Spark Plug Thread Corrosion

cham · Post by **cham** » 19 May 2025, 14:44

I'm about to swap spark plugs and plan on using the factory Volvo 8642660 plugs with the three ground electrodes (N/A 2.4 engine). I was a little skeptical at first since you are not supposed to gap them, and I've had experience in the past with inaccurate gaps from the factory for single ground electrodes. Apparently these don't have this issue for whatever reason. Maybe somebody can chime in.

My problem is I pulled one of my plugs to see what type of plug is in there and found some corrosion between the plug threads and head. I've always been taught to not use anti-seize but I'm a little concerned about just throwing a new set in there without some protection. I've read some people will use a little bit of new oil, but that will alter final torque values as well right?

Also it looks like my old plugs are single electrode turbo plugs. NGK PFR6G, which were discontinued and replaced by PFR6B, both of which seem to only be for the turbo application. I understand this is a little bit of a can of worms topic, but I'm staying away from asking which plugs are the best yada yada yada.

Lastly, what are the torque values! I have seen everything from 18NM to 30NM as the spec.

Post by **BlackBart** » 19 May 2025, 16:48

I always gap plugs. "Sometimes" they are close out of the box, but do it anyway.

I would think if they're chrome or stainless, you wouldn't have that problem with aluminum, but I think some anti-seize is fine, as long as it's not gooped in the threads or close to the end where it might fall into the combustion chamber (or get on the electrode!).

One hand on a 3/8 ratchet handle, snugged up but not horsed, and not two hands for leverage. Maybe 12-15 ft-lb max.

Post by **Blacklab467** » 19 May 2025, 16:56

25Nm or 18 ft/lbs.

cham · Post by **cham** » 19 May 2025, 19:20

@BlackBart Everything I've read says you can't gap these 3 prong multi electrode plugs. You are not supposed to touch them and can't even measure them properly anyways. Also, I'm guessing that maximum of 15 ft-lb recommendation is with anti-seize on the threads?

Vova585 · Post by **Vova585** » 19 May 2025, 19:54

As far as I was taught you should not put any lubrication on the threads. Lubrication will alter your torque spec and you will risk to damage the threads in the aluminum head. All new spark plugs are coming with special anticorrosion coating, so there is no need for any extras. Agree about checking the gap for single electrode sparkplug, but with 2 or 3 or 4 you should not touch it at all.

Post by **erikv11** » 19 May 2025, 19:59

The 3-prong Volvo plugs are best for these engines. Yada yada

I've never tried, or heard of anyone trustworthy, gapping the 3-prong plugs. No reason to mess with them if they don't appear damaged, you didn't drop them off the roof and run over them, etc.

Don't use anti-seize, if you're really concerned then remove the from time to time to inspect.

Re the corrosion, options I'd consider:

(1) Don't worry about the corrosion. You've broken it free without issue, it was between the old plugs and the head not the new plugs. Just install the new ones snugly and move on.

(2) Put a drop or two of penetrating oil on an old plug's threads, run the plug in and out without tightening it at all, and get rid of the corrosion. Then air dry the threads, or clean them with a paper towel and isoproanol and let them dry. Then install the new ones snugly and move on.

Post by **BlackBart** » 19 May 2025, 19:59

cham wrote: ↑19 May 2025, 19:20 @BlackBart Everything I've read says you can't gap these 3 prong multi electrode plugs. You are not supposed to touch them and can't even measure them properly anyways.

Oh, 3-prong, sorry about that, I've never had one of those in my hand.

cham · Post by **cham** » 20 May 2025, 10:45

@erikv11 I think you're right, it's already unseized, just don't leave the plug in there for 6+ years again lol. Turns out these old plugs were discontinued in 2019, don't have records of when the last plugs were installed but I imagine it could be well before 2019 or slightly after since old stock still sits on shelves.

I am curious with your second method of cleaning the corrosion, iso aclohol on a paper-towel would be pretty difficult to get down there. What would be your method of applying the iso? Douse a tube brush with iso alcohol? Or something else?

Also turns out VIDA has a general torque spec page that shows spark plug torque at 30nm or 22ft-lbs. This is higher than what others have said. For whatever reason in VIDA, there aren't any direct instructional pages on spark plug changing, but there is one for ignition coils.

Post by **cn90** » 20 May 2025, 18:54

Re torque setting vs anti-seize...

We discussed that extensively on bimmerfest E39 forum.
In order to strip the threads, one needs to use more than 100 ft*lb torque Just find a junk engine and try how much it takes to strip the threads.

I apply anti-seize and reduce the torque a tiny bit...maybe around 16-17 ft*lb. I have done this for > 20 years.

cham · Post by **cham** » 21 May 2025, 13:19

Finished replacing the plugs yesterday. Ended up installing dry and torqued to about 19 ft-lbs. Now I'm not sure if it's just me being hyper vigilant but it seems to run very slightly rougher. Is there potential for the computer to need to adjust to the new plugs? Would the computer have detected the ignition coils being unplugged and now it's going through a relearn?

In VIDA there seems to be Misfire Counters for each cylinder for monitoring. Drove around for a while while monitoring those parameters and none of them showed any misfires, count stayed at 0 for all cylinders.

I am curious though, is it possible it actually can't monitor misfire count on these early V70s, and the counters are just there in VIDA? I am surprised a 2001 would be able to monitor individual misfire count.