Spark plug cap replacement

by Had Robinson

This procedure is for the NGK replacement cap available from Miniplane-USA.

Parts needed:

RTV silicone cement (blue, gray, or black)
Nylon zip tie

To ensure a secure and moisture proof connection, do the following when replacing the cap.

  1. Remove the old cap.
  2. Using a digital ohmmeter (often <$10 or free from Harbor Freight), measure the resistance of the end of the secondary wire to engine ground. It should be either 17.5K Ohms or 8.8K Ohms, depending on the type of secondary wire used by Miniplane.  If there is no resistance, you will have to replace the entire secondary wire and cap.  You can cut back the wire a half inch to see if there is resistance but that is the limit for cutting.
  3. Use RTV cement (blue, black, or gray) to thoroughly coat the last inch of the wire. Using your finger, put a blob of RTV in the wire opening of the boot.
  4. Insert the wire in the boot until it contacts the threaded spike at the end.  Push firmly on the wire and twist the boot about 4-6 times in order to screw the threaded spike into the wire.  When it is completely screwed in, you will feel a little bit of resistance. DO NOT CONTINUE TO SCREW THE CAP ONTO THE WIRE!
  5. Use the nylon tie to squeeze the rubber boot onto the wire. Using a double wrap of the tie will ensure good contact.
  6. Wipe off the excess RTV.
  7. Let the RTV cure for 24 hours. DO NOT RUN THE ENGINE UNTIL THE RTV CURES!