Hi, I’m newly learning how to do routine maintenance on my car. I just put in new spark plugs, but I’m worried I’ve overtightened them (because I’m paranoid, not because I felt like the plug broke).
What would I feel if the threads had stripped? Is there any way to tell if they’re overtightened? And, oh yeah – the guy at Partsource, where I bought the plugs, recommended I use anti-seize, which I did, and now I’m more paranoid, because I’ve read that antiseize can cause you to over-torque the plugs.
So, four questions, really: What does it feel like when a plug is overtightened? What does it feel like when threads are stripped? Will I be able to notice anything while driving if I’ve overtightened the plugs? And, what are the consequences of an overtightened plug?
Thanks… I really hope this has gone well as I’m hoping to keep learning how to take care of my car.
When you use anti-seize, you only put a small amount on the beginning threads. It will work its way through the rest of the plug as you spin it in. Some people like to liberally coat the threads, but I don’t think it’s really necessary, and just leaves more to squish out and be messy. 
When you put the new plug in, you want to hand-tighten it first. Once you have it hand-tight, you then want to use the wrench and turn it another 1/2 turn (approx). Much more, and you’ve overtightened it. You’ll feel it tighten up, and you stop.
If the threads are stripped (the worst consequence of over-tightening), the plug will just keep turning and turning, and never get tight. It may or may not spin out at this point. Normally it will, but occasionally it needs to be pulled while it being turned. If the threads are stripped, the only fix is a helicoil (that, or replace the head = expensive), which can normally be done with the head installed, so you don’t have to rip the motor apart. It’s a pain, but not an overly expensive process.
If the plugs are over-tightened, but you haven’t stripped the head, then you shouldn’t notice anything peculiar while driving. It’s just too tight. You can’t quite simply loosen and then tighten it again.
If the plug is truly over-tightened, there’s really nothing you can do but replace it. The gasket is a one-time use compression gasket. Your best bet at this point is just to run it for a few thousand (like 20 or so), and replace the set.
Good job on getting in there. Although it may seem daunting at first - and there are some things that are pretty vital - it’s a machine, and most people can do their own basic maintenance with some training, some reading, and a few good questions.
Best of luck!
Chase
Here’s the rule of thumb when tightening spark plugs.
With tapperd seat sparkplugs turn the spark plugs 1/8 turn to tighten them.
With gasketed seat spark plugs turn the spark plugs 1/4 turn to tighten them.
If you followed these rules you’re okay.
Tester
Aye, 1/4…my bad. I don’t even think about it any more, I just do it.
I should add something – I don’t have a torque wrench, so basically, I hand-tightened, then when they were snug, I used a socket wrench to tighten (slowly) until the resistance increased. Basically I stopped when I felt like I’d be forcing it to go further.
Tester – the plugs I bought were NGK, and the packaging said with the gasket, to go up to 2/3 turn past hand tight. Does that sound reasonable?
The gasket design will dictate the how far to tighten the spark plug. I’d be a little leary tightening the plugs 2/3’s of a turn. Especially in an aluminum head. I’ve used the 1/4 turn rule on gasketed seat plugs for over 40 years and never had a plug come loose.
Tester
Thanks for all your advice (and so quick!). One more thing, then, and I’ll leave it be: 1) Can I overtighten without stripping? Is it possible that an overtightened plug will only cause problems after the car is running for a while, or if I’m able to start it up and take it around the block a bunch (which I did), am I okay? And, what are the consequences of overtightening, if it’s not noticeably stripped?
A spark plug can be overtightened where it stretches the threads in the head. Then eventually the threads strip out.
Tester
To answer your four questions:
- If it feels like you are tightening the heck out of them, you probably are.
- If you are tightening the heck out of the plug, and it suddenly gets much easier to turn, the threads have stripped.
- Overtightening the plugs will not cause any kind of driveability problem, unless you used a lot of side-load force on it and cracked the porcelain, which is very difficult to do, especially with a spark plug socket and it’s rubber insert.
- Overtightening the plugs could stretch or pull the threads, causing the threads to be damaged the next time the plugs are removed for replacement, leading to the need for a Heli-Coil set, or, according to some dealer service departments, replacement of the entire engine (inside joke there, don’t worry).
Your risks while installing spark plugs are: 1) incorrectly lining the new plug up into the plug socket and then forcing the plug in, which is why you only insert and initially tighten the plug by hand until the plug is seated, and 2) overtightening the plug. By far, the first thing is the easiest thing to get wrong, and it would be at best a small disaster for anyone.
Any of the suggestions already made are fine to make sure the plug is tight but not overtightened. I have always just used my hand to lightly tap the end of the socket wrench until I feel or hear a slight squeak. That’s it. You tighten the plug as far as it will go by hand, use very light pressure with the socket wrench to make sure the plug is fully seated, and lightly tap the end of the socket once or twice. Same as Tester, I’ve used this method (well, for a lot less years), and have never overtightened a plug or had one back out.
The main thing, especially in very tight engines, is to make sure that you properly insert the plug in the first place and don’t use the socket wrench at all until the plug is well seated. If you feel early resistance while you’re trying to fit a plug, back it out a little and wiggle it and go forward until it goes in with little resistance by hand.
On a related note, I just received my latest copy of Car Craft (June 2011) and they address this stripped spark plug hole scenario based on one of the editor’s project cars.
The article is Ford specific (late model V-8) but it applies to everything. There is also a comment in there about “most shops” wanting to replace cylinder heads when the repair is right there in front of them.
Personally, I prefer to remove all doubt and use a torque wrench.
Here is a tip, to start the spark plug in the hole use a piece of cheap fuel line that fits snugly over the porcelain end of the plug. You won’t cross thread it that way.
Lefty loosey, righty tighty.
You can’t use a torque wrench if it requires the use of a U-joint to gain access to the spark plugs. It results in inaccurate torque readings.
Tester
Yes, I am aware of that issue, but I am pretty sure the OP’s Chevy Aveo is a basic four cylinder car. I don’t believe you would need a U-joint to change the spark plugs on this particular car. Am I wrong about this?
Sounds reasonable. The 2/3 turn is squashing the gasket and making a seal. Now if you remove the plugs to inspect them and replace with the same plug; then the gasket is squashed already and if you went that 2/3 turn you’d be overtighening the plugs.
So, the 2/3 turn is fine for new plugs with fresh gaskets. But, 2/3 turn is bad for putting old plugs back in place after you removed the plug for whatever reason.
chaissos: Why can’t you loosen and re-tighten a spark plug?
What if they won’t stay tight in a aluminum head
I Have never had an issue with plugs loosening in an aluminum head. In my younger days . when every gas syation was a repair shop, weused to clean, gap and reinstall spark plugs fpr 25 cents each at 6000 miles. At 10,000 miles we replaced them for $1.25 each, Never used a torque wrench, always reused yje gaskets at 5000, never had a problem. A;ways started them bu hand or if theu were in tighy quarerss a runner hose.