Spark plug hole stripped

[Palehorse940]

As the title guys. I pulled the plugs to fit new ones and unrated leads all went in great apart from the one closest to the bulk head it pulled some threads and now I got the new plug in but only half way and it seems like a loss in power. What would be the best corse of action to sort this and can I do it with the head still on. Cheers Dave

[dingov70]

Evening , You might get away with using a tap of the correct size /pitch to clean up the existing threads , BUT there is a risk of swarf getting into the cylinder .

I have repaired in-situ by drilling & heli-coiling the head . To avoid any swarf getting into the engine the engine was turned until the inlet valve was opening and compressed air blown into inlet manifold, forcing any metal swarf to blow up & out of the hole . You certainly need eye protection & ear defenders , along with a very large air source .

A helicoil repair is probably your most effective solution

[Ian21401]

I discovered this problem on my 1992 940 when servicing it in 2005 shortly after acquiring it. Same no.4 plug hole. Removed the head and had a local engineering firm fit a helicoil. Still OK after several plug changes but I’m always very careful when fitting and tightening the plugs.
Good luck.

[Dastardly Diesel]

Head off and either helicoil or a specialist repairer. Head off is an easy job on these, so not worth sweating over.

[Palehorse940]

Thanks guys some food for thought looks like I’ll be getting some prices

[Whyman]

My son had his BMW Z3 head helocoiled in situ with no ill effects.

The guy who did it took all the plugs out and spun the engine over to get rid of any swarf and then used a small pipe on the end of a vacuum to try and get any remained out. Was an ally head so any that remained would have bunted off during combustion.

He never had a problem after that but was very careful when replacing the plug.

[Laird Scooby]

You've already had the obvious advice, head off and helicoil it - as a couple of people mentioned some mobile guys will do it in situ but access is difficult so you could easily end up with a skew-whiff helicoil in the head.

In future, once it's been repaired, use a length of garden hose on the ceramic part of the plug to start the thread off before winding it in with a ratchet or whatever. If you cross-thread it, the garden hose will slip so you can simply undo it again and have another try. Safer and often quicker than multiple tries with a spark plug socket!

If you don't have any garden hose, a piece of 6mm bore fuel hose on a #2 Pozidriv screwriver shaft works well but it fits on the pip on the end of the plug where the HT lead connects instead of the ceramic insulator.

As for being down on power, first you're losing compression because the plug isn't sealing properly and second, due to the lost compression, it will misfire on #4 and some of the power from whatever explosion you do get from the fuel/air mix in #4 will be lost past the plug threads.

[xco]

You've already had the obvious advice, head off and helicoil it - as a couple of people mentioned some mobile guys will do it in situ but access is difficult so you could easily end up with a skew-whiff helicoil in the head.

In future, once it's been repaired, use a length of garden hose on the ceramic part of the plug to start the thread off before winding it in with a ratchet or whatever. If you cross-thread it, the garden hose will slip so you can simply undo it again and have another try. Safer and often quicker than multiple tries with a spark plug socket!

If you don't have any garden hose, a piece of 6mm bore fuel hose on a #2 Pozidriv screwriver shaft works well but it fits on the pip on the end of the plug where the HT lead connects instead of the ceramic insulator.

As for being down on power, first you're losing compression because the plug isn't sealing properly and second, due to the lost compression, it will misfire on #4 and some of the power from whatever explosion you do get from the fuel/air mix in #4 will be lost past the plug threads.

This advice is a top tip re refitting new spark plugs as well as spraying spark plug thread with wd 40 prior to seating and torqueing the plug it enables you to have complete control over the re insertion of the plug as you will feel if the thread is seating properly or not.....This was standard proceedure with all my petrol engine cars(especially my much loved and coveted 3 ltr Opel Senator) before I was drawn to the dark side (oil -burners/diesels).

Many years ago I had a much loved Citroen cx 25 ie familiale and the spark plugs were buried deep in the head and each spark plug then had a 4 inch threaded bar screwed on top of that before fitting of the .spark plug lead and the hose pipe locator and turn method was really the only way to Change the plugs without danger of cross threading.

Still a very good tip today to start a plug into an engine head and avoid the need for helicoils especially as a lot of modern day heads are now aluminium.

[Palehorse940]

Great help and advice guys thanks for that. I’ve got a couple of weeks off soon so I’ll have to get it sorted then. Dave

[happy diver]

Seat the plug and then 3/4 of a turn, and a bit


My liberal thread coating of anti seize allays
some of the terror of removing plugs from aluminium heads

[Clan]

Seat the plug and then 3/4 of a turn, and a bit


My liberal thread coating of anti seize allays
some of the terror of removing plugs from aluminium heads

actually the only lubricant for alloy plug threads is paraffin or nothing . grease and oil will oxidise and cause seizure .. Basic engineering principles ..

You can do what you like with a cast iron head , 👍🏻

[Laird Scooby]

actually the only lubricant for alloy plug threads is paraffin or nothing . grease and oil will oxidise and cause seizure .. Basic engineering principles ..

You can do what you like with a cast iron head , 👍🏻

Paraffin is just another hydrocarbon with no anti-seize properties though?

I've always used a smear of copper based grease and never had any problems.

[huron]

I use silicon grease on alloy.

[classicswede]

The best answer is to fit a a timesert from wurth

[Clan]

Paraffin is just another hydrocarbon with no anti-seize properties though?

I've always used a smear of copper based grease and never had any problems.

Plugs don't seize though .. Especially in alloy heads , no lubrication is needed that's what i learned doing my 5 year City & Guilds Motor vehicle engineering courses and the following 40+ years practical experience has proved that right , just make sure the threads are clean and torque the plugs up and no further trouble . Any carbon based grease can react with the plug material and alluminium over time and thousands of heating and cooling cycles and CAUSE the problem . Imaging a clump of grease on the end of the plug in the combustion chamber , that's going to ruin the threads on the end and when you unscrew them it rips the threads out on the way out . Paraffin or WD40 is harmless if you feel it needs some sort of lubrication , but you should be able to wind the new plug all the way in by hand when everything is good no matter how many times the plugs have been changed .

Now cast Iron heads are a different story , a thin smear of oil or similar IS recommended .

[Laird Scooby]

Plugs don't seize though .. Especially in alloy heads , no lubrication is needed that's what i learned doing my 5 year City & Guilds Motor vehicle engineering courses and the following 40+ years practical experience has proved that right , just make sure the threads are clean and torque the plugs up and no further trouble . Any carbon based grease can react with the plug material and alluminium over time and thousands of heating and cooling cycles and CAUSE the problem . Imaging a clump of grease on the end of the plug in the combustion chamber , that's going to ruin the threads on the end and when you unscrew them it rips the threads out on the way out . Paraffin or WD40 is harmless if you feel it needs some sort of lubrication , but you should be able to wind the new plug all the way in by hand when everything is good no matter how many times the plugs have been changed .

Now cast Iron heads are a different story , a thin smear of oil or similar IS recommended .

That's a very vaild theory and holds true as long as the cars are regularly serviced, by experienced personnel who are careful and not hurrying to meet bonus incentives.

However, remove one of any of those factors and you have problems. Plugs that have been in too long can start to weld themselves to the head through electrolytic corrosion.

Get a grease monkey who isn't careful, is hurrying to make his bonus and he is likely to cross-thread or just ignore the last plug.

From all your other posts i've seen, it would appear to me you've been fortunate enough to work on cars that were either new or were serviced regularly and correctly and above all, carefully.
Once you get into the world of cars that have been neglected, the story starts changing, very much for the worst and you'll find plugs that have seized in heads (whether the head is aluminium or cast iron) and sadly i've seen many that are like this. I've also seen plugs that have seized in alloy heads and successfully removed them.

[classicswede]

I most cases you would not add and anti seize to the spark plug threads but certain engines like the ford Fiesta for example they are a real pain and very often snap off in the plug hole


Plugs seizing in is becoming more common with extended plug service life and plugs being buried down inside the engine where water can be trapped

[Clan]

That's a very vaild theory and holds true as long as the cars are regularly serviced, by experienced personnel who are careful and not hurrying to meet bonus incentives.

However, remove one of any of those factors and you have problems. Plugs that have been in too long can start to weld themselves to the head through electrolytic corrosion.

Get a grease monkey who isn't careful, is hurrying to make his bonus and he is likely to cross-thread or just ignore the last plug.

From all your other posts i've seen, it would appear to me you've been fortunate enough to work on cars that were either new or were serviced regularly and correctly and above all, carefully.
Once you get into the world of cars that have been neglected, the story starts changing, very much for the worst and you'll find plugs that have seized in heads (whether the head is aluminium or cast iron) and sadly i've seen many that are like this. I've also seen plugs that have seized in alloy heads and successfully removed them.

Hi Dave ,
i've never seen anyone racing like that in a garage in my whole life ! They used to like their fag and cup of tea in the old days . Bonuses are a farce very few ever meet theirs today without doing what you say ie. not doing something they are supposed to . That won't happen in a volvo garage . They get paid enough to not need bonus .Did you know a BMW master technician was on £35000 25 years ago ! you have no idea what quality control goes on these days . Especially with two people working on the car at the same time they tend to watch each other's work methods .

Spark plugs were changed every 72000 miles not long ago no problems getting those out of a volvo after 10 years . never any problems with the 240 and 900 plug threads and i have done 10's of thousands of those .
Yes i've come across plugs broken off and bodged , I bought a spark plug helicoil kit for the first one i repaired on an old 340 years ago , never used it again .. They had 1/2" reach plugs in the early ones so not a lot of thread in the head and could be damaged with ham fisted hands and wrenches .

Just fit the plugs as they come out of the box , you wont get any problems .

[Clan]

I most cases you would not add and anti seize to the spark plug threads but certain engines like the ford Fiesta for example they are a real pain and very often snap off in the plug hole


Plugs seizing in is becoming more common with extended plug service life and plugs being buried down inside the engine where water can be trapped

yes those ford cast iron heads have the plugs right next to the exhaust ports , you can see the black paint burned off all around the plugs no wonder the threads get a lot hotter than on more conventional engines .

[classicswede]

Its the newer engine with coilpacks that has the biggest problem, they are really deep down making them harder to deal with

[Laird Scooby]

Hi Dave ,
i've never seen anyone racing like that in a garage in my whole life ! They used to like their fag and cup of tea in the old days . Bonuses are a farce very few ever meet theirs today without doing what you say ie. not doing something they are supposed to . That won't happen in a volvo garage . They get paid enough to not need bonus .Did you know a BMW master technician was on £35000 25 years ago ! you have no idea what quality control goes on these days . Especially with two people working on the car at the same time they tend to watch each other's work methods .

Spark plugs were changed every 72000 miles not long ago no problems getting those out of a volvo after 10 years . never any problems with the 240 and 900 plug threads and i have done 10's of thousands of those .
Yes i've come across plugs broken off and bodged , I bought a spark plug helicoil kit for the first one i repaired on an old 340 years ago , never used it again .. They had 1/2" reach plugs in the early ones so not a lot of thread in the head and could be damaged with ham fisted hands and wrenches .

Just fit the plugs as they come out of the box , you wont get any problems .

I've seen the real rush when people are trying to make bonus on servicing, including wiping the oil filter clean (or not even looking at it if it's buried in the engine like certain French cars) to save a couple of minutes fitting a new one.
I never knew of and can't imagine two people working on one car to do a service, it's not really cost effective in my book.

As for the 72k plug change interval, surely they should have been checked and regapped every 12k as well? A plug gap, as you know, increases by ~0.1mm/10k miles so after 72k if not checked/regapped would be 0.6mm bigger than it should be! :err:




yes those ford cast iron heads have the plugs right next to the exhaust ports , you can see the black paint burned off all around the plugs no wonder the threads get a lot hotter than on more conventional engines .

It's not just Fords (i'm guessing you're referring to the Kent engine as we're talking Fiesta and exhaust manifold proximity) that have that sort of arrangement. In fact the Pinto engine (as fitted to countless Capris, Cortinas, Sierras, Granadas, Escorts, Transits and probably a few i've forgotten plus of course the Pinto) has a similar arrangement.
So do many Vauxhalls, the Buick/Rover V8 - the list is endless!

However it does seem the Kent/Valencia engine is particularly prone to it. The CVH wasn't that far behind and as a result, Ford sent out a memo about the plugs in CVH engines shortly after the launch stating it must have Bosch plugs fitted or they wouldn't entertain any misfiring/similar warranty claims.



Its the newer engine with coilpacks that has the biggest problem, they are really deep down making them harder to deal with

Ah, the dreaded Zetec lump! :rolleyes:

[xco]

I had an F reg Sierra sapphire 1.6 gls with a pinto lump in it that thought it was a 2 ltr. ....an absolutely brilliant engine but very fussy about oil changes and plug and tappet settings.

I owned mine for 9 years from a three year old and when I got rid of it it had 144.000 on the clock and could still out put 126 mph.

The only thing that went wrong with it was the diff as it was one of the last rear wheel drive ones which cost me £40.00 from a scrapyard and about an hours labour to fix.

I seem to recall the only reason I got rid of it was because it was one of the last engines fitted in its time that required leaded petrol.
I ran it for a while with unleaded and the additive bur the performance went to sh#.

I investigated the transplant head with hardened seats etc but the cost was not economically viable so we parted company regrettably. ...A sad day.

Then came my Opel Senator.....but that's another story.