tap a spark plug hole

[brock]

just did my 3k maint on my '79xs1100 special. which included changing my plugs. bad news.

i guess the dude before wanted it tight, cause it was a bitch to get out, and when i tried to put the new one in, it seems like the threads on the engine are stripped, cause it gets stuck about 1/4th of the way in. i can get it down pretty far, but i really have to crank down on it to get the plug in.

i got it in pretty much in (stupid, i shouldnt have forced it, i know), but now that side is smoking really bad, it running really rough, and stalling. its gotta go.

so has anyone every tapped out the threads in a spark plug hole? I'm mostly afraid of getting little bits of metal shavings in the cylinder. also i've never done this kind of work. so any thoughts or advice i'd certainly appreciate.

[Geezer]

You can try a sparkplug thread chaser and it may solve the problem but to do it right you'll have to pull the head.

I've met a lot of people that claim you can rethread a sparkplug hole with the head in place and I've also done a lot of teardowns to repair the mess from those that tried it.

Geezer

[brock]

i dont know. you're not really cutting the metal out. just reshaping it, right?

[ratbyk]

Tapping Plug Hole

If you have or can borrow the proper tap go for it. Smear lots of grease into the flutes on the tap to capture most of the cuttings. Blow out the cylinder as best you can and then ignore the rest. It will be very small and few aluminum shavings. Very unlikely to do any damage as they will likely be blown out as soon as you start the engine. A couple of aluminum shavings are not going to do any damage to cast iron rings or a steel cylinder. If any get trapped/caught between the valve face and seat they won' last long before being pounded thin enough to simply disappear.
Ken/Sooke

[rover]

Remove airbox, and timing cover, remove spark plug. Duct tape a compressed air source to the carb where you want to tap the hole. Turn on air SLOWLY. Turn engine CLOCKWISE until you hear and feel air coming from spark plug hole. Turn the threadchaser/tap into the hole. Give one turn and back out 1/2 turn, repeat until chaser/tap is all the way through. Remove air and tape, re-install the airbox and timing cover. Put anti-sieze compound on sparkplug and re-install. Every time you remove and re-install sparkplugs in an aluminum head, use anti-sieze SPARINGLY. And be sure to use adequate eye protection while cleaning the threads.

[prometheus578]

I stripped a plug at work. Luckily, the MAC tool guy was due that day. OK... here's the deal. Most plug tappers work from the outside downward. What you're doing is starting a tap in messed up threads. MAC has a tap that's collapsable. Spread some grease on it, and shove it all the way into the hole. Tighten the nut on the end, and the tap expands to fit the hole/threads. This is INSIDE the cylinder where the threads are still good and not stripped. What you do then is back the tap out(it started easily on the good threads) and it straightens out the bad ones as it comes out, bringing the aluminum chips with it in the grease. Works like a dream. Rather pricey, though, as MAC tends to be. I bought the three tap set. Think it was about $75. Bring him the old plug so he can see the size. New tap should be about $25. A lot less hassle than pulling the head.

[rover]

These plugs are 12 mm by (I think) 125 threads per inch. A regular 12 mm spark plug thread chaser works very nicely with the air. It doesn't take a large volume of air to keep the chips out. Maybe it could even be done using the nipples on the manifold boots, but I have not tried this.

[skids]

I did this with a helicoil tap and it worked out quite well. From the description, it seems like the threads are OK except for the deep ones. If that is the case, the chaser threading tool might work out. Do not put so much gease on it that it prevents the cuttings from flying out of the slots.

Originally posted by rover
Remove airbox, and timing cover, remove spark plug. Duct tape a compressed air source to the carb where you want to tap the hole. Turn on air SLOWLY. (snip)

[brock]

OK. cool. thanks everybody. I think I'll give some of your advice a try this weekend.

Sounds like the threads are messed up toward the bottom, and not the top, since i can get a couple of good screws on it before it starts getting tight.

Should i buy a tap set, or is there something special i should be using. I'm not sure about the one that does it backward, since it sounds like my threads are messed up further down than at the top.

so just a regular old tap? and how do i know what size? i guess i'll take a plug with me to size it.

thanks again. I'll let you know.

[John]

Most tool vendors, and good parts stores sell a thread chaser that has two different threads on it. One on each end. Not expensive. I have one from NAPA.

[ratbyk]

Stripped Plug Hole

You want a 14 mm X 125 tap. Don't take my word for it though. Go to a good auto parts store that also sells tools. Take spark plug with you. Tell the parts guy (Bob) that you want a tap to repair a plug hole this size.
Before using, put a smear of grease in/on the flutes. Start it by hand so that you get it started straight. You want it going straight right from the start. If you try and start it with a socket/wrench you will have it run down a turn or two before discovering that you are not straight.
Once started and turned in about 2/3 turns it will likely start to get harder to turn. Now add the wrench. Go one complete turn then back out a 1/2 turn. Now another complete turn and back out a 1/2. As you back out you are clearing any cuttings out of the threads and back into the fluted reliefs. Keep going until you are well in. Best to look inside before starting to make sure that the piston is down the bore a bit to ensure that you don't run the tap down into the crown of the piston.
Have fun and remember to always start spark plugs by hand before adding a wrench. If you start cross thread by hand you will not be able to run the plug in far enough to cause damage.
Ken/Sooke

[Ken Talbot]

The tap you need will be specific to the brand of insert you use. A few years back I installed a Helicoil brand insert on an XJ650 that SWMBO used to ride. Recently, I used a KD brand insert and it uses a different tap. Here's a shot of the KD insert and its tap:

[IMG]/forum/images/tips/20040926-dscf0669.jpg[/IMG]

And here's a shot of the Helicoil tap (top) and the KD tap (bottom):

[IMG]/forum/images/tips/20040926-dscf0671.jpg[/IMG]

Note that both the diameter and the thread pitch on these two taps are quite different.

FWIW - the Helicoil unit is easier to use. The bit of smaller thread on the bottom does a much better job of keeping the tap aligned as you cut the new threads, and it has a much larger sidecut for clearing out the chips.

BTW - I have done two of these, both without removing the head and both with no problem with chips falling into the motor. For both, I removed the carbs and duct-taped a shop vac hose to the intake manifold. With the vacuum howling away, the chips fly outwards at an amazing speed. This is certainly a 'mandatory safety goggles' activity.

[prometheus578]

Brock:
Sounds like the threads are messed up toward the bottom, and not the top, since i can get a couple of good screws on it before it starts getting tight. No... the threads are messed up at the top, not the bottom. Bottom's still good. What happened was the previous owner, when putting in sparkplugs, cross-threaded one. It started a little bit crooked, and then started cutting it's own threads as it went in. Eventually, the friction of the crossed threads makes it harder and harder to keep going, and that's the point at which people STILL keep going, doing further damage. The threads at the bottom are still good, as the plug got stopped before it could get deeper and screw them up, too. Yes, at the top seems like good threads, but they are the ones that the plug cut into the aluminum on it's way down till it got stuck.

[brock]

but when i took the old plug out, it was all the way down. in fact, the bike ran fine till i changed it. now it wont go in barely at all, and of course with it hanging 1/4th of the way out, it's running like pure crap.

i guess what you're saying is probably that if the bad plug was all the way in, then its probably damaged all the way down, including the top.

[prometheus578]

(a few moments later...) also the prob with the standard tap method is that the tap is going to bite onto the new threads at the top, the ones that are cross threaded in the first place. It will use those as a guide on it's way down, displacing the old original threads. It doesn't really "cut" new threads into the cylinder head, as it's diameter is the same a a spark plug, what it's doing, if started in crossed threads, will be pushing and relocating the old aluminum threads once it gets to the good stuff. This makes all the threads weak, but that's a minor point. Now... if you took the head off, and tapped from the inside out starting in good threads, that would straighten out the few bad ones at the top. But that's a lot of work, too.