BSA All A50, A65 (std plug) 4054 N3-C W5CC 43N
BSA All A50, A65 (hot plug) 4056 N4-C W7CC 44N
BSA B25 4054 N3-C W5CC 43N
BSA B44 4056 N4-C W7CC 44N
BSA Rocket III 4054 N3-C W5CC 43N
Norton 650 4056 N4-C W7CC 44N
Norton Atlas, P11A 51 N6-C W5D6 R41XLS
Norton Commando 4252 N7-C W5DC R41XLS
Triumph Pre-Unit models with cast Iron head 4123 L82C N/A 441F
Triumph Pre-Unit with alloy head 4056 N4-C W5CC 44N
Triumph 250,500 unit 4056 N4-C W7CC 44N
Triumph Unit 650 4054 N3-C W5CC 43N
Triumph 1972-77 Unit 750 Twins 4054 N3-C W5CC 43N
Triumph Unit 750 Triples 4054 N3-C W5CC 43N
Triumph 1978-82 Triumph Unit 750 Twins 4056 N4-C W7CC 44N


Avoid over tightening Spark Plugs, especially when installing them in a hot engine with aluminum heads. A plug that is over torqued when the engine is hot can cause crack(s) in the head as the engine cools down.

Generally speaking, you want to run as wide a plug gap as possible. Having to reduce the size of the plug gap once it has been well established can possibly been an indication of problem manifesting in the ignition system. This is common in the magneto systems as they generally need smaller and smaller plug gaps as the magneto deteriorates.


A spark plug that appears to have a pinkish hue to the electrode has either been overheated or has been subjected to pre-ignition. Pre-ignition is the result of the cylinder firing before the plug fires. Carbon buildup, untrimmed gaskets, sharp edges, too hot of a plug are just some of the causes.


Little black splotches on the insulator of the plug can be caused by detonation, which is basically an explosion in the cylinder after the plug has fired. Shitty fuel, too far advanced timing, engines that have seen a lot of miles, or were improperly assembled can suffer detonation when oil gets into the combustion chamber and reduces the octane of the fuel. If that happens, a rebuild is the only repair, otherwise retarding the timing, and/or using better fuel usually does the trick.


Aluminum deposits on the plugs means that the piston is overheating. This is caused by either a lean mixture, or having the timing too far advanced, or a combo of both.


When you have the head off, check the projection of the plug into the combustion chamber. If there are threads showing you will want to either use washers to pull the plug back up or better yet switch to a shorter plug, if possible.

 Tech Tip: A thin coating of some copper Anti-Seize (Sold on Page 120) on the spark plug will go a long way towards eliminating thread problems with the spark plug hole.