spark plug size change

[mikes31rscoupe]

Hello

Ihave a 31 pontiac that needs the heads replaced. The heads I have use cnvention sized plugs. I have a set of earlier plugs that is identical on the back but it uses the older style lqrger plug hole. Is that a thread adapter made or i can machine one to bush down and fit the newer pllug. Does anyone see a problem with this? Wher exactly should the plug gap {spark} be sitting with relation to the bottom of the head at the top of the cylinder. I can send photos of what I have.

Thanks
Mike

[Commodore]

One thing I can see with doing this is that the heat range of the spark plug will be changed because the adapter will move the plug farther from the water jacket.

[mikes31rscoupe]

Makes sense. I have the machining abilitiy to "adjust" the thread adaptor to allow the new sparkplug to sit exactly where it should. Does this sound reasonable?

[Tinindian]

Unless you are trying to make it look, to a casual observer, that you have the correct head why not just use the 7/8 pt spark plugs. They are almost as readily available as the 18mm ones.

[mikes31rscoupe]

That would be fine with me. I was told that the old plugs were very hard to find and also that the engine does not perform as well as it does with the newer plugs. Have you found that to be true? Which plug would I be looking for to fit the 7/8 pt head?

[Tinindian]

I don't know about "very hard to find". My local NAPA has Autolite 3076 plugs in stock. I also have a set of them that I will never use. I am sure that someone has a conversion chart. If you are looking at Champion plugs the prefix is GC, AC the number starts with 7 ie 76 or 78 obviously the 78s are a hotter plug. You are looking for a 7/8 pt with a 7/8" reach.
I have owned a pontiac of each year that they had the split six engine. The series 6-30B, 401 and 402 were much better running cars but I attributed it to more displacement, more compression and a better carb. I don't think the change of spark plug size would have had much effect.

[nzcarnerd]

Not sure just what size plugs you want to change from and to, but for reasons of ready availability we have put adaptors into the 1926 engine to use modern plugs which are much cheaper to buy here (NZ$6) than the correct type. Check here for a picture - About Quiz # 316: 1926 Pontiac 6-27 (Upd. two sons, two carbies) - PreWarCar and click on the engine info link. The adaptors are made locally and cost NZ$9 each and are available in several different sizes. As you can see the car is not a restoration, more of a just-get-it-going so that the kids can drive it round the farm.