Spark Plug type

[Roger/Sacramento]

My motor is 276 cubic inch with Offie 400 heads, 3-97's, L-100 cam, crab ignition ,Bosch coil and I use non-alcohol gas....what say you?

[drolston]

For the Offy heads you need a plug with a little more reach, like 7/16" instead of 3/8 for stock iron heads. With 284", Offy 425 heads, similar cam, and same ignition, I am running NGK 6BHS plugs with good success so far. If your engine is prone to fouling plugs, you could use the NGK 5BHS, which is one range hotter. I carry a set of the 5BHS as spares, because I learned that once fouled there is not much you can do to get plugs un-fouled. Nobody has the old spark plug sand blast cleaners any more. With good reason, - such cleaning did not last long.

[tubman]

We've been through this before. It seems that the Offenhauser heads are oddballs in that they use spark plugs with a reach that is not made by anyone. We came to the conclusion that to obtain the optimum plug position, they need an "N" series Champion plug or equivalent (3/4" reach) with a .21" spacer.

See this thread on the H.A.M.B. that references another thread back here on The Barn : https://www.jalopyjournal.com/forum/...#post-13327633.

[drolston]

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My mistake, the NGK B6HS is 1/2' thread reach, not 7/16. In my Offy heads they are about one and a half thread from flush with the underside of the head. At that, the electrode protrudes well into the chamber. As I said, they work well so far. Same for the B5HS. "H" is for thread depth of 1/2" at NGK.