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This site was last updated on February 15, 2008.
The plug on the right is a standard glow plug; the one on the left is a Nelson plug. Notice they are different sizes.
- The standard plug is threaded 1/4" x 32 TPI.
- The Nelson plug is threaded 11/32" x 32 TPI.
Note also that the Nelson plug doesn't have a washer -- this is significant. The standard plug seals at the washer, allowing combustion gasses to invade the space around the threads. The Nelson plug seals at the bottom of the plug, where the little taper seats in a corresponding taper in the head. The result is more rpm, i.e., power :-) As a bonus, Nelson plugs have the reputation of being quite robust. They're also reasonably priced.
Of course, Nelson engines use Nelson plugs; but many other engines use Nelson plugs too. Furthermore, Henry will convert the head of your favorite engine for you.