cross threaded spark plug on '80 SC

[snowtire]

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.

[Oh Haha]

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?!

[Porsche_monkey]

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

[Zef]

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

[snowtire]

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.

[Porsche_monkey]

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.

[Grady Clay]

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

Sorry that your first post has to be one in distress.
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.

Best,
Grady

[snowtire]

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...

[spiff]

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.....

[snowtire]

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...

[curiousone940]

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

[Nine9six]

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.

[ruf-porsche]

Originally posted by snowtire
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.

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

[Grady Clay]

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.”



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

[snowtire]

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.

[CliffBrown]

Originally posted by snowtire
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.

[snowtire]

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.

[snowtire]

Continued: Here is the tool. I coated it with my favorite bearing grease to catch chips. This is bicycle bearing grease from Phil Woods.

[snowtire]

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).

[snowtire]

Here it is