Spark Plugs Screw Threads

The threads are machined into the head. The sparkplug threads into the head so the electrode is exposed inside the combustion chamber. Every spark plug has a required torque value for proper seating/sealing against the head seating surface. Some plugs have a metallic ring at the bottom that acts like a gasket, so it's very important to install at the proper torque to get the correct crush on that ring. Other sparkplugs have a smooth angled seating surface that is machined so it mates to the head and seals, but also requires proper torque so the surfaces seal together. NOT torquing the plugs properly has probably allowed them to back out. You wouldn't be the first to have this happen, and it rarely causes damage to the point of a head needing replacement. Usually, by the time it backs out a couple threads, the engine is running badly enough that it gets your attention and gets fixed. TORQUE them, not by hand or by feel, but by using the correct torque wrench.

I'm pretty sure the Gen 3 has the metal ring crush washer. These absolutely, positively, HAVE to be properly torqued to crush the ring.

 

You buy a spark plug thread chasing tool at any of the major auto parts store corresponding to the threads of your plugs which they can look at your plugs at the parts store and screw a bolt and a nut on the plugs it's only like I don't know 12 mm I think or whatever it is then you take that spark plug chasing tool You stick it on the end of a socket and a ratchet and all that nonsense You spray some kroil or something like that on the end of the chasing tool you stick that down in the spark plug hole and by hand you jog that tool back and forth until you feel it catch the threads this is part of mechanical skill. Once you get the chaser caught in the threads then you can start slowly advancing it down with your turning tool ratchet whatever and then you crank that down until it bottoms as the threads clean up the tool will go in easier and it will bottom then you return it remove it you'll see the coil has picked up some aluminum pieces Don't worry about the small minute shavings that have fallen into the spark plug hole. The internal combustion temperatures will make short work of that and will not damage anything I promise you even if you're already oil burning and your cylinders are worn this is not going to do anything to advance retard or otherwise hurt that You're where you are with your engine and that's that now your spark plug should run in almost by finger with your ratchet and the knurled extension and bottom out now affix your ratchet and give it about a half turn. Everyone of these generation twos I work on the spark plugs are ridiculously loose as in I stick my ratchet and extension down there and I barely flick it with my finger and the plug is turning and reversing outward that should not be. It needs to be tight that ring needs to seal the plug to the head That's why we knew you give it that whatever it is a half a rotation after the spark plug ring touches that gives you your seal and all that but that should make that happen pretty quickly there's also a methodology of a helicoil I don't think you're there yet personally.

 

Don't make this more complicated than it has to be use a thread chasing tool for the existing threads in your head let's not worry about milling away things and all of that You're not using a milling machine You're using a thread chasing tool which is used as the way I talked about above you'll be in great shape Yes the washer's on the plugs that you're taking out are squeezed pre-squeezed they were almost double the thickness If you reinstall those plugs you go till the metal touches the head you feel resistance and now give about a quarter of a turn instead of the half or near 3/4 turn that you needed when the washer was new pretty much as simple as that It should take two hands to undo a spark plug out of the head one studying the extension and the other one pushing against the ratchet in the lefty loosey direction there's no way you should be able to just drop your socket and extension and ratchet in the hole with one hand hook up to the spark plug and give a turn and it's undone It's not tight enough that's a pure fact unless you are King Kong.

 

There is no way on earth the plugs fully unthreaded from the head. If they even came close, compression and combustion would have blown the plug and coil off the engine. That plug has probably ~19 mm of thread. Pull 'em out, inspect them and it's very likely you'll just be able to put them right back in. Heck, you could probably just torque them down right now and they'll be good to go.

 

in the beginning of this post I thought it was stated that the original poster couldn't unscrew his plugs or they were very tight or he had them out in the washer was compressed duh obviously something along those lines That's the only reason the thread chasing even came up but yeah try to take him out try to put them back in obviously I thought that was already done but apparently maybe not.

 

Like I say every one of these generation twos that pull into here the plugs are in so loose it is mind-boggling It must be a Prius thing for sure because in Corollas have never seen this I don't think the car is doing it I just think the I don't know the dainty mechanics that work on hybrids you know or just think they touch and turn a little bit and they're done I don't know I mean I really don't It's amazing I like plugs to be buried pretty good not stupidly over toked and not that you need a 4-ft bar to undo them either but they should be in there pretty stout certainly shouldn't be finger tight.

 

Torque is 15 lb/ft, no lube mentioned, so presumably clean/dry threads.

note : the last page of above is way off in another section of the manual, specifically about the valve cover. That’s the only place I found the torque spec for spark plugs; there’s nothing in the spark plug section.

 

Yes and for the plugs you can go by like the machine is handbook for the thread diameter pitch etc If you're torquing spark plugs and you might have some other issues but people do it but like I say almost every two that I take apart some plugs are literally finger tight and it is becoming overly ridiculous I never saw this in the '70s '80s even early '90s I don't know what's going on I don't think they're getting loose in the engine I think they're not being installed tight enough it's actually quite funny.

 

Really ... so when you go looking up the generic tightening torques for those M12 threads on the spark plug, which figure are you gonna use? 28 ft lb? 65 ft lb? 92 ft lb? 110 ft lb?

Or are they fine pitch? 70 ft lb, 99 ft lb, 118 ft lb?

You definitely might "have some other issues" if you try to use any of those generic torques from a handbook on a spark plug made for 15 ft lb.

The torques in generic handbooks are there for a last resort if you have the same kind of fastener the handbook is talking about, joining the same materials the handbook is talking about, under the same conditions, and there isn't any other source to tell you what torque to use.

In any application where there's a repair manual and it says "use this torque here", that's the torque to use.

 

Generally what I use is touch the washer and give it about 3/4 of . I believe in the old original machinist handbook there is a section on tightening glow plugs spark plugs and the like I'll have to pull my I don't know what 1958 version or something.

 

Yes any good metallurgy handbook like The Machinist handbook will talk and discuss various types of steels going into softer metals like aluminum etc etc If you like to read that sort of stuff.

 

Spark plugs are extra special, too, because beyond the metallurgy and stuff, they are hollow threaded metal cylinders with brittle ceramic inside, and stuff.

The torque info for those should be coming from the people who make the spark plugs, or from the people who make the engines that use the spark plugs.

 

I've got a CD-rip pdf version of 26th edition of Machinery's Handbook, searched it for "spark plug", no result.