Real risk of head damage when changing spark plugs

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With a cast iron head, I usually don't use a thing. contact plus 1/4 for the washer, 1/8 for taper seat

Dad's old R16 had long threads and crush washer, and didn't really need a spec (IMO, other than contact plus a 1/4), but in hindsight that machine had "superb" threads, the like I haven't seen on Japanese products...admittedly all second hand, they usually have the majority of threads that feel "scratchy" or "heavy".

Will invest in a tap methinks for future works.
 
I never got into the hose on the end trick. I tried it once, and I felt unconnected with what was going on. If you cross thread a plug when putting it in with your hand, you must of wanted to cross thread it.
 
Originally Posted By: MarkM66
I never got into the hose on the end trick. I tried it once, and I felt unconnected with what was going on. If you cross thread a plug when putting it in with your hand, you must of wanted to cross thread it.



I usually use an extention on my plug socket, so there's a little slop there.

After I get about one thread in, I wiggle my end of the extention up, down, left, right. If the plug is going in straight I get the same slop in every direction... some from the plug threads, some from the extention.

Hard to explain in print, it's one of those "feel" things.
 
I have found from experience on aluminum heads that it is a good idea as removing to gently and slowly move the plug back and fourth a bit at a time, this tends to help brush away any carbon build up from the threads. Also you can use something like liquid wrench or PB blaster to assist in smooth removal. It's a good idea to as you gently remove the plug to spray a bit more as you are turning it to get the penetrant into the threads as much as possible.

The important point is if you are removing and feel significant resistance AFTER breaking the plug free BEWARE! If you are not sure and can't repair stripped threads just retighten the plug and take it to a mechanic and let them deal with it.
 
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I do not think I saw it anywhere in the thread. When seating the plug I start off counter-clockwise by hand with an extension, listening and feeling for the light tick as the thread lead on the plug drops off the lead on the head. A few reverse rotations and you can see exactly where the engagement starts and change direction immediately after.
 
A related question occurred to me on my drive home tonight:

In the case where a crush washer and a torque spec are both present, is there any difference between the first use of a plug (when the washer gets crushed and the plug gets tightened to torque spec) and subsequent uses (where the washer is ALREADY crushed and you tighten to the same torque spec)?

Seems to me like these two cases would be the same as far as pressure on the threads go, because the washer would end up in the same condition in both cases. Right?
 
Originally Posted By: rationull
A related question occurred to me on my drive home tonight:

In the case where a crush washer and a torque spec are both present, is there any difference between the first use of a plug (when the washer gets crushed and the plug gets tightened to torque spec) and subsequent uses (where the washer is ALREADY crushed and you tighten to the same torque spec)?

Seems to me like these two cases would be the same as far as pressure on the threads go, because the washer would end up in the same condition in both cases. Right?



This is a great question..I'd like to know the answer as well.

You would THINK it would require a bit more torque because the crush washed is already flattened from first use. But not sure.
 
I alway do plug change when engine is STONE COLD. Usually first thing in sunday morning.

and when install the plug, I screw it in by hand counter clockwise til I heard a little click noise, then turn it clockwise. turn by hand til it tight then use wrench to tight it.
 
Originally Posted By: sandy_d
Originally Posted By: rationull
A related question occurred to me on my drive home tonight:

In the case where a crush washer and a torque spec are both present, is there any difference between the first use of a plug (when the washer gets crushed and the plug gets tightened to torque spec) and subsequent uses (where the washer is ALREADY crushed and you tighten to the same torque spec)?

Seems to me like these two cases would be the same as far as pressure on the threads go, because the washer would end up in the same condition in both cases. Right?



This is a great question..I'd like to know the answer as well.

You would THINK it would require a bit more torque because the crush washed is already flattened from first use. But not sure.


Take a quick look at the first page of the thread, I posted a link from Denso. They give angle torque specs for plugs, including those with washers. Yeah, the tightening specs are much different for previously installed plugs. If you're using friction torque, you should go on the low side of the spec for a previously-installed plug.
 
Originally Posted By: JOD
Originally Posted By: sandy_d
Originally Posted By: rationull
A related question occurred to me on my drive home tonight:

In the case where a crush washer and a torque spec are both present, is there any difference between the first use of a plug (when the washer gets crushed and the plug gets tightened to torque spec) and subsequent uses (where the washer is ALREADY crushed and you tighten to the same torque spec)?

Seems to me like these two cases would be the same as far as pressure on the threads go, because the washer would end up in the same condition in both cases. Right?



This is a great question..I'd like to know the answer as well.

You would THINK it would require a bit more torque because the crush washed is already flattened from first use. But not sure.


Take a quick look at the first page of the thread, I posted a link from Denso. They give angle torque specs for plugs, including those with washers. Yeah, the tightening specs are much different for previously installed plugs. If you're using friction torque, you should go on the low side of the spec for a previously-installed plug.


Of course angle torque would change, because the plug will seat earlier before the washer is crushed -- i.e. before the washer is crushed the threads will be slightly less engaged at seating time than they will be after it's crushed. I suppose friction torque could be affected a bit by a slight difference in contact surface area between the uncrushed and crushed washer if it smashes out but it seems like this would be a very small effect.
 
Originally Posted By: mechtech2
I rarely use a torque wrench on spark plugs anymore.
My feel is fine.

+1
I got alot of practice on an old Polaris sled I had in high school...
It would foul plugs like crazy so I guess on the one hand, they would never be in long enough to even thing about siezing.
Anyways I bet over the few years I had it the plugs were in and out a 100 times. Never had one come loose and never stripped the threads.
 
What I find strange is that it seems Ford aluminum heads get reports of failed spark threads more than anything else. I wonder why.
 
Originally Posted By: artificialist
What I find strange is that it seems Ford aluminum heads get reports of failed spark threads more than anything else. I wonder why.


I thought that was definitively caused by having too few or too weak threads on some (a lot of) specific engine models.
 
Originally Posted By: rationull
Originally Posted By: artificialist
What I find strange is that it seems Ford aluminum heads get reports of failed spark threads more than anything else. I wonder why.


I thought that was definitively caused by having too few or too weak threads on some (a lot of) specific engine models.


Does anyone know the specific engines referred to?
 
Originally Posted By: sandy_d
Originally Posted By: rationull
Originally Posted By: artificialist
What I find strange is that it seems Ford aluminum heads get reports of failed spark threads more than anything else. I wonder why.


I thought that was definitively caused by having too few or too weak threads on some (a lot of) specific engine models.


Does anyone know the specific engines referred to?

Mostly I hear stories about the DOHC 3.0L V6 and the SOHC 4.6 and 5.4 V8 engines. I hear it plenty of times with 2 valve heads, but I hear it even more often with 3 valve heads.
 
and spark blow out is very common on ford
Originally Posted By: artificialist
What I find strange is that it seems Ford aluminum heads get reports of failed spark threads more than anything else. I wonder why.


on mustang gt word, spark plug blow out is pretty common thing lol.
 
The story with one of the Ford engines is that the threaded part of the hole wasn't deep enough. When installed, a few of the spark plug threads were exposed in the chamber. With use, the threads built up deposits, which were then dragged past the threads and damaged them during removal. The next set of plugs were then inserted into threaded holes that were compromised... plus there were fewer threads than normally seen on engines. A combination of these two conditions led to plugs that either popped out during use, or simply couldn't be threaded into the holes.
 
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