so I would say I'm alright there. I figure why I'v got the motor and my wallet open I would go bigger and better.

here is a list of the parts I have to replace (as of now) if this helps

Head
Cylinder (re-coated)
Piston and rings
Pipe (Its banged in bent and a pain to put back on)
Radiators (same as pipe)

I'm with you. I'm pretty sure I forgot it or it fell off or whatever the case may be. I'm guessing if I go digging around under my work bench I would find it.

Deffinately replace all seals, when the engine gets that hot it can wreck havoc on seals even if it doesn't look like it!

hey that's good advise i defiantly will replace all of them