I've got a REALLY cross threaded plug in cylinder #7 of my 390. I mean someone really put in 110% effort.

Currently, I've just worked on it with a whole lot of penetrating oil and soaking time. I've used a 3/8" breaker bar - which is a very wimpy one - maybe 7 inches. It just stops and won't go anymore.

So I'm wonder what is the best approach. Would love to hear what others have done with this. It's definitely stock heads.

Do I use one of my monster 1/2" breaker bars that I use for wheels? Do I stop and switch to MAP gas? Do I just keep soaking it? And then try to keep moving it back and forth? Or do I purposely shear it and then get a square tap Champion bit (or whatever they're called....not a spiral one) and take it out and then go straight to tapping?

Lastly, is the smart thing to do to NOT do any of this and just pull the head and bring it to a machinist? I'm spending money on this thing like there is no tomorrow and I'd rather not.

Some guys treat a stuck plug as no big deal, others say it is. I've been lucky in not having to deal with this with any of my rides to date. So I guess I was due. Thanks all. John

 

Are you pulling it because you need to or because you want to ? if it ain't broke don't fix it at some point you're going to have to rebuild it anyway. .

There are tools to extract a broken plug and rethread it in frame without filling the cylinder with shavings. but I'm not sure they exist for your plug size and thread. don't assume that's an option until you check.

If you plan on pulling the head if you can't get it out then you might as well force it until something gives you have nothing to lose. worse case you pull the head.

 

Are you sure it is cross-threaded or just really stuck after many years? I have had some that either wouldn't move at all or would move a bit and get stuck presumably due to the carbon/rust build up in the threads. I have had good success with an impact wrench set on a lower setting. I back the stuck plug out with the impact wrench until it stops and then spin it back in. Repeat and it comes out a little further each time. The vibration of the impact seems to be the secret. I have never broken a spark plug this way but you have to use common sense and not get carried away with the impact wrench--be gentle with it!. You could have a greater chance of breaking the plug off by using a monster breaker bar.

 

Well, that's an interesting point. I hadn't thought of just leaving it be. Tho that runs against my grain in terms of worrying about it not 'being right'. Don't forget it's currently about half way out. So I'd actually have to crank it back in, and risk damaging the threads more.

As an FYI, it's a new to me truck, sold as a "Ran Last Time It Was Parked" many many years ago. Currently a non-runner and working my way through it. It's a pretty solid ride and I'm happy with it so far.

I was pulling the plugs to soak the cylinders with Marvel Oil to loosen gunk before I try to hand crank it. The plugs should be replaced. They are heavily oiled with big gaps. It definitely makes sense to change them.

I'm sure plenty of guys on here have run into this situation. I'll just wait a bit and see what pops up here. Thanks. John

 

Make every attempt not to break it off. It doesn't sound like you have it but an oxy/acetylene torch to rapidly heat area around plug is best friend. Mapp gas may not get it done as there is just too much mass there to absorb the heat. Typically heat and cool cycles with penetrating oil will get it free. Since it sounds like its moving a bit, that is a positive sign. You can use a 1/2" drive to get a little more force but I wouldn't use all the leverage. Take it slow and patient.

 

I've got the equipment to do that. I had thought of it, but assumed it would be far too much for a hollow bolt (aka spark plug to handle). When you spin it back in, do you do it by hand? Or by gun? Has this ONLY been the case with firmly entrenched plugs, or have you seen this work with a plug that is truly cross threaded?

I have always just assumed that an impact wrench is a combo of shock + machine muscle....hadn't thought that it might actually be a more gentle approach than human muscle on a bar.

 

This is helpful stuff guys. Mark - if I alternate penetrating oil with Mapp gas....might that be a somewhat effective alternative. Then hit it with the impact gun as mentioned (probably on a setting of "1"...the lowest). I've worried that with cast iron and heat, I will get too much localized heat and get a crack...so I've been hesitatent to try.

John

 

I have had really good results soaking things like this with Kroil penetrating oil. I would let it soak for half a day or so while working it back and forth before applying any heat.

 

Thanks John. I started with a lot of PB Blaster. Then I've switched to SeaFoam Seep Creep (sp), which I've heard a lot of good things about. Kroil is top notch too.

If I don't like what I'm getting with SeaFoam, I'll add Kroil to the mix. It's been soaking a number of days. At some point....I'm going to have to give it a twist. Thanks.

 

Remember that a driveshaft is also a hollow tube. In a twisting motion the outside of the tube bears the brunt of the torque. If it is a 18mm spark plug, it will handle quite a lot of torque--I have never broken a spark lug off in the head in my life. Surprising actually. I have on occasion broken the insulator.

I have to ask. How have you determined that it is cross threaded? Is this your assumption because it is stuck? The reason I am prying at this is if it was cross threaded, it most likely would never have seated properly and there would have been combustion chamber gases leaking past it ultimately causing quite a visible mess. It is actually kind of hard to cross thread a spark plug.

An impact wrench does have the advantage of the rattling action--not to say that the monster breaker bar doesn't have it's place. When using an impact wrench I spin it both directions to work out any carbon or rust that might be locking the spark plug in the threads. The secret is not to hammer on it with the impact wrench (especially on a setting). I have an old no-name bargain store impact that doesn't have too much power so it works well for this. Finesse, not force is what you are after.

Also, if it is cross threaded then that means it is jammed in the head. Penetrating oils likely won't be of much help.

Irrelevant information--I have heard of shops in my area that use impact wrenches to change spark plugs! Yikes! Time is money when you get paid by the piece I guess. They won't be getting any of my money.

 

Well that's encouraging.

I am totally ASSUMING this. And you know what they say about assuming....you make an *** out of Uma Therman (I love that dumb joke). Again, since I've never run it, I can't vouch for how it runs. I can tell you that the engine bay is an absolute mess in terms of oil, grit, weird clay, some sort of dessert fines. I've washed it with a bunch of different stuff just to figure out what all the lumps are in there.

OK, so LOW SETTING plus SHORT BURSTS. Got it.

I'll probably keep soaking just in case. Can't hurt. But sounds like I won't skip straight to HEAT.

 

From your description it sounds like that engine will be needing work before the spark plug has to be replaced. I'd leave it alone until it has to be replaced or the engine needs attention whichever comes first.

 

Well I'm hoping with some cylinder soaking, reworking the fuel system (tanks gunked, I'm sure fuel pump dried out), new wires, points, and the like...it will 'roar' back to life. I'm trying to take it slow, but I will probably give it a shot in the near future. Will have to remember to post back on this thread with the outcome.

 

If the plug is cross threaded, it was cross threaded on the way in, not on the way out. If it was cross threaded, something cut new threads into something, or both somethings did so to each other. If the plug was all the way into the head, I doubt it was cross threaded because to cross thread it, would mean an angle was introduced. These are not two-piece plug bodies, but they can be broken. A 7" long 3/8" drive breaker bar is not a whole lot of leverage measured against 20-30 years rust. If you use a long 1/2" breaker bar, use small opposite directional turns. Once it is out, do run a correct size spark plug thread chaser tap in the hole. Back in those days, plugs often got replaced yearly or in 10-20,000 mile periods.

It is possible someone way overt torqued the plug and damaged the threads I guess. It's gonna have to come out though.

 

Another alternative penetrant is a 50-50 mix of acetone and ATF. If you are not sure whether the plug is cross threaded, compare it to adjacent plugs - they should all be parallel.