Spark plug torque: no more +/-?
- Thread starter vavavroom
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What brand is it? Sometimes they have weird translation.
You typically allow ~ +/- 5% for the torque wrench accuracy at that range. That should be the min taken into consideration, plus more room for any other acceptable or desired range/tolerance for the spark plug torque.
For example they may list x +/- %15 (15 > 5).
Sounds like they want min of 20 but not over 23 and they meant to say 20-23.
Then why not 21.5 lbf-ft +/- 1.5? I guess that is still the question.
I think 20-23 is easier and the standard ... or maybe they missed a minus sign and meant to say 20 -+3?!
You typically allow ~ +/- 5% for the torque wrench accuracy at that range. That should be the min taken into consideration, plus more room for any other acceptable or desired range/tolerance for the spark plug torque.
For example they may list x +/- %15 (15 > 5).
Sounds like they want min of 20 but not over 23 and they meant to say 20-23.
Then why not 21.5 lbf-ft +/- 1.5? I guess that is still the question.
I think 20-23 is easier and the standard ... or maybe they missed a minus sign and meant to say 20 -+3?!
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Don't think I've ever used a torque wrench for plugs. Just follow the guidelines for plugs with or without a seal. Usually 1/8 to 3/4 of a turn after finger tight.
Yeah in tolerancing on blueprints it's not uncommon. I just think of it as here's the desired size, it can be a little larger but absolutely cannot be smaller (or vice versa)
I've not personally seen it on torque specs but would just interpret it to mean no less than 20ft lbs.
Still, it's ultimately a way for engineers to feel good about themselves. Sitting behind a computer screen you can envision a perfect world, but that fails to compensate for real world variables of which there are several here (tw calibration, ambient temp, thread contamination or lubrication etc etc etc).
I struggle to believe at 20ft lbs all is well but at 19 the world ends......
I've not personally seen it on torque specs but would just interpret it to mean no less than 20ft lbs.
Still, it's ultimately a way for engineers to feel good about themselves. Sitting behind a computer screen you can envision a perfect world, but that fails to compensate for real world variables of which there are several here (tw calibration, ambient temp, thread contamination or lubrication etc etc etc).
I struggle to believe at 20ft lbs all is well but at 19 the world ends......
Who would set the torque wrench to a higher or a lower value anyhow? The nominal value is all that’s needed.
I don't see the issue here.
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slo town
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This has been the case of every NGK plug I've installed over the last decade or more. I like it because it takes all the mystery out. The only caveat is that the recommended torque angles are for brand new plugs with the crushable washer.
Scott
Here, I fixed it for you:
in SI 29 +/- 2 N⋅m
This is similar to ASME Y14.5 unilateral tolerancing - often used for press or sliding or exact fit
ex: My Buick 401 engines have a rod bearing insert clearance spec of: 0.0020" +3
My question wasn't in regard to how to torque spark plugs, whether to use a torque wrench or not, or if the specs I posted were based on a misprint.
I see the to-me new way posting torque specs frequently but have not cone across an explanation.
Until recently I have only ever seen +/- tolerances. The way of writing x1 torque + x2 torque makes me think it means a minimum torque of x1 and a maximum torque of x1+x2. If so it's just another way of expressing a range. Judging by the numbers in question this seems plausible.
I see the to-me new way posting torque specs frequently but have not cone across an explanation.
Until recently I have only ever seen +/- tolerances. The way of writing x1 torque + x2 torque makes me think it means a minimum torque of x1 and a maximum torque of x1+x2. If so it's just another way of expressing a range. Judging by the numbers in question this seems plausible.
They don't want you to use your calibrated elbow unless it's accurate to within the stated tolerance, of course.
So how accurate is the torque wrench? Yours may tighten it to 17 ftlbs, your friends may tighten it to 22, when both are set to 20.
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Hand tight + 1/2 turn is how I broke a Beru spark plug and had to extract it. That was when I learned that not all spark plugs are made the same!
slo town
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Sounds like a defective plug or possibly the socket wasn't squarely aligned when installing.
Scott
How would you check the final torque? Outside of high precision tools and factories, the end user has no way to verify the final torque of a fastener. Therefore a range is effectively useless.
Same here. It also eliminates any correction factor due to the use of anti-seize if you choose to use that.
Range is a range which allows flexibility and has to consider the accuracy of the tools.
If for example a drain plug torque spec is 30 lb.ft and no tolerance or range was listed, then you set your torque wrench to 30 and will end up somewhere between 28.5-31.5 and it's all good considering torque wrench accuracy of +/-5% (30 x 0.05 = 1.5)
On the other hand, if the torque spec for the drain plug was 27-33 lbf.ft (or 30 +/- 3 lb.ft or 30 lb.ft +/- 10%), again you set your wrench to 30 and will be in range of 28.5-31.5 and the spec was 27-33 and you are within that range and it's all good.
They can't and won't specify a tighter tolerance than the accuracy of measurement tools. Meaning you'll never see a drain plug torque spec of 29.9-30.1 lb.ft
Read the first part of my post #2.
Think of it as 21.5 ft-lb +/- 1.5 ft-lb.
Is this from the spark plug manufacturer or the service manual for the car/truck/engine manufacturer?
That's my presumption. I have not cone across any source saying something like, "There is a new convention of expressing torque values and tolerance."
I don't know. I think everybody is citing the same source whatever it is.