KTM Twin-Cam Top End Rebuild

Now that KTM uses the late-model DOHC engine in SX and XC 450s and 505s, the numbers of these motors in use is rapidly growing and folks need to start looking inside these engines. Dirt Rider has pulled down an SX and an XC 450, and the internals were in essentially perfect condition. The sole exception was ring end gaps were both well beyond tolerance. Neither bike expelled smoke from the exhaust, but the SX started sluggishly.As always, this article isn't intended to replace a manual but to supplement it. In addition to the normal tools and a feeler gauge set, you'll need a small torque wrench that allows accurate setting under 10 foot-pounds. You'll also need a few Torx bits.Thoroughly wash the bike, then remove the seat, fuel tank, exhaust and subframe. You can loosen the top subframe bolts, remove the bottom ones and tip up the subframe. That way, none of the battery cables needs to be disturbed. Drain the oil and the coolant before starting.The valve clearance will give an indication of where to concentrate your efforts. If the valves check out, then the head should require little attention. If the valves are extremely tight, the head will need to be disassembled and inspected.It isn't shown in the photos, but all parts of a four-stroke should be oiled during reassembly with motor oil. Use assembly lube on the cam bearing surfaces and lobes.It's time to button up everything that was removed to access the engine. Make absolutely sure to remove the bolt locking the crankshaft! Replace it with the original Allen bolt and copper washer. Take special care with the coolant hoses and clamps, the throttle cable routing and the carburetor clamps and vent hoses. Fill the coolant and put oil in the engine. Start the engine and look for any leaks. If everything looks good, break in the bike for a couple of heat cycles, then go ride.

1. The ignition cover timing plug is a 14mm Allen. It's difficult to find, so Clint made one from a handlebar mounting bolt and two nuts locked together. The plug must be removed to turn the engine when needed.Dirt Rider
2. Removing the plug cap is easiest if you pop it loose, then remove the cam cover bolts. Lift the cam cover from the left side to angle the cover to the right. The plug cap should lift out easily. Castleberry left the plug in.Dirt Rider
3. Rotate the engine with a socket or T-handle through the timing plughole. Don\'t use the hex on the end of the cams to turn it! Rotate up the dot as shown. Check the valve clearances.Dirt Rider
4. When that dot on the cam chain gear is up, the exhaust cam lobes should face front and the intake cam lobes toward the rear of the engine. The flats machined onto each cam should align as well.Dirt Rider
5. To hold the engine at TDC, remove the short bolt shown at the engine\'s front on the lower right. The hole will always leak oil. Replace the bolt with a bar mount bolt ground or filed to a smooth point. It will still work on the bar.Dirt Rider
7. Thread the bolt in finger-tight, then rock the crank back and forth slightly as you thread it in fully. The bolt slips into a crankshaft notch and locks the engine at TDC. Clint uses a paint pen to mark all the parts to speed installation.Dirt Rider
7. Break loose the outer nut on the hydraulic cam-chain tensioner, before removing the entire unit. Watch the copper washer and O-ring; there should be no other loose parts to worry about.Dirt Rider
8. Note the additional paint pen marks on the flats of the cam. Loosen the cam cover bolts a quarter turn at a time in a cross pattern. Loosen all the bolts, then remove them.Dirt Rider
9. On the KTM design, the cams lift right out once the caps are removed while the cam chain stays in place. There\'s no need to mark the cams. The exhaust cam will have the auto-decompression mechanism.Dirt Rider
10. Inspect the cam lobes and bearing surfaces for wear or damage. There should be no rough or discolored spots. While the cam bearings still have oil, spin them by hand to make sure they\'re smooth and quiet.Dirt Rider
11. Look for similar damage to the finger followers. Inspect where the cam lobes ride on the top side and the tips on the underside where each contacts the valve adjusting shim. Again, no wear or discoloration is ideal.Dirt Rider
12. It's easy to forget this small nut holding down the head. Check to make sure no coolant hoses or other parts (carburetor) will keep the head from lifting cleanly off the engine.Dirt Rider
13. Loosen the cylinder head nuts in a cross pattern. Work a bit at a time and be careful with the two that are inside the engine. You definitely don\'t want to lose a nut or washer down inside the engine cases.Dirt Rider
14. Remove the cam gear shaft cover from the left side of the cylinder head. It holds the shaft in place and keeps it from spinning. Gently work out the shaft. Try not to mark it up in the process.Dirt Rider
15. When the shaft is out of the gear, it will drop down so it can be removed. Be careful not to lose these bearings as the gear lifts out. Inspect the shaft and bearings for wear and damage.Dirt Rider
16. Lift the head straight up. It will have to get extremely close to the frame backbone before it will clear the studs. Watch for coolant hoses messing up the lift off. The chain is allowed to settle gently into the cases.Dirt Rider
17. The cylinder will resist being lifted. This water junction is a tight fit with the back of the engine sidecase. We loosened the bolts in the water-pump area to ease this connection and make the cylinder come away easier.Dirt Rider
18. This was the only sign of use that the engine showed. The standard gap is 0.2mm and ours had 0.75mm. The piston/cylinder looked fine, so we changed rings, gaskets and O-rings.Dirt Rider
19. Two O-rings in the case opening and another around the cam-chain opening act as a base gasket. Use grease on the cylinder O-rings to hold them in place and allow the cylinder to slide past without unseating them.Dirt Rider
20. Getting the rings into the oiled cylinder is a pain. KTM removes the piston and uses a ring compressor to get it started in the cylinder; both go onto the engine together, and you only have one circlip to deal with.Dirt Rider
21. The junction to the water pump got a fresh O-ring and coat of silicone. The pump/cylinder junction is a tight one. The engine got a new head gasket, and it was installed dry.Dirt Rider
22. Locate the cam gear in the chain with the bearings inside and ease the shaft through. Needle bearings can crush, so don\'t force it. The cover locates the shaft, so the flats must be vertical.Dirt Rider
23. The engine is locked at TDC, so line up all the paint marks and install the cam carriers. Torque the bolts (cross pattern) to 14 Newton-meters. There's no clearance for a 3/8 drive torque wrench.Dirt Rider
24. Compress the cam-chain tensioner until the inner plunger extends 4mm, then screw it back in with the outer cover only. Push the internal plunger with a blunt tool inserted through the inspection hole. You should hear it release. Install the outer nut.Dirt Rider
25. Install the cover over the end of the cam gear shaft. Make sure the shaft is aligned with the dimple on the cover. Did you remember the nut on the little stud below the cover?Dirt Rider
26. Use great care with the cam cover bolts. They thread into the cam carriers, so stripping them will be an expensive pain in the rear. The bolts are torqued to only 10 Newton-meters-and that's a bit past finger-tight.Dirt Rider