I was wondering the other day about the possibility of machining flat the tapered spark plug seats on a '75-up head during a head rebuild, perhaps with a fly cutter, to convert to the regular flat-seat ("gasket seat") plugs. Haven't looked at such a head to see what clearance issues might be involved, check the thread length, etc., so I have no idea if it would work or not. I do have an idea of how I'd do it: Take a piece of round bar stock, centre-drill it, thread the outside to match spark plug thread, add a dink to the top of the threads as a stop so it couldn't go all the way through the plug hole, and slot the top so a screwdriver could install or remove it from the plug hole. Install this pilot bushing, then use fly cutter to machine the plug seat.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Whye?

EDIT: Nevre minde, I juste saw your othre poste!

D/W

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If it ain't broke, fix it!

My advice would be to keep the pilot bushing idea, but not the fly cutter. Instead, I'd use a counterbore, like this:



No adjustable parts to worry about, and not many clearance issues if you get one with a long enough shank.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Good idea. Now the only remaining questions would be:

•How to make the pilot bushing so that it could be threaded into the plug hole as far as necessary to allow the counterbore to cut the plug seat all the way down, but so that the pilot bushing wouldn't be spun out of the hole by the counterbore?

•Whether the other plug parameters (thread length, reach, etc.) are compatible. Oops, the pre-'75 plugs are 0.750" reach, while the '75-up plugs are 0.460" reach. So, looks like this idea dies on the vine. Oh, well!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

How 'bout an adapter? A sleeve, tapered on one end, flat on the other? Slip over plug and tighten.

Seems like a lot of work for a sparkplug though. Are they that much better?


Hmmm... On second thought maybe I should patent that idea?!

Just use the champions that autozone sell you and change them every other week.