Free printableMeasure partsTSP-ME-19
How to measure a spark plug's thread, reach and seat
Thread diameter, reach and seat type decide whether a plug is safe to fit. All three are measurable, and none of them is the heat range.
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TSP-ME-19 · version v1.0 · September 2026. Opens in your browser's PDF viewer, where you can print directly.
The problem it solves
A spark plug of the wrong reach will thread in perfectly and then either leave threads exposed in the combustion chamber to carbon up, or protrude far enough to be struck by the piston. Neither is visible once it is fitted, and neither is prevented by the plug fitting the spanner.
Three of a spark plug's dimensions are purely physical and can be measured: the thread diameter, the reach — how far the threaded section extends — and how it seals, either on a gasket or on a tapered seat. Get any of those wrong and the consequences are mechanical rather than a matter of tuning.
The fourth property, heat range, is not measurable and is not on this sheet. It is specified per engine by manufacturers we frequently cannot identify, the numbering runs in opposite directions between makers, and getting it wrong causes damage slowly enough that you would not connect the two. The right move there is to record the plug that is currently in your engine and running, which is on the spark plug log sheet.
So this sheet measures what can be measured and says nothing about what cannot.
The mistakes this sheet exists to prevent
- Fitting a longer-reach plug because it threads in, which can put the electrode in the piston's path.
- Fitting a shorter-reach plug, leaving unthreaded bore to fill with carbon so the correct plug will not go back in later.
- Mixing a gasket-seat plug with a taper-seat head or the reverse.
- Reusing the sealing gasket, which is designed to crush once.
- Choosing a heat range from a forum recommendation rather than from the plug that is currently working.
Who it is for
- Anyone fitting a plug with a different part number from the one that came out
- Owners of an engine whose correct plug nobody knows
- Anyone who has found the old plug's threads carboned or damaged
What is on it
- True-size thread circles for the sizes used on small engines
- How to measure reach, and why it is the dimension that does damage
- Gasket seat versus taper seat, and how to tell
- A record block for the plug that is currently working
How to use it
- 01Check the calibration rule before using the thread circles.
- 02Stand the plug's threaded section on the circles to find the thread diameter.
- 03Measure reach from the sealing face to the end of the threads, not the whole plug.
- 04Record the part number of the plug that is currently in and running — that is your reference.
Use it with the registry
Other sheets you may want
- Spark plug record and reading logThe plug is the cheapest diagnostic instrument on the machine. A record of what it looked like each time turns it into a trend rather than a guess.
- GY6 no spark diagnostic chartWorking back along the ignition circuit from the plug, one component at a time, without replacing anything on spec.
- Mystery part measurement sheetYou are holding a part. Nobody knows what it is called. This sheet records everything a supplier needs in order to find it for you.
Free to print, copy, and share — including in forums, clubs and workshops. If you find a mistake on this sheet, tell us and it gets corrected. Quote TSP-ME-19 and the version so we know exactly which sheet you mean. All printables.