KTM Twin-Cam Top End Rebuild - Dr. Dirt

Tech How-To
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.

01 The ignition cover timing...        read full caption 01 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.	02 Removing the plug cap...        read full caption 02 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.	03 Rotate the engine with...        read full caption 03 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.
04 When that dot on the cam...        read full caption 04 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.	05 To hold the engine at...        read full caption 05 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.	06 Thread the bolt in finger-tight,...        read full caption 06 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.
07 Break loose the outer...        read full caption 07 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.	08 Note the additional paint...        read full caption 08 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.	09 On the KTM design, the...        read full caption 09 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.
10 Inspect the cam lobes...        read full caption 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.	11 Look for similar damage...        read full caption 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.	12 It's easy to forget this...        read full caption 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.