Re: Spark Plug Protrusion into Head?
If your 3.5 threads short, your exactly 1/4" short of full tread engagement.
I'm not sure if Ford was first with the extend reach plugs but, the reason was to keep them clean, over a variety of conditions. The porcelain being exposed to keep deposits, burned off. Plugs will usually, first foul at the porcelain to electrode join. Deposits build in that area. FH's never came with that style plug but, can benefit from them, if they clear the valve heads.
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Frank
'35 Ford Model 51
'48 Ford F3
'54 Ford Tudor Mainline
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