Thread pitch info needed. U 67 spark plug hole...

[Pacific coast Datsun]

A few months back i somehow cross threaded the threads on the #4 spark plug when i was changing the plugs. I need to drill it out & insert a "helicoil". My ?'s are does anybody know the thread pitch for the spark plugs on a U 67 head & what size drill bit should i use to initially drill out the bad threads? I can get the bit & helicoil insert @ Sears so thats not an issue. Just want to make sure i buy the right parts.I know the best way to do this repair is with the head off..but i may do it with the head on using a few "tricks" to keep the shavings out of the cylinder. I plan bringing the piston all the way up then dipping the drill bit in grease & drilling slow, the shavings should stick to the grease. I can also vaccum out the cylinder hole after im done.

[Farmer Joe]

my dad had to do this on the truck before i had it. the #1 i have to be really careful with now.

 

i wouldnt worry a whole bunch over the aluminum shavings. theyre soft and shouldnt do anything really bad. lol i havnt had any problems at least... knock on wood tho... lol

 

good luck!

[datzenmike]

I think sealick used a vacuum with a McDees straw.

[Sealik]

I think sealick used a vacuum with a McDees straw.

 

 

Correct...shop vac.....it sucked bigtime.... :)

No need for any lube... :lol:

[Pacific coast Datsun]

Niel do you remember or can you ask your dad what size drill bit he used & the size of the helicoil he used? Also..i need to know what size tap to use / thread pitch on the head.

[datzenmike]

The thread is not metric and the same as most spark plugs. I've run 318 Dodge plugs in the L series before. Hey, it's all that was handy at the time.

[RustyRat4x4]

Had to do intake plug #4 on my '83 Z24/Z22 once upon a time. Same thing, cross threaded mine bad!! The plug brought out most of the aluminum from the plug hole in the head. Just take in a good spark plug for the shops reference (Pretty sure the BPR6ES is a 14mm). The helicoil set comes with everything you need except for grease or vacuum. The 'Master Set' (get ready for sticker shock) was almost $130 but included helicoils for every length of spark plug of that diameter. There's no drill required. It's a piloted tap that goes into the original spark plug hole and drill's/cut's the new larger threads needed for the helicoil installation all in one tool. Complete instructions included.

 

Be careful with the Piston up all the way up when you tap out the hole to the larger size (nicked the top of my piston slightly, no big deal though). Also be careful of the depth that the helicoil is set into the head with the tools' depth collar (in too far = hot metal piece in combustion chamber PING PING PING/ detonation problems!!!! )

 

For the preventative type keep in mind a spark plug re-threading tool from the auto parts store before you tap it out for the helicoil, they work very well. If the Spark Plug Hole isn't too bad, it might be able to be saved. Mine is dual size,14mm & 18mm Those should be about $10, money well spent!!!

[Pacific coast Datsun]

TY Rustyrat. I dont think the threads are too bad on my head. The spark plug can still be threaded in easily, it just barely wiggles once its all the way in. Its almost like the hole is oblong & not perfectly round anymore. I hope i can save the existing threads...maybe retap it to the next bigger size & run a diferent plug. Is that possible ?

[RustyRat4x4]

If the plug is still loose once it's tightened down crushing the crush ring washer (not sure what it's actually called if I'm wrong, and too lazy to grab a box to look) as noted on the spark plug box, then you should put in the helicoil for engine compression reasons.. a loose plug will most definitely let out cylinder compression; thus that cylinder would not work at the same compression ratio as the other cylinders. You could try to run the re-threader then replace the plug with a new one with good threads, tighten down the plug as directed. Then with the engine running take a piece of vacuum hose about 3-4 feet long. Place one end of the hose close to your ear and the other end around the base of the spark plug. Move the end of the hose around the spark plug base to listen for a leak (like a DR. uses a stethoscope for an exam). If you don't know what to listen for, place the end of the hose near the carburetor throat. That wind sound is what your listening for. If that sound is heard around the spark plug base then I would put in the helicoil for the sealing aspect of compression (partly dead hole at that point, or at least a weak hole). Could try to tighten up the plug a little more for a tighter seal but steel pulling on aluminum is very soft as you and I have found out. Then the helicoil will be a needed purchase for sure.

 

As far as taping out to a larger size plug, that I'm not quite sure about. The helicoil set solved my problems keeping everything as stock as possible.

 

If anyone has info that I am messing up, please correct me..