5 replies to this topic

[marion5drsn]

On very old spark plugs such as the ones called "Metric" about 1930, has anyone the information on the "Reach", which is the distance from the end of the threads to the mounting "Flat" These old plugs were 18 mm with a 1” hexagon,, A.C. G-1 to G-14 and had a terminal nut of 3/4"(?). But no place can I find the reach on those old plugs. Champion (? guess) J-12, J-9, H-9, J-7 had a 7/8” hex. I found some of the information in an old Audels 1951 but no mention of the brand or the reach.

http://www.corollape...sparkplugs.html

The old Audels 1938 might have it in there?
Also see sketch.
M.L. Anderson

[D-Type]

William Boddy, former editor of Motor Sport used to write about his collection of historic spark plugs. It might be worth dropping him a line c/o the magazine.

A letter to the main manufacturers might elicit a response.

[davidsmanson]

A difficult question, perhaps not properly for this forum (try the Antique Automobile Club of America site). Plug design was getting standardised by 1930 but many manufacturers offered different lengths in the same range, giving 'hot' or 'cold' results, well into the post-War era. Only a manufacturer's catalogue (or a sparkplug nutcase) will give you a comprehensive answer.

[xkssFrankOpalka]

Some of those plugs used a tapered pipe thread and some a straight thread. Many plug collectors in the US. Different brands and designs of plugs are very many. There is a plug collectors society.

[McGuire]

In the Champion system, the "J" letter prefix means 14mm thread, 3/8" reach with gasket seat, and 13/16" wrench hex. The number which follows indicates heat range, while the suffix letter indicates the gap/nose type. If you consult the spark plug catalogs for Champion and others you can decipher all the various codes for each manufacturer.

[marion5drsn]

The spark plug in question is an old 1930 AC "G" series. The problem is finding the old spark plug charts. I have spent several weeks trying to find the old chart and finding one back to 1950 is no problem, but finding one back to 1930 is very near impossible as all of them on the net stop at 1950 as they do also in the dealers computers charts. Finding 14 mm also is no problem as they run into the 1950's but finding the 10 mm of the late 1930's is also very difficult as the few lists list them as 14 mm due to GM's success at drilling them out to 14 mm from 10mm seems to have been a complete success in that respect.

As some people know the 10 mm of that time were a complete failure By GM-AC and the standard “FIX” was to drill and tap them out to 14 mm. Not too unlike FORDS problem of their 10 mm plugs in the last several years.
But anyway, it is the finding of the old charts to find the whole story of just why the 18 mm “Metric” plugs were dropped so quickly is the puzzle.
M.L. Anderson