Boring KTM Rebuild Thread

[schlepp29]

Can't say I blame you. Personally I like the orange frame look and it would look great on a 3 wheeler. I made the Same mistake last year on the 1990 kx 250. While I was doing some minor welding to foot peg mounts I didn't figure in the wide foot peg dimensions. Oh well I figured I can just weld some add on pieces to the stock pegs instead of re powder coating.

[El Camexican]

Last night we began the process of prepping the frame for sandblasting.

After removing all hardware from the frame we wipe off as much grease and oil as possible before threading bolts into all the holes to protect them from sandblasting and powder coat.

In theory there is no danger in sand-blasting the threads, but I have experienced sand getting stuck inside the threads on some grease or oil residue and then ended up seizing a bolt inside the thread and snapping it off when trying to remove it. It’s just not worth the hassle unless you are sure that you can get all the sand out of the threads.



The next step is to take some carburetor cleaner and try to spray it around all of the areas where welded parts are in contact, like where the engine mounts are welded to the frame. These areas trap grease and oil which can bleed into your powder coating once the temperatures get up to 400F.



Once we’ve cleaned up as much of the oil as we can get in these areas the frame is pressure washed to remove the remaining dirt and oil prior to sandblasting.



Now is a great time to go over the clean frame and look for any welders spatter or sharp edges on the metal stamping’s and remove, or buff them out. Same for and gouges or scratches. These small details are what will set your frame apart from anybody else’s, and will lessen the chance of you cutting yourself while wrenching in tight places.



A tip for anyone who isn't sure the sandblaster got all the paint off the frame (it happens a lot around welds) Ask the PC guy to put the frame in the oven without paint for a cycle. This will cure most conventional paints and allow you to aplly PC over them. You can cure the paint with a torch as well if you notice a small spot of paint after you leave the sand blaster's.

[El Camexican]

So two weeks later and the frame is back at the sand blasters due to a piss poor first attempt at removing all the paint.

Meanwhile the closer I look at the cylinder the less I like what I’m seeing. There’s some weird wear patterns on the wall, like the cylinder isn’t round. Lots of blow-bye in the sides of the piston and almost no wear on the sides of the cylinder.





Coincidentally I just received back a cylinder with a new sleeve and unlike the one that was on the bike I know this one was done right. I removed the rings on the piston and dropped both the old and new cylinders onto the cases to see how the piston crown lined up with the bottoms of the ports. Interesting the piston drops almost .080” below the ports on the old cylinder and on the new cylinder the piston is only a few thousands below them.



Guessing whoever did the old cylinder didn’t sink the liner into the cylinder as far as they should have. So now the sleeve on the new cylinder needs to be matched to the ports and chamfered.





Stay tuned for more exciting posts!

[knappyfeet]

Once you come to grips with the fact that you are going to have to do this alone, it is always recommended that you loosen any hardware that you will eventually have to take off in a less convenient position, like with your frame laying sideways on the floor.


In this instance I will be removing the low profile retaining nut on the top of the triple tree so that I can pull the bars off before I remove the engine.

Attachment 258642


The key to not damaging a low profile nut or bolt is to just leave it alone, but if you must remove it use a socket which has been ground flat to remove the taper that normally exists to facilitate placing a socket over the head and clearing the slight radius where the hex meets the head on a normal nut.

Attachment 258641

Awesome tip!!

[El Camexican]

We begin to match the liner and ports to each other using a variety of carbide tipped bits.



The idea here is not to deviate in anyway from the factory port configuration, we simply want to match all the holes and angles up and chamfer the liner. Anyone with my porting skill level who thinks that they can improve upon the port map of a modern two-stroke likely suffers from a God complex and should not be allowed to play with these toys.





Once all of the rough shaping is done, the same carbide tools are used to rough chamfer all the port windows and edges of the cast iron liner.

Then comes the most critical step two engine longevity, polishing the chamfers.

I like to start with these little zirconium tipped bits and do the final polish with sanding drums. I should probably mention that if you are working with a plated cylinder you should not try to do the initial chamfer with carbide bits as you may ship the plating. In that situation you would start with these less aggressive stone bits, or sanding drums.



Remember, every edge that could possibly make contact with your piston or rings must be chamfered, that includes the base of the cylinder.



The final chamfering step is to make sure that every single edge of the ports are slightly rounded, especially the tops and bottoms. CC Specialties has some good videos on this subject.



When you think that you’re finished, get in there with your bare fingers and feel the edges of every port. If anything feels rougher than a wet clit you need to get back in there and polish it.

Chamfering is one area where the hobbyist porter can improve upon what comes from the factory. This task takes a lot of time and is cost prohibitive, but done properly can make your rings last considerably longer. Even when you pay to have your cylinder bored, most shops just knock off the edges with a carbide bit and call it done, so keep that in mind the next time you have a cylinder bored, or plated.

[El Camexican]

The next step is to hone the cylinder. I specifically ask my guy not to do it, that way I can clean up any little marks I make during the porting process.

There seems to be a lot of opinion about how this should be done, and I’m not saying my way is right, but I lube up the cylinder and stones with WD-40 and run the hone in and out for about 15 seconds before checking my progress.



It is critical to enter and exit the cylinder with the hone already rotating to avoid vertical scratches. Everyone will have their own style, but I like to hold the cylinder in one hand and the drill in the other to allow the process to center itself. I also like to yell out “oh baby oh baby” when stroking the bore.





Almost forgot to add the compression bleeding hole above the exhaust port. These things are real son of a gun to kickstart without them.

[El Camexican]

Being that this was originally billed as a boring thread it is mandatory that we share photos of the gasket removal process.



I assume most of us have read in our respective manuals to avoid allowing the connecting rod to come into contact with the cases when the cylinder is removed from the engine. As sinister as these warnings may appear in bold print, the reality is that short of forcefully ing on your connecting rod the only danger that contact with the cases can result in is a slightly proud indentation where the hard steel rod has struck the soft aluminum as has happened on this engine.



If we leave this high, or proud spot there is the possibility of an air leak forming in this location. To remove the spot I like to lay a flat file on the gasket surface and gently slid it back and forth to remove all high spots. As you can see by the shiny spots there were quite a few places that were proud.





In order to correctly set the deck height we must first install the piston. To prevent small parts and debris from falling into the crank case I like to fold a heavy duty shop towels, or rag and snap the center. It is then pulled down over the connecting rod much like a dental dam.

[El Camexican]

Now if this were a Wiesco piston we would be sanding all the sharp edges, but because it’s a Wossner it comes ready to install.





This brings us to one of my least favorite part about working on engines and that is the installation of cir-clips. If there is an easy way to do this I am unaware of it. My personal multi step operation involves bleeding, swearing and intense flashlight and magnetic tool assisted searches underneath my bench. You’re on your own here kids, all I can tell you is that it’s way easier to install one of the clips in the piston before you assemble it onto the connecting rod and that it is imperative that the open end of the clip either faces straight up, or straight down to keep it from compressing at high RPM.

I also make sure that I can rotate the clip slightly in the groove after I “think” it’s in place. Once it rotates a little you know it’s properly installed.

[El Camexican]

Now comes the part where we use an assortment of base gaskets to set the deck height of the cylinder. KTM specifies a tolerance +0.00” and minus .002” or something along those lines and sells gasket that range from tissue paper thickness all the way to .035”.



Having measured the original clearance as well as the old gasket thickness we knew that we wanted to lower the deck by .002”, so a .018” gasket was selected and we now have perfect deck height.



Hey! What’s that on the top of the cases boys and girls!?!?! Why it’s our old friend FREE Horsepower!!!

That gasket is protruding over .070” into the crankcase, exactly where the air is supposed to pass up through the transfer ports. No matter how nice a job you do on your ports, it’s all for nothing if a detail like this is ignored. Carefully remove the excess material with a razor and be prepared to bleed.







If you’re really serious about doing a good port job on an engine you should match the cases to the cylinder transfer ports while you are doing the port work. Unfortunately KTM uses the same cases for the 250, 300 & 380
cylinders, so there is some mis-matching built right into the design. I addressed this some years back on another engine by using epoxy to fill the void’s in the cases that were meant for the 380 cylinder. I’ve never had it on a synod, but I can feel a big difference at low rpm on that engine engine compared to any of the other 300’s I’ve built. It really feels crisp down low and likes leaner jets than the stockers.

[El Camexican]

Our last task for the weekend will be to transfer the power valve mechanism from the old cylinder to the new one.

These tiny little screws are notorious for not wanting to break loose,but I’ve gone through this enough times now that I take every precaution possible not to strip the heads. On this cylinder three of the screws came out easily, but there was one that didn’t want to, so I heated it up with a torch and that didn’t seem to work, so I gave it a smack with a blunt ended punch and it came out.





Always a good idea to take photos of the gear tooth positions before disassembly.



A clean power valve is a happy power valve.

[El Camexican]

It’s been ridiculously hot here these past few days. I don’t even want to go into the shop. Temperatures aside I managed to get out and finish the cylinder on the weekend. First by setting the ring gap which on this bore should be .013”. I don’t know if the cylinder was warm, or the bore was a little on the big side, but right out of the package the gaps were .016”, so I my want to order a “B” or “C” piston next time around. Wossner offer A, B, C, D, sizes for the standard bore.



I always take the edges off the ring ends with some fine grit paper.



Next I set the “Z” dimension which is KTM’s fancy way of saying power valve height. In this case, 46mm from the deck is recommended. The valve “over-opens” at high rpm, so you have the option of going lower without adversely effecting your top end power. I should take a minute to brag on this system a little. Aside from the adjustable stop on the PV the actuating rod is adjustable, there are three spring weight for the centrifugal governor and an adjustment screw for the pre-load of those springs to set the precise RPM that you want the valve to open. I’m not familiar with many modern systems, but this is a far cry better than the older Yamaha systems I’ve seen.



Time to paint the cylinder, All I had was some Painters Edging Tape, which isn’t sticky enough for metal. The regular stuff is much better for this task.



Next we bolt the cylinder on to the cases and call it a night.

[El Camexican]

What a show it was to get this frame coated. Literally had to fire the first sandblaster after his second attempt at removing all the paint. The guy had zero pride in his work, left patches all over the frame and didn’t seem to even try to get close to the welds.

Took it to another place on Saturday and they had it ready late Monday. They charge a lot, but they specialize in perfection. Dropped it at the coaters last night at it was done at 5:30 today. Probably the best job they’ve ever done for me. Took over two weeks, but it was worth it looking at this wannabe pumpkin.



Time to get this project back on the rails!

[El Camexican]

Rather than risk taking boring to a whole new level I decided to skip the process of switching out the non-kickstand frame for the freshly coated original frame and engine with a new top end on it. We can now skip straight to setting the drive chain tension.

Every factory manual has a spec for this, but if you seek the ultimate in chain tension the best way to do it is to rotate your wheel until you find the tight spot on your chain and then disconnect the shock and run the swing-arm up and down through its travel range until you find the tightest point which will indicate that the rear axle is at its furthest point from the center of the transmission output shaft.



You can now set the chain to be “tight” as in barely move it more than 1/2” in the center with a firm push. Between heat and sprocket wear your chain should never be tighter than this baring a dip into wet clay, or deep sand. Once it’s together make a note of how it feels and measures in a convenient location and use that for future adjustments.

[El Camexican]

Next is the part that shut us down a few weeks back when we discovered that there was no mounting provision for the kick-stand.



Took long enough.



While we’re here I should point out a little modification I made to reduce the noise generated by the chain slapping against the guide on the underside of the swing-arm. It’s just a rubber washer I cut out of a piece of old tire tube, but it makes a huge difference to the amount of racket going on down there over rough terrain at low speeds, which at my age are average speeds.

[El Camexican]

Now it’s time to switch out the shock spring from the stocker which suits a 160 pound rider and was installed to facilitate my 140# buddy, back to the 200# lardass rated spring.



Anytime I remove a shock I always take a moment to grease the Pivot Works shock spring bearing. If you aren’t familiar with these I highly recommend trying to find one for your vehicle. They add a plushness to the compression stroke that has to be felt to be appreciated. I always mount them at the highest point so as to avoid as much water and dust as possible. They come with rubber seals, but a fair bit of grease escapes anyway. They would have to be built a lot thicker in order to be perfect, but then you would have a lot of trouble getting the spring back on to the shock body.





Getting there....