My kx 125 1998will not start..i just bought the bike yesterday an the guy said it had a crankshaft seizure he rebuilt the top end suposively but he said he didnt have the right tool to replace the crank..what is all involved with part wise and how much would it cost?

Yeah their great, its a 2 stroke though so it will scream, but you'll get a lot of top end speed out of those little guys. It all depends on how big the person is riding it. I rode a kx 100 when I was about 14 and kept it for about 3 years, and was fast as hell, always popping wheelies everywhere. So the answer is yes, their a great starter bike for younger racers.

check your belt and clutches

http://www.repairmanualclub.com/motomanuals/index.php

piston kit, wrist pin bearing, top end gasket kit and a honing tool to clean your cylinder is about it I would probably buy new air filter and spark plug if I was doing it, but not essential ?

sounds like a siezure, tear down the top end and inspect it!

hopefully its just the piston and not the crankshaft.

remove the piston and see if the rod has nice smooth movement, and no up and down movement on the big-end bearing

Now, you need to find out what caused the siezure, so you dont sieze it again after its rebuilt

Kinda sounds like your needle valve is sticking in your carb and it is giving it to much fuel

Cylinder bolts @ 25 ft/lbs (34Nm)
Cylinder head nuts @ 18ft/lbs (25Nm)
Check these nuts again after first warm up and again after first two rides to let gaskets bed in and seal. Cheers.

I'd bet you 10 to 1 it's not the coil. It sounds like it is starving for fuel and needs a carburetor adjustment. When you rebuilt the top end, did you put a bigger jug and piston? If so, your problem most definitely is the carburetor needs to be re-jetted.

What caused you to rebuild it? Did it seize, or quit running? Was it losing power before the rebuild?

That's where my money is but if you still believe it is the coil, you can check it with a spark checker. This is a handy little tool you can find at AutoZone, Harbor Freight, etc. It's a one piece tool that connects between the plug and the coil. The plug stays in but you remove the wire and put the checker on, one side connects to the plug, the other side you connect the plug wire. There is a light in the middle that will flicker when you crank the engine if it has a good coil.

Put the checker on and start the bike and see if you are losing spark...if the light stops flickering at top end, then you do have a coil issue...but like I said before, having seen this problem hundreds of times, my money will be on re-jetting the carburetor.

Chances are the flywheel key is partially sheared or fully sheared off causing the spark to be out of time.. The crankshaft could be misaligned due to the top end blowup.
Can I get a “very helpful” rating on this answer?

I don't know what may be wrong with the top end, (if anything), but I wish I could come up with a line of BS like these guys have. I could maybe get rich by constantly servicing just one bike. What happens when the bike "breaks down"? What does the shop say is wrong with the top end such that it needs to be rebuilt? What do they do to fix the breakdown each time? What do they charge you?
Get back to me by posting a comment.