Faulty Spark Plug Thread Repair

[jrbartlett]

I have a problem with a faulty spark plug thread repair in an aluminum head. Several years ago I had my Auburn engine rebuilt by one of the big name shops. The head was a repro, only about 15 years old, and I had always been very careful when installing and removing the spark plugs and was satisfied with the situation. But when I had the engine rebuilt, the shop (without my knowledge) installed NAPA helicoils in all the spark plug holes. 7 have been fine, but one has caused problems. It is apparently too loose, as the spark plug apparently loses ground and stops firing. If I remove the plug to replace it, the insert comes out with it, which always poses the question of whether any of the coil threads have broken off and fallen into the combustion chamber, requiring a removal of the head to check. I've replaced the insert several times, and tried Loc-Tite to hold it in place, but that seems to worsen the loss of ground issue and the temperature is probably too high for the Loc-Tite anyway. The NAPA helicoils are still available, and are in fact specifically made for aluminum heads. Does anyone have experience with addressing a similar problem?   

[joe_padavano]

As you've found out, Loctite is not conductive and not heat resistant. Consider going to a Keensert or Timesert. Either of these are solid inserts (vs the coiled wire Helicoil) and are larger in diameter that the Helicoil. These solid inserts also use locking stakes that are tapped into place to hold them and prevent backing out.

 

[JACK M]

53 minutes ago, joe_padavano said:

As you've found out, Loctite is not conductive and not heat resistant. Consider going to a Keensert or Timesert. Either of these are solid inserts (vs the coiled wire Helicoil) and are larger in diameter that the Helicoil. These solid inserts also use locking stakes that are tapped into place to hold them and prevent backing out.

 

 

Learned something new again.....

 

 

[joe_padavano]

13 minutes ago, JACK M said:

Learned something new again.....

We use Keenserts in the aerospace industry because the Helicoils back out. GM recommends the Timeserts in their newer aluminum motors.

[Frank DuVal]

James, you have a picture or web link to the NAPA Helicoils? Is it this:

 

https://www.napaonline.com/en/p/BK_7703223

 

I (and it appears other people here too) always think of Helicoils as the thread only portion to repair stripped holes, but Helicoil does make the linked solid insert for spark plug (14 mm x 1.25 thread pitch) which is available from NAPA. 

 

I've always had great luck with the old K-D solid insert type, that simply screws in, after using the special tap, and has a wide section to prevent it form going below the hole surface. The Helicoil lacks this, and requires use of the expansion tool to lock it in place. Maybe you need to use the expansion tool again? Yes, I know, you do not have that, the shop had it.

 

There is also a Big Sert for when other repairs failed and now the hole is bigger than Time Serts can fit. 

 

And then (assuming you have 14 mm spark plugs, which is not always a given on older cars🤔) there is the "get-er-done" method of taping the hole to 18 mm threads and using a larger base plug, but you did not hear that here!😲 I can tell you Champion tech help actually answered my question of what 18 mm plug would be correct for a Corvair engine!👍 Well, after they stopped laughing.

Edited June 18, 2022 by Frank DuVal (see edit history)

[Larry Schramm]

I too would recommend TimeCerts.  As said, they are the recommended thread repair for GM aluminum block engines.

 

They are solid inserting threads that have an shoulder on the insert that when the special tool is used to prep the aluminum part, the aluminum part has a depressed shoulder that the timecert goes down into.  It makes a very good and strong repair.  GM started using them in the aluminum blocks when the head bolts stripped out the threads.  

 

From left to right, the insert, correct size drill, tap for timecert threads in part, shoulder cutting tool, insert tool.

 

Picture below.

 

Edited July 2, 2022 by Larry Schramm (see edit history)

[dodge28]

I admire you guys, a wealth of experience/ Congratulations.

Harry.

[Angelfish]

I picked up an old Yamaha Seca 750 motorcycle for $150 because someone had hogged out the spark plug threads and then pounded in an oversized bolt to plug the hole.  It was so oversized that none of the spark plug inserts were large enough to repair it.  I ended up using a steel pipe nipple.  I cut pipe threads into the head and spark plug threads inside the nipple.  I doused an old spark plug in anti-seize and used it to thread it in.    I have to admit I haven't put many miles on it but it's still holding and by the appearance you'd never know it had been repaired.

 

But that may not be a solution for an Auburn. 

 

 

Edited July 8, 2022 by Angelfish (see edit history)