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Thread: Spark Plug Standard Thread Form
03-28-2006, 05:38 PM #1
You would think this data would be easy to find...
I am doing a little machining on some spark plugs for a friend and need to chase the threads. There are enough plugs in the job that I could invest in a die and run it over the body to clean up the threads but I can't find thread standard data for the pitch of the various diameters. In other words, do American plugs use a 5/8 NF thread form or the equivalent in metric? If it's a 14mm diameter, what it the proper pitch - etc? Thanks for shedding any light or directing me to the right website.
03-28-2006, 05:55 PM #2
It's a metric thread, although I can't remember which. 18mm sticks in my head for some reason, but I don't remember why.
03-28-2006, 05:57 PM #3
According to Machinery's Handbook (24th ed pg 1650), there are four standard SAE spark plug threads, all metric: M18 x 1.5; M14 x 1.25; M12 x 1.25; and M10 x 1.0
03-28-2006, 08:34 PM #4
Is it not surprising that all sparkplugs are metric thread. My 34 pontiac takes 7/8, but since the 40's plugs are metric. Yet all rachet drives are english 1/4, 3/8 etc, buy a metric tap and die set it has a 1/8 pipe tap and die(snap-on does anyway) FWIW
my wheels don't slow me down
03-28-2006, 09:06 PM #5
The threads for spark plugs have a special tolerance, different from a standard metric tap. Don't go order a metric tap with matching thread specifications, specify a spark plug tap. Years ago, I used to use Besley. Any decent tap manufacturer should have them on the shelf.
The threads are metric for the same reason standard ball bearings are metric, they were invented by a German!
03-28-2006, 09:07 PM #6
Thanks for the info.
03-28-2006, 10:30 PM #7The threads for spark plugs have a special tolerance, different from a standard metric tap.
03-29-2006, 12:20 AM #8
the correct info for a 14mm plug is 14X1.25mm
03-29-2006, 01:23 AM #9the correct info for a 14mm plug is 14X1.25mm
03-29-2006, 08:37 AM #10
Ok, there seems to be some misunderstanding here about spark plug threads. My take on this is that the only thing special about a spark plug thread is the diameter to pitch combination,which means any 14mm X 1.25mm tap is the right one for a 14mm spark plug. Other than that spark plug threads follow the standard metric system. If this is not true someone please explain.
03-29-2006, 08:49 AM #11
You shouldn't have any trouble gettin "spark plug" taps, not sure about a die, and of course you can get helicoils for the not unusual job of redoing threads in aluminium cylinder heads (removing plugs with carbon on the end damages the threads).
03-29-2006, 10:10 AM #12
I found a metric thread file at MSC with a range of sizes that includes 1.25mm and plan to buy that for chasing the threads.
03-29-2006, 10:40 AM #13
No, Mike72, your take on spark plug threads is not correct. The tolerance from nominal size is different on a spark plug than a standard metric thread. I don't remember whether it is larger or smaller, but it is not the same. For a backyarder it may not matter, but don't complain if your sparkplug strips or seizes.
03-29-2006, 02:03 PM #14
So far I searched the internet for about one hour to find if a spark plug thread of 14mm X 1.25 requires a special tap other than a 14mm X 1.25 tap and cannot find it. I'll keep looking.
03-29-2006, 02:48 PM #15
Mike72, I suspect spark plug thread form is a SAE specification, not an ANSI specification.
03-29-2006, 03:08 PM #16
I just ordered a spark plug tap the other day. 14 x 1.25mm. I also ordered a 18 x 1.25mm. My taps are Brubaker I believe.
03-31-2006, 09:04 AM #17
I have quit looking for now and added this to my mystery list. There are things you just cannot find on the internet. A complete explanation of spark plug threads and what is special about the taps used for them is one.
03-31-2006, 10:10 AM #18
That was exactly my problem and what prompted the question in the first place. Aside from buying taps and testing, this forum was the best source of info for this question.
03-31-2006, 10:36 AM #19
Do you have a O'Rielly Auto Parts store in you part of Texas? My Local O'Riellys here in Missouri has 14mm and 18mm taps on the shelf. I do not know if the have dies.
06-14-2007, 06:22 PM #20
SPARKPLUG 7/8"-18 UNS-2B
MACHINERY'S PER MACHINERY'S
BOOK 1949 BOOK 1996 DIFFERENCE
PITCH DIA. HIGH .8384" .8449" PLUS .0065"
PITCH DIA. LOW .8343" .8389" PLUS .0046"
MINOR DIA. HIGH .8068" .828" PLUS .0212
MINOR DIA. LOW NONE .815" ?
MAJOR DIA. HIGH .8750" NONE ?
MAJOR DIA. LOW .8668" 0.875 PLUS .0082"
One thing that we all must remember is that old Cars that used these sparkplugs are not of high combustion pressures and the taps using Unified threads are likely made on the low limit of the pitch diameter tolerance. One must also remember that the Minor Diameter is not cut by the tap but by the drilling of the hole previous to tapping. The Major and Minor diameters are not a functioning portion of the tapped hole but are clearance. Only the angled faces of the thread are truly functioning parts of the threads.
.8449” - .8343” = .0106” a lot of cast iron to remove by hand!
To cut these threads at the high limit of the tolerance from the low limit of the old threads would take a lot of torque.