Re: Why the 10mm spark plugs?

Posted by Owen_Dyneto On 2017/1/9 12:31:42

The 10mm plug movement was initially promoted by GM's AC division with support from Buick and the use was extended to some other GM makes (Cadillac for one) and somewhat sporatically across the industry in general but with exceptions; the were promoted by AC as being more compatible with higher compression ratios. The 356 engine also used 10mm plugs though after it was discontinued Packard did produced a 356 head in 14mm.

Plenty of garages at the time made some extra money by drilling and retapping heads for 14mm plugs. That was not without it's problems however as the thickness of head at the plug threads corresponded to the threaded length of the 10 mm plug and when modifying for 14mm, you now had several turns of thread exposed in the combustion chamber.

If you scout around advertising material of the era, you'll find AC ads promoting the virtues of the 10mm plugs.