Spark Plug Torque


#1

Just a comment about tightening spark plugs, a topic that comes up pretty regularly.



As some of you may know, I do not believe in using a torque wrench for the following reasons:

1. At low torque specs the readings may not be trustworthy.

2. Many torque specs are way too high in my opinion.



Overtightening plugs, especially in aluminum heads, may pull the threads at the least or strip them outright. With the former this could come back to bite someone later when the plugs are removed again or if one blows out.



I maintain that a simple snug is good enough and is a far better and more reliable method than a torque wrench.



So this evening I threw a new set of Autolites in one of my cars and while I never read the fine print on the package, I did this time.

Autolite states that:

Tapered seat plugs should be run down finger tight and then given 1/16 of an additional turn.

Gasket seat plugs should be run down finger tight and given about a 1/4 turn extra.



So I concur with Autolite on their recommedation.


#2

Your snug and my snug may be vastly different. The first time I changed head gaskets, without a torque wrench, I was sure I had everything tight enough. When I started it up I had oil and coolant all over the place. Most specs for plugs in aluminum heads are pretty low. Just be sure you have the right specs for your application. I worked at a trucking company that had all coarse thread lug nuts that torqued at 300 ft. lb. We got some GMCs that had fine threads and using 300 lb. the mechanics pulled the studs right out of the wheels.


#3

We’re only talking about spark plugs, right?

I use my torque wrench when I install new plugs, but I have to wonder how accurate it is at the low end of the scale. I’ll bet money I could do just as well without the torque wrench.

My cars have gasket seat plugs, and I’ve also read the “1/4 turn” instructions on the package.

I guess the question is “finger tight?” Your finger tight and my finger tight might vary. The torque wrench is always the same. Or at least I hope it’s always the same.


#4

For my part, I’ve only ever done the “bump snug” method and have never had a single problem with a plug blowing out or being difficult to remove. I wasn’t leaving the plugs in there for eons; they came out for inspection about every 10-15K, so that has a lot to do with it as well.

Bumping the plugs snug - and using a thread chaser to clean the threads in the cylinder head while the plugs are out - shouldn’t cause any problems…if done properly.


#5

I go by the torque wrench. We’ve all had those jobs where we tightened a part correctly, then gave it that little extra torque “just to be sure” and then felt that sickening feeling of stripped threads. I have 3 different torque wrenches. I’m covered from from in-lbs to “get the pipe” torque.

While we’re talking spark plugs, I want to mention that you should ALWAYS use anti-seize on the threads.


#6

Everybody is free to take the thread wherever they want,we don’t have a electron shortage but when the OP wants to talk about sparkplug torque and his first response is into headgaskets and wheel lug torque, well thats “streching” (get the joke HA, HA) it.

Never used a torque wrench on a sparkplug in my life.


#7

For some readers I would comment that there is an inch-pound torque wrench. You can convert foot-pounds to inch-pounds if you multiply by twelve. Nothing wrong with those instructions printed on the spark plug package.


#8

what is a thread chaser?


#9

Like a thread tapper. With existing threads, it cleans out any dirt and deposits that may clog up the threads.


#10

NGK prints torque specs on their packages. OK4450 and McP, I’d bet my morning muffins…for an entire month…that you both have torque wrenches in different ranges. I do, and I’m just a simpleton!

My perspective on the use of torque wrenches is that while I spent a few decades not using them for sparkplugs, I now think they’re a good idea, especialy for newbies. As oldtimer mentioned, our perception of correct torque by simply judging by feel differs. I know you guys are all more familiar with barrel micrometers than you’d perhaps like to be, and you all know that two different people not using the “breakaway” end clicker (whatever the correct term is…I forget…now where DID I leave my Starret book?) get two entirely different readings. That’s quantitative evidence that we perceive torque differently.

Having said that, I’ve no doubt whatsoever that most of us (excluding myself) can safely install spark plugs in any head without a torque wrench. That “feel” can be developed over countless installations. But many who read the forums have yet to develop that “feel”. I’d trust you guys to safely install my plugs anytime, but that’s because I’ve grown to respect the knowledge and experience. For anyone I don’t know, including a young tech in a shop, I want to see a torque wrench in his hand.

Bottom line: unless I know therwise, I’d prefer to assume that a torque wrench is necessary. And for myself, I trust the wrench now more than I do my own “feel”.


#11

Ant-seize is a no no in some cases. If the torque figures are for dry threads, anything on them will cause over tightening. If you use plugs with nickel-plated threads, this is the anti-seize.


#12

JMO, but it’s not just the fact the torque wrench is not entirely accurate at low specs but it also involves specs that I feel are too high and some of those specs are given a lot of leeway.

Example.
On my Lincoln (4.6 DOHC) the manual says 7-15 Ft. Lbs. That’s pretty much a shotgun approach if you’re essentially doubling the torque spec by giving a figure like that.
Not only that, but those plugs are 14 MM long shank but the holes are only threaded for half the length of that shank. The thought of putting even 10-15 Ft. Lbs. of torque on about half a dozen rounds of aluminum cylinder head thread makes me cringe a bit.
Ten foot pounds is plenty enough to pull threads even if it doesn’t strip them.

I’ve always found the safest method is to use 3/8" stubby ratchet and simply palm the plugs snug. Never had one come loose or leak yet.


#13

I’ve NEVER used a torque wrench on any spark-plug I ever replace…Never had a problem by NOT doing it either.


#14

I never use a torque wrench on spark plugs. The reason? If there is a torque spec for the plugs it’s usually a dry torque spec. If you apply anti-sieze compund to the threads of the plugs you’re now doing a wet torque. This can cause over-torquing of the spark plugs causing damage to the threads in the head(s). Especially in aluminum heads.

I just use the snug-em-up by feel technique.

Tester


#15

Almost all damage to threads in an aluminum head is caused by cross threading not over torquing. I use a small piece of vacuum hose slipped over the end of the plug to start them. If you’re cross threaded the hose will slip on the plug before it does any thread damage. Also invest in a thread chaser and clean the threads before installing every time. It takes only a few seconds per hole and it cleans out any grime or dirt. It’s a lot harder to cross thread clean threads.

I like a small amount of anti-seize on the leading edge of the plugs threads. No glops or glumps. Apply a thin coat with a brush and get it down in the threads.

I do use a torque wrench on all plugs. You’re well in range of the wrench IF you use the right wrench. As stated, most applications have a torque range. Use the low number for wet(using anti-seize) and the high number for dry(no anti-seize) and you’ll be OK. You’ll get a uniform clamping pressure on all plugs.

In my experience the biggest problem with plugs is leaving them in too long. I like to pull them and look at them every 15k miles. This can be a good indicator of how an engine is running. I suppose if you even pulled them every 30k miles you’d be OK.
But how many plugs are out there that haven’t been removed in 60-90k miles? Ugh, I hate working on those…

Benzman


#16

Gasket compresses after the first use so 1/4 turn for the second installation of the same plug may be too much.

If you don’t trust your torque wrenches to be accurate, then you can have them checked by a scientific instrument calibration service or possibly the Snap-On man can help with this; worth an ask.

I agree, an experienced hand can do a pretty good job of installing spark plugs w/o a torque wrench. I long for the good old days when heads were cast iron, at least for the spark plug threads. My long gone BMW R50-US had threaded steel inserts cast into the aluminum heads for spark plugs, a nice touch.


#17

I believe you are over-thinking this. Looking up the torque specs and using a torque wrench makes it so easy that even I can do it without messing it up.