I understand that people buy and use washers when indexing spark plugs but what I don't understand is why they don't use a nut instead? With washers, you'd need a multitude of different sizes in order to get it right but with a nut, you screw it onto the spark plug and then you screw the spark plug into the hole. Spark plug angled too far? Bring the nut closer to the top of the spark plug and retighten. Spark plug not angled enough? Loosen the spark plug and the nut then re-tighten once again.

It would just seem like that this would be the simplest solution to the problem of indexing spark plugs.

 

A 'nut' must have some reasonable thickness, and placing a nut between the plug body and the head would withdraw the plug electrodes further into the plug orifice into a position where the spark would be partially shrouded by the orifice, which may affect how well it ignites the mixture.

This 'recessing' of the spark will also cause the ignition timing to be effectively retarded because the spark is igniting the mixture from a point that is physically farther away from the piston crown. It may affect the degree to which heat conducts from the plug into the head casting (i.e. may cause the plug to run hot).

I also suspect that achieving the indexed angle as you want it using a lock nut won't be as simple as you're envisaging, with the nut slipping a you tighten it etc. Besides, there is so little to be gained from indexing plugs that it's just not worth the bother (any bother, no matter how you do it).