[1965tripleblack]

Smallblock Chevy 327 with Crower Sportsman rods. Forged pistons. Engine makes peak power at 6300 RPM and shift point at about 7000 RPM. Blueprinted engine with all 8 cylinder quench within .001" of .035". Replacing standard spark plugs with extended tips. If I correctly index the plugs with the opening facing the center of the chamber (ground electrode facing piston crown), there is about .020" clearance. I figure that I have three options:

1. Go with the .020" clearance.
2. Index the plug by turning it 90 degrees so that the electrode won't interfere with the crown.
3. Add washer(s) under the seat and index as in (1).

What is the conventional wisdom for recommended clearance here?

Thanks.

[Fred Ficarra]

I'd go with #1. That's plenty for a plug. You just don't want interference that closes the gap. And I also think that indexing is a waste of time.

[BillK]

Joe,
I used to have to do that with my 355 in my 67 Drag car. I used to buy 3 or 4 sets of plugs and mark the electrode position on the porcelain. Then I would try them one at a time until I got a whole set in with the electrode to the top. That was the only way they would not hit the domes, and that was not even with extended tip plugs ! I never liked the washers although plenty of people use them.

That being said, .020" should be ok unless the piston does a lot of rocking around.

[Greg O]

Are you heads up class racing this thing? Otherwise I am not sure how much you think you will pick up going to extended tip but I bet it's not worth the aggravation IMHO.