First, while you might be able to purchase a single spark plug and still get a couple of pence back from a 20 pound note, spark plugs are not usually purchased individually. They are purchased as 4, 6, 8 for your typical vehicles, corresponding to the number of clyinders – V8, 6 (as in the ‘Flathead’ 6 which some of us old timers will remember), 4 cylinders as your beloved Colt Jon (that’s his car, not a gun for you NRA types). There’s even a couple of 2’s – as in the Criteron CV-2. Or it could even be 10 or 12 if it’s a real high end car (think Viper, Aston Martin, etc.)

Second, to get your plugs changed you’d have to add another zero (at least!) to that 20 and that’s if we’re only talking about a 4 cylinder.

But then, maybe your garage just takes the plug out, cleans it (yes Jon, they can be cleaned) and takes your 20 for the trouble.
And as you might say sir, “Robert is your father’s brother’.

I mean, every time I take my car to the garage and they say it needs a new spark plug, that’s what I hand over and it seems to fit just perfectly to fix the engine…

Yep, BJ, you’re right. I have *no* idea whatsoever!