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Next problem: stripped spark plug hole + complication

 
Old 09-30-2017, 06:36 AM
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Johnny G Pipe
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Default Next problem: stripped spark plug hole + complication

OK, this is the neglected French '79 again. A big hitch has appeared.

Time to change the plugs and ignition leads. The very first plug (front right, cyl 5 I think) - immediately on unwinding the plug wrench I get that unmistakeable sensation of deeply unhappy bolt threads destroying themselves.

So, the threads are stripped

That's OK, I guess I could put in an insert, but on removal of the shrapnel, I see that there is what looks like a helicoil in there (see pic) - I am guessing a previous repair which has not worked.

So if I try to ream this socket and put in an insert I will end up with steel helicoil fragments in the head, which is not going to be great.

So, the head is coming off, right? Am I safe to screw the plug back in best I can, and nurse the car 60 miles to my mechanic? Or is there another way out of this?
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Old 09-30-2017, 07:59 AM
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Hard to tell on mobile but the helicoil looks okay. I would check for debris. Vacuum it out and replace the plug. The plug looks to have suffered the worse of the damage
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Old 09-30-2017, 12:01 PM
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Not sure if this will help but I used this in my Land Cruiser and it's pretty stout. They claim that factory used this in repairing production engine.

Amazon Amazon
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Old 09-30-2017, 12:20 PM
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From the pictures, it looks like the inner end of a Heli-Coil came out with the plug through the outer end, or there is no Heli-Coil at all and that's just head metal. To get to your mechanic, the suggestion to vacuum out the debris from the cylinder is a good one if driving it is your only option. There's always a flatbed transporter or trailer. Rotate the crankshaft until that cylinder is coming up towards the top of the exhaust stroke, maybe 30║ before top dead center on that cylinder but one crank rotation from ignition TDC. Vacuum first, hoping you get most or all the debris out. Then compressed air through the plug hole to try and push any remaining debris out that open exhaust valve. Inspect using a small inspection camera. There are some interesting small inspection cameras for cell phones for under US$30. Make sure the camera head is small enough to fit through the hole. Then fit a new plug in there carefully in the remaining threads, with a little anti-seize on the threads.

A Heli-Coil is a wire insert used to repair a damaged hole. The repair process begins by cleaning, then drilling and re-tapping the threads in the hole with a spacial tap. It has the same pitch (threads per inch or mm) as the original but a larger diameter. Then a spiral-wound wire (heli-coil) insert goes in to take of the difference in diameters. The insertion process includes a tool that grabs a tang in the bottom section, so that the twisting as you thread the insert into place causes the insert to get slightly smaller as it is pulled into the threads in the head. Once in place, the tang is broken off and withdrawn back through the hole. Depending on the hole, it's sometimes appropriate to use a little thread locker on the outside of the heli-coil insert to help it stay in place. Usually, a spark plug hole gets no thread locker though.

There are other better threaded inserts available these days. Time-sert is one of the favorites in my garage. There's a bit of a challenge fitting a thin-walled solid insert where a Heli-Coil was previously fitted, as some of the supporting metal has already been removed during the prep for the Heli-Coil insert. Your mechanic and maybe his machine shop will tell you what will work best in your situation.

FWIW, there were particular problems with the threads on certain American brands of spark plugs in the earlier days of Porsche in the U.S.. Seems like the plug manufacturers figured out how to make the pitch right, but the angle and therefore the depth of the threads wasn't the same as the way the heads were threaded. Fitting a Champion plug an an aluminum 356 or 911 head just once would damage the threads enough that either that plug would seize and pull aluminum out with it, or the metal in the head would be weakened so that a subsequent plug, even if it had the correct thread, would still pull metal with it on removal. Adding Heli-Coils or other threaded inserts was a very regular repair. The local Porsche dealer where I grew up would regularly install Champion plugs as their preferred brand. They also favored a particular Pennzoil that had very high paraffin content. Between those two, there were always engines sitting on a rack waiting for varying degrees of overhaul. Bottom line for this is to stick with the Bosch or Beru plugs that were originally fitted. NGK would be the next choice, but I wouldn't stray any farther than that from the truth. I've "repaired" way way too many air-cooled heads due to casual ignorance by owners and "mechanics".
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Old 09-30-2017, 12:43 PM
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Thanks guys - it is hard to photograph but there is definitely a clearly visible coil of wire beyond the base of the hole, I guess it is indeed a helicoil (which maybe has become unseated - it wouldn't usually protrude that far under the hole.)

Thanks for the tip about the mini-boroscope. They look really useful and cheap..who knew!

https://hobbyking.com/en_us/2m-mini-...___store=en_us

After I clean it up and thread in a new plug, I guess the worst that can happen is that it blows the plug back out

I guess head removal without dropping the engine is a lot of hours...oh well.
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Old 09-30-2017, 02:52 PM
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Actually it is tempting to grab the helicoil wire I can see with some pliers and see if I can pull it all out. I wonder what the chances of that working would be..

If I got it out there is a better chance then of an insert working (+adhesive that some folks are using on youtube for the inserts).

Have ordered a mini camera!
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Old 09-30-2017, 09:51 PM
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Dr Bob has it almost right.

Use your shop vac, and put the hose on the outlet, so you are BLOWING high volume air out the hose. Connect it to your exhaust pipe with some duct tape, block off the other exhaust pipe if there is one, so all the air is blowing into the exhaust.

Turn the engine over by hand until you get a high volume of air from that cyl plug hole. This will have the exhaust valve fully open. The piston will be mid-stroke on that cyl.

Now, in this condition you can repair the thread with a drill, and an insert, and all the chips will be blown OUT of the cyl and will not fall inside.

Once you get the insert of your choice installed, it's best to use a compressed air line, with a long slender 3/16"(5-6mm) copper blower nozzle on the end. Stick the copper nozzle in the cylinder, and blow around the edges of the cylinder to dislodge any debris that was in there. You will bend the copper nozzle a bit to go around the circumference a few times.

I've done this more than 10 times, and so far all are working perfectly, no failures. When you cut the threads for the insert, it's best to back the tool off several times, and clean the debris off the tool.
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Old 10-01-2017, 01:15 AM
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Doc Mirror has it almost right...

I've never had good luck getting air through the exhaust to blow junk out of the chambers. Maybe it's my technique or I can't get enough mass back through the exhaust. Regardless, do it the way that gets the stuff out. Ok to do it two ways if the first doesn't work for you.

On using the tap to prepare the hole for another insert: Grease the tap with Vaseline. Turn the tap clockwise a quarter turn cutting, then back it out 3/4 of a turn to clear the chips. Then back, plus another 1/4 turn cutting. A few of those, then carefully withdraw the tap. Clear the grease and chips, remembering that the Vaseline comes off with detergent and water. Then more Vaseline and go at it again. The Vaseline will help keep chips from falling into the chamber, but if a greasy one does go in it won't come out easily with just air.
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Old 10-01-2017, 01:49 AM
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Originally Posted by dr bob View Post
Doc Mirror has it almost right...

I've never had good luck getting air through the exhaust to blow junk out of the chambers. Maybe it's my technique or I can't get enough mass back through the exhaust. Regardless, do it the way that gets the stuff out. Ok to do it two ways if the first doesn't work for you.

On using the tap to prepare the hole for another insert: Grease the tap with Vaseline. Turn the tap clockwise a quarter turn cutting, then back it out 3/4 of a turn to clear the chips. Then back, plus another 1/4 turn cutting. A few of those, then carefully withdraw the tap. Clear the grease and chips, remembering that the Vaseline comes off with detergent and water. Then more Vaseline and go at it again. The Vaseline will help keep chips from falling into the chamber, but if a greasy one does go in it won't come out easily with just air.
With a good shop vac, and the other exhaust port blocked off or plugged, I can get gale force wind going up out of the plug hole. Rock the crank so the exhaust valve is mid-stroke, and you'll get gobs of air coming up.
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Old 10-01-2017, 04:48 AM
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So maybe this isn't a disaster. I love places like Rennlist, when a bunch of guys gather round and talk through the problem.

The big BUT here is the helicoil in situ. which as I said at the start will mess up the tapping process. But with a bit more research, it seems that these things ARE removeable. There are a few techniques online, and tools like this one:

http://www.engineering-supplies.com/...SABEgKvfPD_BwE

So, I have a big job interview tomorrow, but after that I'll feed back how I get on. Cheers!
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Old 10-01-2017, 09:14 AM
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Also - Doc mirror, which kit have you used?

Over here I have a choice of timesert kits or similar looking 'solid insert' kits, much cheaper..

Also do you use adhesive on your inserts?
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Old 10-01-2017, 01:54 PM
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I would very seriously recommend having the car moved to a shop, not driving it. If you don't already have AAA extended road service (something I consider a must as a long term owner of vintage sports cars) you should upgrade now and, if necessary, wait for the coverage to begin.

I wouldn't risk trying to move the car under its own power. But if you do, and you destroy the block, let me know. I have a spare in excellent condition I could sell you at a reasonable price...

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Old 10-01-2017, 02:04 PM
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DocMirror writes: "if a greasy one does go in it won't come out easily with just air. "

And therein lies the problem; any residual oils or even water in the combustion chamber will make vacuuming out detritus unreliable. If you search these pages for accounts of what happened to people who used media blasters to clean up intake manifolds on these engines, but weren't surgically precise getting the residual media out of the intakes before use, I think you'll have a better understanding of the risks.

These are meant to be sealed systems with micron dimension filters between them and the outside world. They need to be very, very clean and it just doesn't take much to really mess them up.

This has been a Civil Defense Engine Paranoia Alert Test Message. Had this been a real emergency, your engine would have seized and your pistons would have welded themselves to the cylinder walls.
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Old 10-01-2017, 03:27 PM
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Well, just - eff it. I tried to help. I've been doing this for years, but whatev. Take the head off, take it to a shop and be done with it.

http://www.blownoutsparkplug.com/
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Old 10-01-2017, 03:35 PM
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Originally Posted by docmirror View Post
Well, just - eff it. I tried to help. I've been doing this for years, but whatev. Take the head off, take it to a shop and be done with it.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What he said.
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