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Engine Rebuild


Engine Rebuild
Mark Correll

Thoroughly checking the engine of an early MX bike during a restoration goes without saying. During their time in competition the MX engine was subject to severe stress, and, if not maintained properly, would unlikely be in good condition.

Although the internal condition of an engine can be ascertained to a certain extent, a competition engine must be fully disassembled and inspected. The problem with MX bikes is that the pistons and bores are often scored/scratched due to the ingress of dirt during competition. A prudent owner will have had the engine re-bored periodically, but most engines (cylinders) can only be bored three times before the sleeve would need replacing.

New engine parts on this particular bike included:

  • NOS (New Old Stock) crankcase assembly
  • NOS KTM crankshaft main bearings
  • New KTM 85.5 piston on fresh bore: second oversize (0.5mm)
  • New wrist pin bearing
  • New gaskets and seals
  • New transmission bearings
  • Low hour crank assembly and transmission
  • NOS clutch actuator arm
  • New fasteners all around (most SS)
  • New springs for K/S and shift detent
  • NOS case protector
  • NOS Hi-Point shift lever w/folding tip

When considering a MX bike for restoration, it should also be noted that some MX bikes (such as the CR Honda’s) have chrome bores. These cylinders must be replaced if damaged.

At an early stage of the restoration, the owner must decide if he or she is going to keep the bike original, or update certain parts. Although updating can reduce the value of many classic bikes, few MX bikes remained true to their original specification. Most owners would fit after market pipes (expansion chambers) during the bike’s first season of racing, as an example.

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