cross threaded spark plug on '80 SC
 

Hi there,
I am a newby to this forum, but have owned a few Porsches in my time. I bought a 1980 911SC a few weeks ago. Long story short, I didn't have a thorough inspection before purchase (shame on me), and when I was accelerating hard two days ago I heard a lound bang followed by ominous sounds. It turns out is was a spark plug flying out. It is cylinder 3, the farthest forward on the driver's side. I have tried to very carefully and gingerly start it back in its threads, but it gets about two turns on it and I run into enough resistance that I am afraid to proceed. Looking at the spark plug itself, there are no burnt threads or other obvious signs of damage.

As I see it, I have four alternatives:

1) force the plug in and hope to get through the bad threads and find the real threads (seems like a bad idea).

2) attempt to chase the existing threads with a tap

3) try to do a timesert with engine in car. Not sure if this is possible.

4) drop motor and do it right with a timesert. I am hoping NOT to do this because my workspace location is not ideal and frankly I really hate dropping motors.

Has anyone successfully timeserted a spark plug hole with motor in car? Thanks in advance for your help.

Where are you? A local pelican may be willing to help you with this.
My first question would be why did the plug come out?!

2) probably won't work but it won't hurt anything. Put some grease on the tap to collect the chips.

4) is the right way

The best solution is to install a new helicoil in that hole...Do it right...do it once ...

In response to what I think happened: My guess is that the previous owner got a plug cross threaded and just cranked it as far as he dared. It ran fine for a few hundred miles. I am in Boston.

It might not have been in properly due to a damaged thread. Buy a tap and clean it up first. You do not need a lot of thread depth to hold in a spark plug. DO NOT over-torque the plug when you reinstall.

Welcome to the Forum.SmileWavy
You will find lots of help here.

Sorry that your first post has to be one in distress.:eek:
The value of a PPI can be immeasurable.

1) “force the plug in and hope to get through the bad threads and find the real threads (seems like a bad idea).”
I agree – bad idea. This is probably what PO did and only a couple of threads was insufficient to hold the sparkplug. See if you can find someone local with a ‘bore scope’ to visually inspect the threads.

2) “Attempt to chase the existing threads with a tap.”
You don’t want to use a tap. There are specific ‘thread chasers’ that rearrange the thread without cutting a new thread or making chips.

3) “Try to do a timesert with engine in car. Not sure if this is possible.”
A Timesert isn’t the correct product. A Helicoil is the proper repair. This can be done in the car by someone skilled and experienced. It involves cutting new threads so all the ‘tricks’ need to be used. That includes removing the exhaust heat exchanger and doing the procedure with the exhaust valve open. Yes, you use grease on the threads, cut only a little and then remove the tool for cleaning and fresh grease. This still doesn’t preclude getting half way and needing to drop the engine.

4) “Drop motor and do it right with a timesert. I am hoping NOT to do this because my workspace location is not ideal and frankly I really hate dropping motors.”
Clearly this is the best way. With the head in your hand, you can clean and inspect easily.
I recommend you remove and replace the other five sparkplugs first. If more than one feels incorrect, there isn’t any doubt that you should drop the engine. You might consider finding a honest and competent shop to do the engine removal and reinstall. They could store your car while you deal with the engine.

I recommend you do some more investigation before you commit to any path. You should do a cylinder leak test and cranking compression test. This will get you important information about the condition of the engine. I would remove the valve covers and put an Allen wrench on the head nuts. You don’t want to be surprised if there is a broken head stud.

Are there any oil leaks that should be attended to when the engine is out and easy? Use the opportunity to inspect the clutch; perhaps install just a new disc. Some regular maintenance like valve adjust, rocker arm shaft torque, etc. are easy when the engine is out. Certainly you will want to do some cleaning and detail work. 911 engines are a work of art.

Once the engine is out and you can inspect the #3 sparkplug hole you are going to have to decide whether to repair the head in situ or disassemble the top end. There are pros and cons to each. Clearly it is more work and expense to disassemble even one side. On the other hand the job will be done the best possible.

How many miles on the engine? Is there any information of prior repairs, rebuilds, service, etc?

There are quite a number of Pelicans and PCA members in Boston area. What facilities do you have available? What mechanical skills and tools? This Forum can help you through almost any project.:D

Best,
Grady :)

Thanks for the advice. I had on my checklist to pull the other five plugs to check whether they are bunged up in the same way.

I have completely rebuild a VW diesel engine before, and have had the motor out of two VW Vanagons and a '66 912. So in terms of skill, I am not afraid of too much. It's just that given my work and family schedules right now I am not relishing the thought of pulling a motor and spending the next few months getting dirty every night after the kids are in bed, ordering parts, and feverishly looking at this forum every five minutes. I do have a mechanic who would possibly let me drop the motor at his place.

I think for now I will try the least invasive first step, which is to chase the threads very carefully and look for other plugs with the same problem. If I find others with the same problem, I will have to reassess. I was hoping to drain the oil on Friday night (HOT) and adjust the valves on Saturday morning. This spark plug deal takes precedence, though, not least because I can't get the oil hot with only five spark plugs...

Before you chase the thread from the outside in, there's a product that a friend used that actually cleans the thread from the inside out. From what he described, it's a collapsable tap that you insert into the bottom of the spark plug hole, then expand the tap to mate up with the good threads at the bottom. With grease previously added, you unscrew this to clean the threads out and the chips get pulled out instead of pushed in. I'll try and find the name of the product. He said he found it on the web. Specifically made for stripped sparkplug threads. Cost was about $60. Saved him a whole lot of hassle that you describe.....

Spiff, thanks for the idea. I looked around online and found it. It's called the KD Tools Back-Tap. I was actually envisioning this sort of thing as I was thinking about the problem, and it turns out someone has already made it. It is a split tap that goes down into the spark plug hole, expands, and then is turned and backed out. It starts on the good inner threads and straightens the bad ones on the outside as it goes. Also, less possibility for dropping crud in the combustion chamber because it is backing out. I want one ASAP, so I had a local parts place order one for delivery tomorrow afternoon for a premium--the tool costs around $65 this way as opposed to $45 from Amazon (delivery not included). I am excited to try it...

A post-operative report on your re-threading project with this tool would be appreciated.....I may be facing the same prospect myself.

Very good info here guys...Thank you!

Grady as usual, is spot on and concise with his procedural problem solving techniques and direction.

Spiff, thanks for the info on the Back-Tap. I did not realize such a tool existed, but always dreamed of something of this nature.

My car runs perfect and I'm on this forum every day and night.

I just ordered one from my local auto parts place. They
said they will match the $45 Amazon price, no shipping,
3.8% sales tax, should have it next week.

“KD Back-Tap 14mm Internal Spark Plug Rethread Tool.
Patented design allows for chasing spark plug threads
from inside the cylinder head and working outward.
This tool uses the better threads at the base of the
spark plug port and greatly reduces the chance of debris
falling into the cylinder. Works on 14mm spark plug
threads. Length of 6" allows for deep well plugs. Tool
can be used with 1/2" socket or wrench. Availability: In
stock Usually ships within 24 hours. Ships via UPS.”

http://forums.pelicanparts.com/uploa...1158192816.jpg

They say it is 6" long.

Next week I’ll take some pictures using it on a disassembled
head and try it on a complete engine. One issue with a 911
is that it make the turn and fit down the sparkplug opening
and clear the chassis without removing the intake valve cover.

This looks like a tool that belongs in everyone’s tool box. I’m
going to see if it is suitable for regular sparkplug thread
maintenance. This has always been a difficult issue.
Perhaps it is somewhat solved.

Best,
Grady

I am worried about clearance (getting the tool in) as well as being able to manipulate it once I am in there. I am pulling off the valve cover, and I think this will help on both counts.

I think Grady can best answer this but I thought I'd bring up the option.

If you have a hard time using the tool you might think to do a partial shift of the motor. I'm suggesting you support the motor, disconnecting the axles, one transmission mount bolt and both rear motor mount bolts, then coax the motor to the side to increase the amount of clearance between the motor and the chassis. Might be better to remove the rear mount crossbar bolts rather than the mount bolts

Shouldn't take all that much time and can be done safely by yourself.

VICTORY!!
 

Well, I hope I am not speaking too soon, but the new tool worked. I decided to remove the valve cover, and it gave me enough clearance. I will post a series of photos with commentary. The first photo is of the scene of the crime. I have removed heater hoses, spark plug wires, and valve cover. It's a little dirty down in there.http://forums.pelicanparts.com/uploa...1158281670.jpg

Continued: Here is the tool. I coated it with my favorite bearing grease to catch chips. This is bicycle bearing grease from Phil Woods.http://forums.pelicanparts.com/uploa...1158281775.jpg

Here I have inserted the tool in the spark plug hole. It took a bit of searching. The piston, by happy coincidence, was nowhere near the top of the cylinder. The tool was able to sink in all the way until its shoulder seated. Here I am using my wife's makeup mirror, and you can see the tool sticking out. This will have to go in the next message (I accidentally posted a photo of my 2-year-old and have to delete it).

Here it ishttp://forums.pelicanparts.com/uploa...1158282142.jpg

It was quite difficult to get the tool engaged--you have to twist the knob while holding the tool still. Doing this in the close confines of the engine compartment is not easy. I was juuuust able to do it one-handed, but then figured out how to get two arms in there. I then gingerly tried to back the tap into the threads. It wasn't easy--I kept referring to the mirror to make sure the tool was centered in the spark plug well. Finally, I got about a full turn on it but then ran into too much resistance to continue by hand. I stewed about the decision of whether to put the ratchet on it and apply more force. Finally after checking and rechecking that it was centered, I did it. It resisted a bit, but not so much as to cause me to worry. I backed it out slowly and---voila. The tap was covered with aluminum chips:http://forums.pelicanparts.com/uploa...1158282554.jpg

I started the spark plug back in the hole without any further cleaning or messing around. I figured that I could cause more trouble than I would fix by blowing compressed air, etc. I put a very light amount of torque on the plug, wimping out when I got it just barely tight. Probably like 5 or 10 ft-lbs. I bolted everything all back together, checked the other plugs, put the hoses back on, and....

And then I got in and turned it. It fired right up, no loud noises. It sounded awesome, nice and raspy just like it should. Even though it was stone cold, I revved it quickly, blipping it a few times to 5000 RPM or so. My thought was that this would help blow crud out of the cylinder if I had inadvertently dropped stuff in there. I am really happy about this. I will be adjusting valves this weekend or next weekend, so I will have another chance to check torque on the plug.

I want to thank all of you for your suggestions, help, and moral support. I will keep the list apprised of further developments.

thanks for the post, that is a great tool to have.

Better to take your time at inserting the spark plugs correctly then to go out and buy a tools to fix up your FUCHS UP.

Nice writeup. Posts like these make this board invaluable.
Good luck -

Wow! I feel honoured to have made a suggestion that Grady is going to use and that helped. I forgot about the clearance issue as my friend that used this tool did it on an old cavalier with lots of straight above clearance. He has no mechanical ability, but was able to make it work so I thought it wouldn't be too bad.

Great write up with photos and I'm glad there was enough clearance for you to use it. Hope I never have to do this, but I also hope others will take advantage. Maybe this is a product that Wayne should stock? Again no affiliation, but seems to save a ton of work....

I agree this would be a great part to stock. Is there a way to suggest this to Wayne?

I don't think I mentioned, in defense of the Fuchs Up comment, that this was done by the previous owner. I think this was a shade-tree mechanic tune up deal, and he cross-threaded the plug. A good pre-purchase inspection with compression test would have caught it (shame on me for not having one done). Given the ominous possibilities, a potential buyer might be able to chisel a few thousand out of the sales price because of something like this...

Yes, this is a good story and a responsible approach. Of course, if the plug was really being stubborn, I would also have been afraid to force it. Probably.

But I will say these things: I have, twice now, had my engine spit the #6 plug out. At least one other Pelicanhead has reported the same thing. Perhaps it just hadn't gotten torqued properly. I strongly suggest you monitor that plug.

My #6 plug threads in the head are fine. I rebuilt my engine recently, and have a clear recollection of inspecting those threads. So, you do not need buggered-up threads to spit a spark plug out. don't assume that.

And finally, while I, obviously, accept the premise that the spark plug threads on a 911 engine can be cross-threaded, I have to believe it is not easy to do. The recess is long and narrow. So much so that it is difficult enough to get the plug and tool down in there. The additional challenge of getting it way down in there, and then get it cocked sideways enough to cross-thread.......can't be easy.

I think the problem is that they go in at a slight angle, not straight perpendicular to engine. I find I have to refer to a spare head I have to remind myself of the angle to aim for.

I always seem to have a problem with plug #1, not #6. anyone else have problems getting the plug installed correctly on plug #1?

Art,

I'm not sure if this answers your question.

Always start the plug by hand. You can use a length of thick (and relatively stiff) fuel line. I would recommend maybe 8" to 10" hose that snugly fits around the outboard end of the spark plug. That makes a nice flexible extension, which allows you to turn by hand, but also ensures you don't cross thread the head.

Along the same lines, I use a nice snapon spark plug socket that has a built in universal joint and an extension with a locking mechanism (so when I pull the tool out the socket doesn't separate from the extension).

I never use a ratchet to start until I've spun the plug in the threads as far as it can go by hand - which is usually 6 to 8 threads deep. Torque gingerly with a torque wrench to factory specs. Tightening too tight can ruin threads, too.

I alway use anti-size compound applied just on the threads of the spark plug, too. Not liberally, just lightly.

Doug

Doug,
Thanks for the useful tips!!