Metric Plugs?

[Steve Braverman]

When did spark plugs go to metric sizes, and why?

[Guest De Soto Frank]

Dad's '30 Chevy has 18mm plugs... most of the cars listed for my MoToR's manuals going back to 1935 have 14mm or 10mm plugs...

However, Gravely tractors continued using 1/2" pipe-thread spark plugs well into the 1960's...

Sorry that I can only furnish annecdotal evidence...

[mlander]

As with most things made to sell the real reason was spelled in green as in money. If you have one of each you will find that a 7/8”-19 plug weights more than an 18mm and so on down the line to whatever size you choose. Weight spells material, material spells money. The difference in size between an 18mm and a 7/8”-18 plug is .067” in diameter slightly over a sixteenth of an inch.

The reduction in diameter reached its zenith about 1937-1940 when GM’s Delco AC division went to 10mm as did Packard, Chevrolet and Buick all having some trouble with the plugs fouling up due to slow driving and the lack of gap between the insulator body and the inside diameter of the sparkplug shell. This induced a lack of scouring of the insulator and obviously failure of the plug in service.

Large sparkplug manufactures in Europe had been making 18mm plugs for some time . I have an 18mm KLG plug that was used in World War One. And I am sure that they were made before that time. Bosch claims to have been the inventor of the modern sparkplug but my view on this is that sparkplugs were a developed device and not truly invented by any one person or group of persons.

This big changeover took place around 1930-1932.

Yours, M.L. Anderson

[Steve Braverman]

Ok, mlander, your explanation makes sense, but only explains why the plugs were made smaller. I still don't understand why they used a metric size back when most Americans had no idea what a millimeter even was.

[1937hd45]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sdbraverman</div><div class="ubbcode-body">When did spark plugs go to metric sizes, and why? </div></div>

My guess would be 1900, Terry Bond will have more info on this topic. The 18MM plugs were standard on most motorcycles built up to 1940. Note the logo on this circa 1905 De Dion plug, upsidedown by todays standards buy easy to read when installed in that early automobile.

[Terry Bond]

Bob is right - the 18MM goes back to the turn of the century and was adopted here as a 'standard' along with 7/8" and 1/2" pipe thread which needed no gasket. The acceptance of 18mm here was because of the large number of import plugs and engines to run them in before 1910. A lot of very early cars used either the 1/2" or 18mm. So, the short answer to your ? is the 18mm came from Europe, along with everything else during the early days of the automobile.

Terry

[ak]

Most ball bearings back to the turn of the previous century were sized in metric.

[DonMicheletti]

Ball bearings still are metric (for the most part)

[Dandy Dave]

Did anyone notice this is from 2007? 

[1937hd45]

On 7/26/2007 at 9:14 AM, Steve Braverman said:

When did spark plugs go to metric sizes, and why?

In the late 1890's the good high end cars were built in Europe, they used the metric system, still do. 

 

Bob 

 

Thanks Dave, I rarely check dates, just post replies. 

Edited March 24, 2022 by 1937hd45 (see edit history)

[Dandy Dave]

LOL. It's Ok Bob. Sometimes I talk to myself. Sometimes I even answer myself back with mostly sound advice. 🤪😜 On this website sometimes it is like a echo from down a long hall that took 15 years to return. 🤭Dandy Dave!  

Edited March 24, 2022 by Dandy Dave (see edit history)