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Triumph Repair/spark plugs


My 1965 spitfire calls for 2 different lengths of spark plugs in the books at the auto parts store.  1/2" and 3/4"   When I took out the plugs from the engine I think there were 3/4" for the middle 2 cylinders and 1/2" on the outside ones.  Can this be the case?  2 different lengths for the same engine.  At least I know that the 3/4" didn't bottom out on the cylinder head as I've been driving it for a year.

Why would it call for the different lengths in the MK2?

Thanks John

Hi John,

While different length plugs are used on many cars they are the same length in each. No manufacture would put two different lengths in the same engine. On many of the British cars a different year/model head can be bolted on to a block so it is possible to have the wrong year head on an engine and thus the required length plug could be different then the year model of the engine. When a parts store shows two different lengths it is because they were unable to figure out which engine took which length plug. In the Spitfire 65 Mk II they only used 1/2 in plug reach. In 1967 there were some heads that used 1/2 in and some used 3/4 in but never a combination of both sizes.

You should NEVER put the wrong length plug in any engine. It is very easy to tell what length is required. Just take a piece of stiff wire and bend a sharp 90 degree bend in a 1/4 inch of the end and use that as a dept gauge to check the dept of the threads. You will not find any difference between 1,4 then 2,3. If you do then someone machined the spark plug area for some unknown reason (Stupid reason).

The heat range of any sparkplug is drastically affected by the contact area of the plug threads and the head threads. This MUST be matched exactly. Most race engine builders go so far as to make it match better then the engine manufacture did by machining the gasket surface so that the length matches exactly.

Even though some highly skilled mechanics can alter the heat range of a plug to match a specific engine to the driving conditions, engine conditions and location, They will never change the length of a plug.


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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