View Full Version : TR2/3/3A Spark plug threads

12-07-2014, 10:02 AM
Spark plug threads, number 4 plug hole has some drag or minor cross threading on the last couple of threads. I just find it unsettling mostly because I pull the plugs a lot to evaluate things. When I pull number 4 out, there is resistance at first and resistance at the end going in plus I can see scaring on the spark plug’s thread when removed. I was going to chase the threads with a tap with a glob of grease on the threads of the tap like I was shown to catch any metal, but I do not seem to have that tap. Moreover, I always like to hear from others on something like this. Oh and thanks Tex for the oil suggestion; it does seem like a waste to drain so quickly. The oil and exhaust are or so clean on this rebuild that I like to contribute it to that 4th ring below on the piston, but it is still too early to tell anything and pride doesth proceed the fall.

Geo Hahn
12-07-2014, 11:28 AM
something we did when chasing those threads on a friend's engine was to take a length of thin rope, smear grease on it and stuff it into the cylinder with the piston an inch or two down. after the chase we retrieved it with forceps. grease on the tap too as you noted.

the tap is, of course, available most anywhere - I think I got mine at Ace hardware.

12-07-2014, 12:15 PM
The auto parts stores will have a thread chasing tap jut for this task. Unfortunately, most the clerks won't know what you are looking for. At places like Advance Auto they are up front in the tool section. Do NOT let them sell you the HeliCoil thread repair kit, that is not what you need. You just need the thread chaser tap. It will have the large 18mm tap on one end and smaller 14mm tap on the other with a hex in the middle to drive it with a socket or box end wrench. Pack the tap with grease and use your wrench to run the tap in and pull it back out.

Sidebar note: The 18mm tap is the same thread size used on oxygen sensors for modern cars. If you are ever working on your modern car and find the O2 sensor has damaged threads, the spark plug chaser is a must-have to clean up the bung on the exhaust pipe to install a new sensor.

Picture below from Advance Auto. Their part AC544, $8.50 as of 12/7/14.


12-07-2014, 12:46 PM
While we're on the thread chasing subject does anyone know what size would be used on the oil pan drain bolt hole?:indecisiveness:

12-07-2014, 01:37 PM
IIRC they are 3/8"-18 NPT...tapered pipe threads.

12-07-2014, 06:28 PM
I like it Geo, smart. So the threads are metric? That is what confused me. I was thinking they would be 9/16 or something, but I could see that my 9/16 tap was just enough wrong to be a real problem. I have done it before on aluminum head many years ago, but I cannot find the tool, and I think it was metric.

12-07-2014, 06:38 PM
Earlier cars (like mine) used straight threads on the oil pan drain. 5/8 NF according to the SPC and standard hardware catalog (plug is V7535).

Also FWIW, there are other styles of spark plug/O2 thread chasers available. Eg
https://c.shld.net/rpx/i/s/pi/mp/1434/445909?src=http%3A%2F%2Fzeusd1.shopzeus.com%2Fimag es%2Ftoolweb%2FLIS20020.jpg&d=8205ec355392af09a0e84aece1c41cabe246b871&hei=546&wid=546&op_sharpen=1

Geo Hahn
12-07-2014, 07:51 PM
...So the threads are metric?...

for reasons I have never understood, plugs and tires seem to be both 'inch' and 'metric' irrespective of the system in use in the country of manufacture at the time.

plugs: metric thread but the reach is stated in inches

tires: metric width but the rim diameter is stated in inches

I suspect the French are behind this but have no proof.

12-07-2014, 08:04 PM
Interesting. Never knew that.

The French are behind it? Must be the Napoleonic goad.

(sorry, couldn't help myself)

12-08-2014, 08:45 AM
Also FWIW, there are other styles of spark plug/O2 thread chasers available. Eg

There are indeed other tap styles such as the one Randall posted a picture of. When I needed one quickly I was only able to find the double-ended type tap locally. With this knowledge, SP53 (the OP) can look for either type to address the cylinder head threads on his car's engine. Just don't be talked into getting the HeliCoil kit which (based on the description of the problem) is not needed.

12-08-2014, 11:50 AM
Hey Steve,

I've been pondering your situation, and I think if I were in the same position I'd pull the head to chase the threads. The problem is, no matter how careful you are, there is always a chance of getting metal shreds in the cylinder. I just picture those, wedged between the piston and cylinder sleeve, grinding away.

If it were An older engine that was already installed in the car, I'd lean towards taking the risk of chasing in place. But not for a brand new rebuild, still out of the car.

Anyway, that's JMHO. I'd pull the head so I'd never have to worry about it again.