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Stripped!

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Greg Riutzel

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Jul 9, 2020
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Lebanon, Oregon
I have a K341 with a stripped spark plug hole. I've used various thread repairs over the years, but I'm leery of them for spark plugs considering they're a routine in 'n out maintenance item.

Heads for these monsters are hard to find, at least checking used and new parts supply places. I'm gravitating towards Time-serts and they advertise the process can be done in place or in the vehicle.

My question is are spark plug inserts okay for the long haul or would it be better to keep looking for another head?
 

kharvey

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Kendal Harvey
Put the insert in to keep it running while you look for a good used head.
 

mgonitzke

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Matt Gonitzke
Properly done thread repairs should be stronger than the original threads.

Regardless of which method you use, I'd pull the head. No sense risking getting metal debris in the cylinder, and the head probably needs to be sanded flat anyway.
 

Greg Riutzel

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Put the insert in to keep it running while you look for a good used head.

Yep, felt pushed to do that today with all the work needing done between the rains. I'm glad you mentioned it.

I used Heli Coil's Sav-A-Thread on the engine. Shaving about .030" or so off the tap end gave just enough to do the head thickness before contacting the valve under it. I also used Rapid Tap's Heavy Duty which is very thick and trapped the chips. I pulled the tool for cleaning every half turn of new thread and with a shop vac and homemade "tool" the engine stayed clean. The insert went in without a hitch.

I'll see how it goes; 6 months, 6 years whatever buys the time until another head.
 
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tkhoffman

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Tony Hoffman
There are different styles of helicoils. The one I used on a snowblower was robust. The auto store let me borrow his tool that semi threaded down the mutilated threads and then cut new threads for the slightly larger helicoil. Then used same tool to thread in helicoil and Swage(flare) it over on the inside of head. Yes, the head had to come off to do this. I pull the plug once a year and I must have done the work 12 years ago.
 

Greg Riutzel

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That reads similar to the one I used. The tool has a regular tap on the end followed by beveled cutting flutes and then the larger hole tap after that; all on a 3/8" square drive shank. The insert threads in the new hole leaving an ever so small ridge on top that swages flat with a swaging tool. The threads are sealed with a hi-temp silicone.

The threads were sketchy from the beginning when I first got the tractor. I would use cleaning chasers, never seize, oil, but it was always difficult to remove a plug whether cold or warm, (never hot). after a month or several days of work. When it did go, it left a smeared black hole, no sign of stripped metal; mostly black gritty stuff (carbon?) with some dark looking metal stuck in the spark plug's threads.
 

Todd Utherland

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Phoenix, Az.
Couple of ways to go in my experience.

Machine to smooth - weld up - machine down to a start base dimension - thread to plug.

or

Cut a superior plug portion from an existing head - remove the entirety of your threads to a matching dimension - weld in the threaded piece from the scrap head. Trim & machine.

Either one should be about $225 or less from a good shop. Less still if you attend to more of it yourself/

Todd.
 

Todd Utherland

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Great idea Todd, considering they're unobtainum new. A bone pile of them would be resource.
Watch though that some shops will want to pre-cut for matching tapers with the slightly flawed logic of a greater mating surface area. I've found the two sharp edges in this technique to be problematic. I prefer a straight cylindrical cut.
 

dfrisk

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Dennis Frisk
I did a very complete and expensive rebuild on a 12 HP Kohler, when the shop finally got done with their machine work I went and picked it up, they set it on the counter haphazardly, and it fell over, remember, a 12 hp has an oil sump on the pan. I didn't notice it when I checked the engine, but the riveted tab on the governor rod was hit hard enough the tab broke off. They refused to replace it, they forgot the fallen engine, after all it was 5 days later. Then the spark plug brought the last of the threads out of the head with it, I went to see if that shop had a Helicoil kit for 14 mm threads. I bought a D-10 plug, 18 mm threads, ordered a set of spark plug taps from McMaster-Carr, blew compressed air thru the carb and into the combustion chamber, and all the filings from the tap blew out into the muffler. Installed the slightly modified D-10 plug and started the Cubbie and mowed the lawn. I ran that 129, 12 hp Kohler 10-12 years. I did get a replacement used head at a plowday from Travis. It's still in the shop if my old 10 hp or 14 hp needs it.
I've heard Time-Serts are WAY better than Heli-coils. I know my success rate with Heli-coils is right around 50%, I forget what it was, not even sure which car or truck it was for, was aluminum, had 8-10 small tapped holes and 4 of the Heilicoils spun out when I installed it. Might have been something SON & I installed in a car stereo system.
I use anti-sieze on ALL spark plug threads, even cast iron. Yes, it's sometimes messy while you assemble something but just think how much nicer it will come apart when you remove it.
 

Greg Riutzel

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Lebanon, Oregon
Heli-coils have a place, I've had luck with them in iron, steel, brass; but aluminum or magnesium is too soft in my book. Heli-coil's Save-A-Thread is a Time-Sert knock-off, with a small difference in how they're secured. I have been burned with spark plug inserts 40 years ago, so time will tell and I still will look for another head as back up.
 

Dana Vandehey

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Mar 30, 2022
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Location
Rainier, Oregon USA
Properly installed Heli-coils hold as well as the others as long as you do not intend to remove the fastener. If you remove the bolt you have a 70/30 chance that the Heli-coil will come out with it. The best and simplest I have found is the Keen-sert.
All the years I worked on old Harleys the most common failure was exhaust pipe stud holes
in the aluminum heads.
There is only one stud to hold the pipe to the head and they constantly strip out.
Any time I head the heads off I would install Keen-serts in both heads even
if the heads were brand new. I've installed them with the head on the bike,
access to the hole is just a bit more challenging.
Drill and tap to next size, clean the new threads with electrical contact cleaner
or brakeklean or something that leave no residue. Put red Loctite on the insert
and screw it in.
They make a driver to drive the lock pins in, but you can drive them in one at a time
with a hammer and punch but you have to do it carefully as the pins are hardened steel
and if you hit them sideways you could snap them off.
Tap. Tap, Tap. No 4 pound hammer and a round house swing, we ain't killin snakes here.
Once installed correctly they are there for the duration, they will not come out unless you rip the threads out of the head or drill them out.

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