Technical Heads and helicoils

I have a set of old aluminum Edmunds heads for a 346 Cadillac Flathead. One head is good with all spark plug holes intact- the other head has two helicoil repairs that were done professionally however they look to be a different size than the stock plug holes. Question(s) is-
Am I able to use two different spark plugs on the same engine or would that effect performance/ general running of the engine?
Or
If two different plugs cannot be used is it possible to have the helicoils removed, welded and repaired? (If that's the choice any recommendations on where to have that done)
Or
Helicoil both heads so all plugs are the same sizes?

 

See if you can find a plug to fit the repaired hole that has the same reach and heat range as the other plugs.

 

I would have the heli coils removed, and replaced with the proper sized heli coil, or have all other holes changed to the different size. Keep them all the same.

 

two sizes? as in 14 mm and 18 mm?

 

I think some of the Cad flatheads had 10mm plugs!

 

The engine won't know the difference.

 

That's what I was wondering. I'd rather not mess with the head and run two different types of plugs. As long as there no weird imbalance I could care a less if it looks different

 

Hawker driver, would that be a Hawker Hurricane ?

 

Fix it while it's off, helicoils get funky sometimes....

 

I wish. I used to fly HS-125 Hawkers years ago. Now it's G550's.

 

I would think you could find the same heat range plugs that would fit the different thread holes, and if you did, would be ok. BUT I could not live with that...I would just have to repair the damaged holes to fit the same plugs. I guess the "repaired" holes are the larger size ones? if so, could you redrill and thread all the others to the larger size? just a thought. But I think I would make the larger holes the size of the other smaller ones. Can you make or get someone to make adapters? I'm thinking 18mm holes down to 14mm holes.
Man, I would love to have them heads on my '37 Cadillac.

 

Word. It comes down to how AR you are about stuff like that. Even though I have a touch of OCD, I would leave it alone as long as the inserts were properly installed.

 

346 Cadillac flatheads used 10mm plugs in the original iron heads, but I thought the Edmunds heads were out of the box with 14mm holes.

 

The repairs were done correctly. So that's my dilemma. Unless someone actually looked carefully it's very hard to tell which ones were repared. That being said I too have a slight case of OCD and it does bother me a bit that two different types of plugs will be used BUT if the repare(s) are over the top then it's best to leave things as is.
Because these heads are so damn rare I'm reluctant to screw with them. The helicoils were done back in the day and are done right so as long as there's to weird imbalance I'll leave things the way they are.

 

Are you sure they are Helicoils? Better option is to use TimeSerts, they give a better seal and stay put.


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I ran a flathead for some years that had had a couple of its plugs taken out to 18mm in the dim past, never caused any issues. Depends how anal you are about things like this, in a low comp engine, if it lights the fire when the dissy tells it to you should be ok.

 

They did use the two size plugs, what, 10mm and 14mm or 14mm and 18mm don't know the correct sizes but the smaller plugs were in the newer engines, also don't know the year of the change so I guess I'm not much help.
anyway my '37 346 motor runs the AC 45 plug, same a the Champion J8.

 

Fix it while you have it apart - use the threaded bushing TimeSerts - much better situation than helicoils in heads. The last thing you want is an issue down the road (new head gaskets, always at the worst time, etc). I never use helicoils in spark plug threads - just too much drama over the years and a lot of head gaskets and time wasted once you have an issue.