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Guide To Four Stroke Motorcycle Engines, Part 8

Updated on December 31, 2009

Valve Spring Removal

Most bikes require a special motorcycle valve-spring tool. However, your local welding shop can usually make one for you out of a "C" clamp if necessary. Compress the valve springs just enough to free the keepers. Then gently un-clamp the spring and store the keepers, retainers, washers, and spring until later.

If the end of the valve is mushroomed or peened over, file the edges until it slips freely through the guide. Test the valve springs for height with an accurate scale. If you can detect at the valve seat more than four or five thousandths of an inch of wobble in the valve guide, the guide should be replaced.

These guides can be replaced by using a special drift to remove the old guide after heating the head in an oven to about 350 to 400 degrees F. The same drift can be used to install a new guide.

Inspect the valve seat and valve face for burning or pitting. Sometimes motorcycle valves can be lapped in with an abrasive compound and will seat perfectly. Often, however, a new valve is required and it must be lapped into the old seat. These steps are required to lap a valve to its seat:

  1. After cleaning the seat and the valve, sparingly apply some lapping compound to the face of the valve.
  2. Insert the valve in the guide, being careful not to contaminate the guide area with compound.
  3. Using either a suction-cup stick on the head or a piece of small hose on the stem, rotate the valve against the seat. Lift the valve 1/4 inch occasionally to help the lapping compound work.
  4. Check the contact pattern made on the seat and face. There should be a dull gray pattern completely around both the valve and the seat.
  5. Clean the seat and valve thoroughly with solvent or soap and water. Again, be careful not to get any compound in the guide.
  6. Apply oil to the seat face, stem, and guide before reassembling the head.
  7. Check your job with Varsol or kerosene in the ports to see if more lapping is required.

If the seat is in poor condition, it too must be refaced or sometimes replaced and the new seat refaced. It is more common to replace the valves in a motorcycle than to grind them as is done in automotive head reconditioning.

Be sure to clean the valve thoroughly before replacing it. Don't use abrasive on the valve stem area since any material ground away will increase the clearance between the valve and guide and cause the guide to wear faster.

Lubricate the valve and guide before reassembling. Special molybdenum disulfide grease for engine rebuilding will insure minimum wear when the engine is first restarted. If the engine uses valve seals, slip them over the valve stems before installing the springs and retainers.

Check your valve job by pouring Varsol or kerosene into the ports and seeing if the valves seep any fluid. If so, disassemble that valve assembly and lap it until the valve seals properly.

Spark Plug Service

You may want to repair a damaged spark-plug hole while you have the head off. Thread inserts are an easy, cheap way to fix damaged plug holes. The procedure is to run a special reamer-tap through the old hole until a new, larger set of threads are cut. A coiled wire insert is installed in the new threads that duplicates the original threads. Break off the "tang" with a pair of needle-nose pliers and you're done.

Continued in Guide To Four Stroke Motorcycle Engines, Part 9

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