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How to find out thread spec

I was looking at torquing specification for the spark plugs but it lists 2 different threads. How would I know the difference. Do I need to get one of the fan thread measuring tools? And how do you read thread specs?

model, year car?

You can take a plug to your car parts place. They usually have sample charts you can compare your plug against or you can count the number of threads per inch.

I just want to know how to measure thread and spec. This was just an example.

Just by chance . . . your owner’s manual doesn’t list the spark plug torque?

I know this may sound laughable to some reading my comment, but I distinctly remember that some owner’s manuals did have such useful information

I take it you’re not comfortable tightening the spark plugs “by feel” . . . ?

Thread pitch gauges are quite inexpensive, by the way, and you might be able to find them at sears, auto parts stores, or home improvement stores.

On the newer BMWs nothing is listed in the manual really. But I don’t need it, I’ve got the BMW TIS which is pretty much the service manual the dealership technician use this software to do the work. I just find it odd they list 2 torque spec.

This was mostly an example I just want to learn how would I identify the thread to match the spec listed and know.

Ah so that’s the name of the tool, thread pitch gauge.

Now I suppose you have metric and non metric specs for this to right?


many torque wrenches will have metric AND standard scales, so you don’t need to convert

otherwise . . . will convert just about anything you can think of

Sorry, I was talking about the thread pitch gauges. :slight_smile:

And yeah, I’m very OCD. I torque spec everything :grin:

There’s both standard, metric, and gauges that do both. Check out the link

Ok thanks!

So using these on the thread will give the second number right. The first number M14, is that diameter in milimeter of the top of the bolt?

No, not the top of the bolt. It is the outside diameter of the threads.

An M14 x 1.25 thread is 14 mm nominal outside dimension with 1.25 mm distance between threads. English is the opposite. Still the bolt diameter is first an the thread is in the number of threads per linear inch of the bolt. The wrench you use to tighten is can vary as needed… Big, little, allen, Torx, whatever is needed.

Oh okok yeah, lol, definitely not the top of the bolt. Don’t know why I was thinking that :joy: doesn’t make sense

Ok I understand now, thanks!

I was looking at spark plugs this afternoon and if you just want to know what the threads are so you can determine which is which, the plug number should identify the threads if you go to the web site or just google the number and stuff will pop up.

I thought you were talking about telling the difference between two plugs, not bolts.

Well it comes down to the same right? Measuring the thread on the bolt or spark plug should relatively be the same?. Both have threads. I just used the spark plug situation as an example.

I have 4 torque wrenches, never used one on a spark plug yet and I have changed a LOT of spark plugs, When I first started working in a gas station in high school, spark plugs got changed at 10,000 miles and got cleaned and gapped at 5000 miles. We used to charge 25 cents a plug to clean and regap them. New plugs were $1.25 each installed. At 19,000 miles you needed points and condenser, set the dwell, then the timing, Then you could adjust the carb. If the valves were noisy, you would adjust them too. Unlike today, those iron block and head side valve engines never seemed to lose clearance.

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12mm is slightly less than 1/2".
14mm is almost 9/16".
Get a 1/2" open-end wrench, and attempt to fit the threads of the plug inside the wrench. If it’s 12mm, it’ll fit, with a little extra room to wiggle. If it’s 14mm, it won’t fit inside.

Damn uh, 1.25$ a spark plug. That’s cheap!

Thanks for the trick. I still went ahead and got a thread depth gauge.

Well when you only make $1.65 an hour $1.25 each for eight plugs gets a little pricey.

Don’t 12mm plugs use a 5/8" socket and 14mm a 13/16 socket?

But there’s a lot more to it than that. Tapered seats vs seal rings. Extended reach. Aluminum head vs iron.

Also, how accurate can a torque wrench be when used in confined spaces with an extension and swivel?