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Since Im fairly new to dirt bikes I have a stupid question. I just bought a 99 kdx 200 and Im rebuilding the top end. Should I order the top end kit first or tear it down and look at the cylinder first. The guy I bought it from says the top end is all stock.

sounds more like a partial statement than a dumb question

I would remove the top end and have the cylinder measured at a cycle shop just to be sure it is stock. Then, go ahead and buy your rings and piston.

I guess what im asking, can the cylinders be bored out or do they have to be replated? I didnt want to buy all stock parts and turn around and send them back if for some reason i couldnt go back to stock. Like i said im new so any advise is helpfull.

As carpenter's say, "measure twice, cut once." You should measure cyl and piston and then buy needed parts. I got a new to me '93 model three months ago. Took the top end off and found the piston and rings to be in perfect, near new condition. Didn't need those parts. I did need new KIPS valves, though. Check those at the same time you do your measuring.

There are different sized "stock" pistons. We're talking thousandths of an inch but some were still stock sizes.

IMO, I would pull the top end and measure first and inspect to see if it needs to be replated. If the plating looks good just order the correct sized piston and put it together. Now, if it needs replated, I would go a head and buy the smallest piston and then ship it off with your cylinder to the plater so it can all be matched up.

there are a few "basic" stages of wear on a cylinder
the first is no signs of wear at all, slap in a new piston and go
the second is notable wear that require a replate
third is notable wear that is so deep it requires an overbore (and this automatically requires a replate)
forth stage has wear that is so deep it can not be removed by overboring because there isn't enough meat left on the cylinder to safely do so, and now you need a sleeve
fifth stage (mostly found in 220 with stock pistons) is like a bomb went off in there and it is all blown to bits and you need a new cylinder altogether

(now, keep in mind that these stages can follow each other, if your on your 3rd rebuild it could be that each time you had stage 3 wear, and now you need a sleeve)

what I always do, is send it to a reputable replater (if replating is needed) and buy the piston from them
no point in my trying to match parts to machine work that someone else still needs to do

I agree with DIY and SS109, take it off, look at the condition, (most likely) just replace the piston and go. If by some strange chance that your "all stock" cylinder needs more attention and you don't have anyone close by to ask for advise (make sure you trust their mechanical ability and knowledge), then ship it to Millennium Technologies to replate (ship a new piston kit or purchase one from them). When replating, you MUST, no you should, send your new piston in to insure proper fit. When buying a piston, go with Weisco or similar aftermarket pistons.

Ok. Thanks for all the input.

Some replaters offer to repair gouges and missing chips by filling in with weld, then drill, sand, etc. to get it back to shape.

Jason wrote:Some replaters offer to repair gouges and missing chips by filling in with weld, then drill, sand, etc. to get it back to shape.

This! It's cheaper than sourcing a new(used) jug and having that repaired or buying a new jug.

Chilly wrote:I guess what im asking, can the cylinders be bored out or do they have to be replated? I didnt want to buy all stock parts and turn around and send them back if for some reason i couldnt go back to stock. Like i said im new so any advise is helpfull.

It's not like a car motor. If it's bored over size, it still needs to be plated afterwords. What you are worried about (piston size in a worn/used cylinder) is moot if you use a forged piston (weisco & etc.). They will work fine in a used cylinder as long as the plating is good.
I rode my 93 on a weisco for 5 years (no replate)....After that I had to replate the cylinder, but there was no "damage" to the cylinder at all. Understand that the 93 was my first bike and I didn't ride it hard the whole time. By the time I was riding it hard, it had pretty low compression.
Now (after a replate and new piston) it's a "screaming banshee" and really fun to ride hard.