1994 KTM 250 SX Project

[JCuneo]

I am new to the forum so I would like to introduce myself first. My name is Jared, and I recently purchased a 94 KTM 250 SX for $300. By no means is this bike in bad shape; actually it's in terrific shape considering it's almost 15 years old. When talking to the original owner he explained that it's just a top end job, so I am assuming a few burned up rings, a bad piston and jug. Maybe a quick top end job bore 20 or 40 over and buy a new ring and piston kit, however I am by no means a mechanic, but I do have a good friend who is. Once I got the bike back to the house I am thinking this is too good to be true. A dirt bike that retails for $1500, that has new tires, sprocket, and chain only needs to have the top end rebuilt, which needs to be done on a regular basis anyways because of the 2-stroke mechincal characteristics.

Well here it is... My friend with the “trained mechanic ear” told me he can hear knocking when turning the crank. Now a top end job has turned into a possible bottom end job also.

You know what I say, “So what,” this will be the learning experience I have always wanted therefore I will rebuild this bike and engine. I will be posting to this forum on a regular basis to show you the progress the bike is making. Please, if you have any input or comments provide them, I need them! As I will be doing a lot of research to figure out how to rebuild the engine.

Finally I want to inform you I have not pulled the head off yet, so we still don’t know what we are dealing with 100%. Here are a few pictures of my current progress, I hope you enjoy my project:

[compmoto]

Welcome ! Better check with your dealer on that cylinder before you have it bored ....it's coated , unless it has been previously sleeved...

[JCuneo]

Originally Posted by compmoto

Welcome ! Better check with your dealer on that cylinder before you have it bored ....it's coated , unless it has been previously sleeved...

Are you saying that after I bored the cylinder it will need to be re-coated? It's a 94 I would hope this isn't the first top end job on this beast. What should I look out for when pulling the jug off?

[JCuneo]

Alright guys I made some progress on the bike last night. I removed the radiators and all its connecting hoses, while draining all coolant from the water pump accordingly. This prevented any liquids from entering the cylinder head and getting into the block. Once I had the radiators off, I noticed a big hole in the left radiator at the bottom right hand corner (please take a look at the picture below).

Question 1: Could this effect the cooling cycle of the bike? Also would the hole prevent antifreeze from filling the radiator entirely? Meaning the radiator will only fill up until it reaches the hole…

After I removed the radiators and all it’s components I tackled the head of the engine bay. Honestly from my personal opinion I think it looks very clean on the inside. There is a chip in the piston head which ended up scrapping the front of the cylinder wall. I wasn’t able to locate the chip of the piston anywhere in the head of the engine bay.

Looking at the scrapes of the front cylinder wall, the jug will have to come off in order to have it bored out properly.

Question 2: Is there anything I should look out for when boring out the cylinder? I know that a machinist will have to do this, expect I had this comment provided to me “Welcome! Better check with your dealer on that cylinder before you have it bored ....it's coated, unless it has been previously sleeved...” Does anyone know what he means by coated? Do I have tohave the machinist recoat the inside of the cylinder wall once bored, if so what with?

Once I pull the jug off I am going to have to inspect the internal guts of the bottom end to make sure this chip of the piston wasn’t sucked up through the intake and has had the opportunity to create some Swiss cheese out of the casings or the mechanical workings (i.e. crank, rod, etc…)

Question 3: Any ideas on the location of the chipped piston?
Question 4: From the pictures below do you guys think this cylinder will have to be re- sleeved?

Finally, when looking at the jug I noticed a power valve installation. I have not been able to find a service manual, in English, which could help me figure out how this could effect the removal of the jug.

Question 5:Can someone explain or provide a diagram on how the power valve is set up so that I don’t make any fatal mistakes while removing the jug? I found a jug online for cheap, does anyone recommend buying a new one? Or will this coating and boring process is something done on a regular basis on these KTM 2-Stroke Engines.

Again please provide me with any knowledge you may have regarding this subject! I really want to learn! Thank you for you’re time

[IMG][/IMG]

[JCuneo]

[compmoto]

we will be learning together. I too , have come into one of these, although mine is supposedly a '93 model. It needs a topend rebuild also.I happen to have a line on a complete engine.I hope to own soon.But I'm doing some research as well. I'm trying to ID the VIN now, and will look into acquiring a service manual.My bike does not have a light now, so I am assuming for now, it's a SX model 250.Anyone know of VIN search tool,to look it up?Anyone had one these 250s ? Mine isn't quite as clean as yours,JC, but it ain't too terrible.I got it with a YZ 125 I bought for an extra playbike.The guy said it was running great, just before it seized.That sounds kinda familiar.The engine is not seized at the moment, but I'm sure it prolly needs a rebuild.How's yours coming along?What service manual did you end up with?I have my KTM dealer working on VIN ID, and then a service manual too. I'm adding this: I just found out that the bike I have is '92 model EXC, not a '93 model as I was told by the guy I got it from. There's a VIN tool on the KTM Hutt site linked to this website. I don't know how comparable my engine is to the later model 93-97 types.I'm still learning.

[whansen02]

Hey JC
It looks like you have your work cut out for you. Good luck. I'm looking forward to the updates.
Wayne

[sector999]

Hey i joined this forum because i am doing the exact same project i bought the same bike for the same price. i just pulled the head off mine and got the piston out and its got a huge chunk out of it. and the head is coated with a nikasil and you will probably have to send it out to have it done it cost about 200 and it takes a few weeks. i went to a ktm mechanic and he said the head is not boreable. keep me posted if you find another way around this and ill do the same

[TurnKey]

Let me see if I can help answer a few of your questions.


Cylinder - Most cylinders starting back before your bike were built have liners that are coated with Nikasil. Nikasil is a very very hard surface and for the most part Nikasil coated cylinders are not bored but they are sent off to places like US Chrome or Millennium technologies to be recoated. The advantage of the coated cylinder is that it lasts much longer and I also believe it is slicker so there is less friction but don't quote me on that. They also deal with heat better or create less heat through less friction.

It is not likely that you can even find an over sized piston for your motor for this reason. They likely sell an A and B sized piston. the B being ever so slightly larger. What most folks do is after they have a cylinder recoated and setup to use an A size piston on the first rebuild when they replace the cylinder they will use a B sized piston to take up the slack.


As for the motor in general if I were in your shoes I would pull the motor completely out of the frame. It's really not that difficult and from where you are now you could probably get it done in 30 minutes or less. This will help you with the rest of the restoration.

As for the missing piston piece. It most likely went out of the exhaust. I for one would wonder why it broke off?


However, if you have removed the engine from the frame it would allow you to make sure that it did not find it's way to the bottom end.

Crankshaft - on a bike of that age I would be very suspect of the lower end crankshaft bearings and the seals. I would advise rebuilding them for piece of mind. Not to mention that replating your cylinder will run you a couple hundred bucks and if the lower end goes it will ruin the cylinder along with it when the bearings start sending metal pieces into the top end.

That's all I will say for now but keep us updated and take lots of pictures.

TurnKey.

[TurnKey]

One more thing, as for the radiator as long as it does not leak I wouldn't be concerned. Truthfully you will not know until you fire that bad boy up and get it up to full operating temp and the radiators are under pressure.