The nipples are universal, they are NOT a different size between "car" and "bike" plugs - because cars and bikes often use the same model of plug.

Just buy the cheapest, most bog-standard type of spark plug you can find and take the nipple off it.

Plug nipples are certainly NOT "universal". Hardly anything is "universal"

How many types of "Universal Serial Bus" cables are there?

"Universal" my a***.

I dunno if the size varies, but there are at least 2 basic types of nipple.

One type is shown in the linked photo above. (A picture isn't worth a thousand words if you don't look at the picture, so heres a thousand words).

This one was removed from a used plug by a mechanic in a local car garage and given to me. He used quite a lot of force so its possible this type isn't intended to be removed. It has a "male" threaded portion which screws into the top of the plug. This type does not fit the end of the Gunson adapter, which also has a male threaded portion.

The other type of nipple is often used on motorcycles here. This type has a "female" threaded hole in the base which accepts the "male" threaded rod that is in the top of the complementary type of spark plug. I THINK these spark plugs are often supplied without the nipple, but I've only bought one once, so this practice probably isn't "universal" either.

Apparently some plug leads, on older cars (and perhaps on contemporary Taiwanese motorcycles) are designed to engage on the threaded terminal of the plug, which is then used without a nipple.

forum.retro-rides.org/thread/177176/spark-plug-nee...d

This "female" type of nipple seems to be required to fit the "male" end of the Gunson adapter, but I don't know whether the size and thread pitch (which may not be "universal") are necessarily going to be compatible, and, as noted in the initial post, I've been told that they aren't.

Here's a description of just some of the many varieties of "universal" spark plug terminals.(bottom of the page)

www.sparkplugs.co.uk/technical-glossary/

"Both of the links show exactly what I described: a removable nipple which threads onto the male threaded terminal at the top of the plug,"

I didn't say it doesn't exist. I said it isn't "universal". You did, and it isn't.

"I've been buying NGK spark plugs for my bikes for 35 years and they ALWAYS, ALWAYS come with a removable screw-on nipple. "

As described in the original post, they don't ALWAYS, ALWAYS come with one here, because I bought one a few years ago and it didn't have one, just the bare thread. The usual car plugs here don't have that type of nipple, as shown in the linked picture.

As I said in the original post, I'll probably be able to get one eventually after CNY (everything is currently closed), though I have been told (by Taiwanese mechanics, who do tend to be full of it) that it won't fit.

Failing that I'll have to make something, as outlined in the original post.

Edited by edlithgow on 17/02/2018 at 03:28

There are more than one size of spark plug nipple thread. I found this out when working on a '30s car that had thumbscrews holding the HT leads on the plugs.

There's also a plethora of plug diameters and lengths.

Back in the '60s and '70s when colourtune developed there were probably only a couple of common sizes.