Big-Bore Continental Spark Plug Installation

[Fly Boomer]

Having trouble (re) installing spark plugs.  Mechanic not too concerned, but it seems to me that the match-up between the plugs (Champion massive) and the helicoil is not ideal.  In my experience, the threads on fasteners (think cap screw or machine screw) have a little lead-in where the first thread or so is slightly smaller in diameter than a couple of threads down, making it easier to start the screw in a blind hole.  This spark plug thing seems to me an invitation for a cross-threaded plug, and a lot of heartache.  What say the group?  Ever notice this?  If so, what is the cure?

[N201MKTurbo]

Yea, spark plugs are hard to start sometimes. Just make sure they are going in correctly before you put a wrench on it.

Make sure you use a bit of anti-seize on the threads. If they are really tight, you can run a thread chaser down the hole to clean the threads, but I would be careful with that.

[MikeOH]

If the plugs are used, make sure their threads are clean before installing.

[Fly Boomer]

10 minutes ago, N201MKTurbo said:

Make sure you use a bit of anti-seize on the threads. If they are really tight, you can run a thread chaser down the hole to clean the threads, but I would be careful with that.

Anti-seize for sure, and I understand that a tap in the cylinder could be as bad as a cross-threaded plug.  Any specific techniques for using a tap in the cylinder or a die to clean the plug threads?

[Fly Boomer]

3 minutes ago, MikeOH said:

If the plugs are used, make sure their threads are clean before installing.

Any reason not to run a die onto the plug threads?  Gently, and by hand only?

[MikeOH]

Just now, Fly Boomer said:

Any reason not to run a die onto the plug threads?  Gently, and by hand only?

Perhaps, but I'm not sure the actual thread type and if there is any taper?  I use a light/thin rag and a small jeweler's screwdriver: put the rag on the threads, press lightly thru it with the screwdriver and slower rotate the plug; the screwdriver 'walks' down the thread cleaning as it goes. Takes 15 seconds per plug.

[N201MKTurbo]

Cleaning the threads with a bristle brush and solvent usually gets the job done. You could use a bristle brush with a little solvent on it to clean the holes. I wouldn’t use a tap or die on them. They make thread chasers for spark plug holes. 
 

Taps are made to remove metal, thread chasers are made to clean and straighten threads.

[Fly Boomer]

1 minute ago, MikeOH said:

Perhaps, but I'm not sure the actual thread type and if there is any taper?  I use a light/thin rag and a small jeweler's screwdriver: put the rag on the threads, press lightly thru it with the screwdriver and slower rotate the plug; the screwdriver 'walks' down the thread cleaning as it goes. Takes 15 seconds per plug.

I'm no mechanic, but my first reaction is Why not use a wire brush?

[MikeOH]

Just now, Fly Boomer said:

I'm no mechanic, but my first reaction is Why not use a wire brush?

I found a wire brush still leaves residue; I suppose if you brushed long enough...

[N201MKTurbo]

If they are real cruddy, but usually not necessary.

[Fly Boomer]

2 minutes ago, N201MKTurbo said:

Cleaning the threads with a bristle brush and solvent usually gets the job done. You could use a bristle brush with a little solvent on it to clean the holes. I wouldn’t use a tap or die on them. They make thread chasers for spark plug holes. 

Now I'm thinking maybe an appropriately-sized bottle brush or test tube brush with a little Kroil on it?

[Fly Boomer]

I guess my takeaway is:  Do a little cleaning using the least invasive technique.

[StevenL757]

Correct.  Everyone has given great feedback here.  I’m assuming by “big-bore Continental”, you mean an IO550.  If this is the case, the plugs should go back in at 30ft/lbs (360 in/lbs), with a fresh gasket; and as Rich suggested, with a small amount of anti-seize (on the first 2 to 3 threads from the bottom).  Personally, I’d spend the extra $$ on fine-wire plugs when your massives go out, but I won’t re-start that dogfight here…again.

Steve

[Schllc]

6 minutes ago, StevenL757 said:

Correct.  Everyone has given great feedback here.  I’m assuming by “big-bore Continental”, you mean an IO550.  If this is the case, the plugs should go back in at 30ft/lbs (360 in/lbs), with a fresh gasket; and as Rich suggested, with a small amount of anti-seize (on the first 2 to 3 threads from the bottom).  Personally, I’d spend the extra $$ on fine-wire plugs when your massives go out, but I won’t re-start that dogfight here…again.

Steve

Until you swap them out and see for yourself, it’s hard to appreciate the difference they can make. 

[Shadrach]

I once saw a grey beard clean up threads with an old spark plug that had channels cut perpendicular to the threads just like a tap. Paper towel in the plug hole and lots of grease on the modified plug.

[N201MKTurbo]

10 minutes ago, Shadrach said:

I once saw a grey beard clean up threads with an old spark plug that had channels cut perpendicular to the threads just like a tap. Paper towel in the plug hole and lots of grease on the modified plug.

Poor man’s thread chaser.

[Guest]

KD Tools makes an 18mm spark plug thread chaser, but must be used with great caution. You don’t want to damage the threads or dislodge the Helicoil.

 

[N201MKTurbo]

For the Cadillac of thread chasers:

 https://www.aircraft-tool.com/shop/detail.aspx?id=CT449
 

only 1/2 an AMU

[EricJ]

44 minutes ago, Shadrach said:

I once saw a grey beard clean up threads with an old spark plug that had channels cut perpendicular to the threads just like a tap. Paper towel in the plug hole and lots of grease on the modified plug.

Yeah, the flutes in the thread chaser are supposed to capture the crap so it doesn't fall in the cylinder, and grease in the channels in the home-brew device does the same thing.

Taking both plugs out and blowing some clean shop air through the cylinder afterward might help eject any remaining crap.   A blue shop towel probably helps, too.

 

[Fly Boomer]

10 hours ago, N201MKTurbo said:

For the Cadillac of thread chasers:

 https://www.aircraft-tool.com/shop/detail.aspx?id=CT449
 

only 1/2 an AMU

I have had my eye on that, but 0.5 AMU for occasional use is hard.

[Hank]

I've always cleaned spark plugs threads with a wire wheel on the plug tester.

And I've recently changed from the goopy copper-based anti seize to a nice graphite based liquid that works very well.

[EricJ]

56 minutes ago, Hank said:

I've always cleaned spark plugs threads with a wire wheel on the plug tester.

And I've recently changed from the goopy copper-based anti seize to a nice graphite based liquid that works very well.

Lycoming SI 1042AH suggests using a copper-based compound and says to not use (in capital letters) a graphite-based compound.    I don't know why but they did put "DO NOT" in capital letters.

 "Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound."