Crossed spark plug thread

MrCostington · 01-08-2020 10:31pm #1

I'm embarrassed to post this, but I think I damaged my cylinder head today on my W123.

I pulled the plugs to check them, they all came out easily. Putting the first one back (no 1), I thought I got a few turns by hand, then put the socket on it. After a few turns (too many) I decided it did not feel right, too hard. I went ahead with the other 5 (M110 engine, ali head) to compare, they all went in as expected, very easy. I then removed no 2 and tried that plug in no 1, just in case it was the plug, no joy.

My mate is a fitter, I rang him and he suggested a cleaning tap. We had a really bad phone connection so could not finish the conversation, will try again tomorrow. But in the meantime, what's your thoughts? I'm worried a tap will drop filings into the cylinder.

I know about heli coils (sp?) but any suggestions without removing the head?

Could I take a chance with a tap?

Yours. kicking himself, in Dublin

Roger the cabin boy · 01-08-2020 10:36pm

MrCostington wrote: »

I'm embarrassed to post this, but I think I damaged my cylinder head today on my W123.

I pulled the plugs to check them, they all came out easily. Putting the first one back (no 1), I thought I got a few turns by hand, then put the socket on it. After a few turns (too many) I decided it did not feel right, too hard. I went ahead with the other 5 (M110 engine, ali head) to compare, they all went in as expected, very easy. I then removed no 2 and tried that plug in no 1, just in case it was the plug, no joy.

My mate is a fitter, I rang him and he suggested a cleaning tap. We had a really bad phone connection so could not finish the conversation, will try again tomorrow. But in the meantime, what's your thoughts? I'm worried a tap will drop filings into the cylinder.

I know about heli coils (sp?) but any suggestions without removing the head?

Could I take a chance with a tap?

Yours. kicking himself, in Dublin

I think your mate is right.

Go give it to a car mechanic who has the right sockets and tools. There is a knack to finding the correct start of a thread that's damaged.

kadman · 01-08-2020 10:43pm

Thread chaser for clearing the thread, but as I am not familiar with MB engines , you will have to wait
for the MB experts to chime in.

I know from working on VW engines, that we used thread clearing taps , and oversize taps on damaged
plug threads. And often bored out 14mm damaged plug threads to 18mm.

Back in the day it was place sash cord into the chamber,bring up the piston, and tap the thread.
Any swarf acollected on the sash cord which was puled out with all the crap.

This was before helicoils.

But Like i said, best to get the advice from the merc men.

good luck with it.

MrCostington · 02-08-2020 10:27am

Roger the cabin boy wrote: »

I think your mate is right.

Go give it to a car mechanic who has the right sockets and tools. There is a knack to finding the correct start of a thread that's damaged.

Thanks! I think I'd be able to trust my mate to do it, he looks after stuff in a plant hire company?

MrCostington · 02-08-2020 10:39am

kadman wrote: »

Thread chaser for clearing the thread, but as I am not familiar with MB engines , you will have to wait for the MB experts to chime in.

Sounds like there might be hope to fix without removing the head?

Does it matter what make the car is? I only mentioned the make as I was not 100% sure the head is alloy. I think it's safe to say you know your stuff!

kadman wrote: »

I know from working on VW engines, that we used thread clearing taps , and oversize taps on damaged plug threads. And often bored out 14mm damaged plug threads to 18mm.

Wow, I hope I don't have to do that!

kadman wrote: »

Back in the day it was place sash cord into the chamber,bring up the piston, and tap the thread. Any swarf acollected on the sash cord which was puled out with all the crap.

Is this a sash cord as used in house windows? So, if I understand, you push the sash into the cylinder and let it drop in there, then tap, then pull it out with a needle nose pilers?

PS great work on the VW!

kadman · 02-08-2020 11:51am

Heads on the VW are cast aluminium alloy, so soft enough to thread easily.

Soft enough for misaligned plugs to cross thread as well. Its common enough in the VW world
as the plugs may not always be easily accessible being a flat 4.

I am not Merc savvy, so I cant advise on the best course of action for you.

But if it can be done with engine in place on a flat 4 vw engine, I reckon an upright engine
is far easier.

I remember back in the 60's my dad boring out a 14mm cross threaded hole, and re tapping
an 18mm plug into the head on an old 1200 beetle. I thought nothing of it at the time,
but looking back with the tools available then, it was a feat.

Car drove for years after, I must ask him the full details of what he did then, and he could still do it now at 88:D

MrCostington · 02-08-2020 12:15pm

Is your dad free this afternoon?

Would you mind explaining the sash thing a bit further please?

I've just been looking at some videos where people seemed to put in helicoils without concern about debris, including Scotty Kilmer if you know him. He just started the engine to blow it out. I won't be doing that. But in the comments, it was suggested to put grease on the tap and clean after every few turns. My mate said that too.

https://www.youtube.com/watch?v=c_t2mz2tN3w

MrCostington · 02-08-2020 2:45pm

I took some pics with a cheap endoscope, so not great quality.

The first one is the bottom of the thread showing the good part, second is the damage:

kadman · 02-08-2020 3:41pm

MrCostington wrote: »

Is your dad free this afternoon?

Would you mind explaining the sash thing a bit further please?

I've just been looking at some videos where people seemed to put in helicoils without concern about debris, including Scotty Kilmer if you know him. He just started the engine to blow it out. I won't be doing that. But in the comments, it was suggested to put grease on the tap and clean after every few turns. My mate said that too.

https://www.youtube.com/watch?v=c_t2mz2tN3w

I just happened to post up pics of me placing case savers into magnesium alloy engine case.
I also used plenty of grease on both the drills and the tap, and placed grease
all near the hole.

With a cross threaded plug hole.
The sash cord was greased lightly and packed into the plug hole, with the piston near TDC. So that you did not need lengths of the cord. Then any tapping or drilling is done extremely gently and with care not to go beyond where you need to fix.

I have worked on vw engines with helicoils that gave trouble with helicoils loosening. This was because the early helicoils were basically wound wire. And this type have rounded edges where they grip into the new thread, so there is always the possibility of moving.
If you look at the case savers in my thread you will see how sharp the edges are, this gives a better bite into the new threaded hole.

As I have already mentioned, I am not merc savvy so in your best interests you should contact one of the merc savvy guys here before you proceed with anything and I am sure they have come across this problem before.

You wont be the first person to cross thread a plug:)

MrCostington · 02-08-2020 4:43pm

Hey, many thanks again.

I just looked at your pics, interesting. I hope I will get by with a cleaning. It will be Tuesday till I can get a tap.

Did you ever have problems getting the sash out?

I might post this on the MB club forum, but I've been pestering them over the last few days with some other questions!

kadman wrote: »

You wont be the first person to cross thread a plug:)

haha thanks

kadman · 02-08-2020 5:09pm

MrCostington wrote: »

Hey, many thanks again.

I just looked at your pics, interesting. I hope I will get by with a cleaning. It will be Tuesday till I can get a tap.

Did you ever have problems getting the sash out?

I might post this on the MB club forum, but I've been pestering them over the last few days with some other questions!

haha thanks

Never had an issue, once you dont move the piston after you put in the cord.

Dont worry about posting questions on the MB owners forum.

Questions , questions, questions is what forums were made for, ask away...:D

MrCostington · 02-08-2020 5:36pm

Thanks!

OK I'll post there too, if they kick me out I'll say kadman made me do it

MrCostington · 02-08-2020 8:42pm

I got some replies on the MB club forum, mostly along what you already said (including your sash trick, but with a rag).

Suggested this tool, which is cheap and local

https://ige.ie/thread-chaser-set-2pc-for-spark-plugs/?gclid=EAIaIQobChMI0t7-94r96gIVSrDtCh0wFgzwEAQYASABEgL3ZvD_BwE

And this which looks like the winner, I think as it may, I think, push the debris upwards

https://www.sealey.co.uk/product/5637192178/reverse-action-spark-plug-thread-chaser-14mm

I'll contact them tomorrow to see if I can get one here.

kadman · 02-08-2020 11:19pm

Reverse action tool is an ingenious bit of kit.

Let us know how that works for you,

good luck with it.

MrCostington · 03-08-2020 4:13pm

Thanks kadman

Vinny Byrnes have the Sealey tool so I will pick it up tomorrow (I'm off this week). That is great as they are 15 mins from me.

Turns out the guy on the MB forum (mainly a UK club) that suggested it is from Galway!

UPDATE they have to order it for me, will be Thurs/Fri

LollipopJimmy · 03-08-2020 4:30pm

If it was me I'd remove the head. However I seem to love hardship

MrCostington · 03-08-2020 5:30pm

That's the exact thing I'm trying to avoid!

kadman · 04-08-2020 5:36pm

MrCostington wrote: »

That's the exact thing I'm trying to avoid!

Any suggestions from MB forums??

Exodus 1811 · 04-08-2020 6:11pm

Would a small magnet pick up any bits rogue in your cylinders, after the contingencies laid out in previous replies?

welder · 04-08-2020 6:43pm

Exodus 1811 wrote: »

Would a small magnet pick up any bits rogue in your cylinders, after the contingencies laid out in previous replies?

Its an aluminium head, so any swarf generated won't be attracted to a magnet.

MrCostington · 04-08-2020 9:42pm

kadman wrote: »

Any suggestions from MB forums??

Hi, I think I posted that already? This site is acting up the last few days, there is a thread about in After Hours.

Basically, much the same as you have said really. One guy suggested this tool:

https://www.vinnybyrne.com/reverse-action-spark-plug-thread-chaser-14mm-html-9398.html

I have it ordered and will be in Thur/Fri.

ROCOL-53023 RTD Compound was also suggested for lube.

Also : 'Following advice I took an old plug & filed ‘flats’ on two sides to effect a thread cleaning tap.' McGyver must be an MB owner!, but good at a pinch!

Exodus 1811 · 04-08-2020 9:50pm

welder wrote: »

Its an aluminium head, so any swarf generated won't be attracted to a magnet.

Ah. Didn’t realise we established that.

Although the swarf would be minimal if just cleaning the threads, I’d be wary!

MrCostington · 04-08-2020 9:59pm

Exodus 1811 wrote: »

Ah. Didn’t realise we established that.

I was not 100% sure in the OP but looked it up in a book later.

I'm sure if it was iron, it would be possible to magnetize the tool which should work well I'd guess

kadman · 04-08-2020 11:27pm

You have 2 choices, tackle it your self, or get someone else to do it

When you made up your mind on that, 2 more choices.

Head on , or head off. Simples.

Do you feel confident enough to do the work, including engine strip down. If you do go for it.

If you dont, get a quote from a reputable guy. Which I suspect would be close to a four figure sum.

But at this stage you want an expert if you dont do it yourself.

kadman · 04-08-2020 11:30pm

If you are using a reverse thread chaser, you are minimising your risk of
swarf into the chamber, and there wont be bucket loads of it anyway.

Can you get access to any old scrap engines or heads to try your self first.??

Were MB forum any help

MrCostington · 05-08-2020 10:16am

kadman wrote: »

If you are using a reverse thread chaser, you are minimising your risk of swarf into the chamber, and there wont be bucket loads of it anyway.

Can you get access to any old scrap engines or heads to try your self first.??

That's the hope!

I don't have aaccess to any old heads unfortunately. I plan to try the tool on a good thread first just to get a feel for how it operates.

kadman wrote: »

Were MB forum any help

Did you see my post #21?

MrCostington · 05-08-2020 10:19am

kadman wrote: »

You have 2 choices, tackle it your self, or get someone else to do it

When you made up your mind on that, 2 more choices.

Head on , or head off. Simples.

Do you feel confident enough to do the work, including engine strip down. If you do go for it.

If you dont, get a quote from a reputable guy. Which I suspect would be close to a four figure sum.

But at this stage you want an expert if you dont do it yourself.

Yes, about a grand was my gut feeling to have the head off, plus transport costs. So I'm going to try myself, with the head on. Although my timing chain is good, it would be madness not to change it if removing head so that's another cost.

kadman · 05-08-2020 11:06am

MrCostington wrote: »

That's the hope!

I don't have aaccess to any old heads unfortunately. I plan to try the tool on a good thread first just to get a feel for how it operates.

Did you see my post #21?

I can give you a vw magnesium alloy head if you want to try it on that.

You can fek up one of the plugs first, then see if you can fix it.:)

kadman · 05-08-2020 11:13am

Sash cord was used because its firm enough that when you pull it out,
it all comes out, and does not get caught. Unlike a fabric type of material,
you may leave bits in there. I suppose lawnmower starter cord is much the same thing.

Fingers crossed these smeared in grease should catch all the material.

Youtube proper use of reverse chasers before you start.

I am surprised that no other classic car owner has had to repair a damaged plug thread,
because in the beetle world i have seen a few.

MrCostington · 05-08-2020 12:01pm

Hi, re the heads, not sure if you are joking, but if not where roughly are you?

I looked around a few places for sash cord, no luck. I made up a circle of cotton and attached thread to 4 'corners' of it so it's like a parachute. This will enable me to pick it out, and, as I pull on the threads it folds up to trap any debris. I made a simulation head with a plastic bottle and it works ok.

I'll post some pics aright. OK, I'll try a video too. You will have me on Tick Tok next lol

kadman · 05-08-2020 12:25pm

I am deadly serious about the heads , I have a few scrap ones.

I am next door to Kilbeggan racecourse.
Parachute cord will do too, or lawnmower pull cord.

Of course you wont be able to leave a pull cord in the plug obviously, no matter how light.

Might work.

Would you have to come far??

Crossed spark plug thread

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