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Old 09-14-2002, 10:17 AM   #1
Fast68Chevy
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what size drill bit and tap for 3/8 heli-coil

anyone know what size drill bit and thread tap are required when installing 3/8 x 16 tps heli-coil ?

and if anyone has either laying around that they would sell me,that would be cool
thanks for anything
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Old 09-14-2002, 10:50 AM   #2
71GMC_3/4T
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Randy,
The Helicoil tap is special. The drill size is listed on the tap, usually. You should be able to pick up a helicoil set for 3/8-16 from a local hardware or autoparts store. Try to get 1 1/2 D helicoils. They length is sized by diameters, hence the D.
Make sure you run the tap in straight and don't wobble. I assume you're doing a tranny tailshaft? I found it easiest to use the electric drill to run the tap in and back out slowly. Use WD-40 for cutting fluid in the aluminum.
If you can't find a tap, let me know. I have one laying around somewhere in my toolbox, with helicoils I could loan you.
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Old 09-14-2002, 12:36 PM   #3
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I should be a salesman for this company. This is a product that is simply a threaded sleeve with an epoxy coating that bonds once tightened and is stronger and easier to install than helicoils and can be installed with standards taps and no winding tools.

heres a link to the one you need

http://www.mcmaster.com/param/asp/ps...10835;161=1448
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Old 09-14-2002, 12:41 PM   #4
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i would be interested in renting tap from you, if possible, and i need to know what drill bit i need, type of bit and diameter of bit, and im doing this in a cast iron head for exhaust manifold mounting holes, the bolts are snapped off in head, two of them.

not working on or planning on working on a tailshaft, no

tell me more about this threaded sleeve dealy, what is needed to prepare for and install one ?

thanks guys
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Old 09-14-2002, 01:02 PM   #5
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might I suggest try useing a left hand drill bit first to try backing it self out.Try about 3/4 of the thread size in case u have to redrill. You are going to have to drill any way, No guts, No glory.
Just try to drill straight no matter which way u choose.
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Old 09-14-2002, 03:00 PM   #6
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attempting ot back them out will do no goo,d they are frozen permanently to the head, thats why the snapped in the first place.

thanks

Last edited by Fast68Chevy; 09-14-2002 at 03:04 PM.
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Old 09-14-2002, 04:44 PM   #7
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Fast. many times when a bolt is torqued it stretches and then seizes into the hole. Try drilling 2/3 to 3/4 the bolt size all the way through. This sometimes releases the stress. Heat a little around the bolt and back out.
I just did all 6 on my truck and tapped them to original 3/8 x 16 no problem.
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Old 09-14-2002, 07:05 PM   #8
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well that may work on the oen thats to broken off flush, but the other one is busted off inside head,

i dunno how i can possily and accurately manage to drill out center of a steel bolt like that, gonna have to find a really cool drill bit ?

thanks@!
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Old 09-14-2002, 09:49 PM   #9
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All you do with the tap sleeve is drill abd tap the old bolt hole out to whatever the OD thread of the sleave is . For a 3/8 you'd go to 1/2-13 or 7/16-16 depending on what sleeve you had. The you screw it in the newly tapped hole. Thats it . Make sure you accurately center punch the middle of the bolt before you drill it out. Not rocket science here. A little bilt one way or the other wont really affect something like an exhaust bolt. I assume its still in the truck.
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Old 09-15-2002, 12:03 AM   #10
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Well fast,
Your post is somewhat confusing.

I think what your asking is for a 3/8 helicoil tap? Which the answer would be a letter "X" drill (.397 diameter). And a 3/6-16 helicoil tap is the tap you would use. Those Items are usually sold together in one package (along with the actual helicoils) for consumer use.

Try your local hardware/automotive store. If they don't have them try a industrial supply store.

I sure would try a few different ideas first.

Is it sticking out any? If it is, grind two flats on it and try to get a wrench on it.

Heat it up real good and put some wax on it. As it cools the wax will seep into the threads.

Anyway, what I'm saying is. Helicoils are for after you screwed up the hole. Not before.

Good luck.............
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Old 09-15-2002, 12:06 AM   #11
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dang it,
forgot to mention there's always the good ole' "easy outs".

Just don't bugger up the hole before you've tried everything.
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Old 09-15-2002, 12:15 AM   #12
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helicoil kits are available at every parts house I've ever been in, they will include the drill, tap, helicoils and installer, but I would agree with Mikep, those little threaded sleeves from McMaster-Carr are far superior to helicoil, that's all we use at the plant, helicoils aren't even available in the parts crib anymore.
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Old 09-15-2002, 09:17 AM   #13
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I apoligize for the suggestion. I thought you might be able to get to it or could accurately center punch it and drill it.
I have really cool drill bits and I know what I am doing and I take my time and do it correctly.
You can do whatever it takes. Machine shop?
Does this sound snotty ? Yes. Just trying to help you out here and can't read your mind! If it was here I would do it for you in 5 minutes. Wanna ship it?
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Old 09-15-2002, 04:22 PM   #14
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ok things are getting twisted around here,


these are hole sin head that are on engine that i dont need to tear apart just to fix two holes in,
the engine is fine otherwise.

the bolts are not gonna come out any other way, i will have to dril them out and tap and re thread them

all i simply need to know is what size and type of bit to use and what size tap for 3/8" coil or also for the 3/8" sleeves


yes, NAPA has a 3/8 coil kit but they want a small fortune for it and i coud probably get a sleeve kit for less than a coil kit and end up with lots better repair, i plan on keeping this engine, so.,,,,,

so for a 3/8 sleeve I'd go with a 1/2-13 or 7/16-16 bit and tap ??
thats not very definite... how can i find out for sure what i need ?? all i need is simple/accurate answer/info,,

im still at square one ? help ?


thanks.

Last edited by Fast68Chevy; 09-15-2002 at 04:31 PM.
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Old 09-16-2002, 12:17 AM   #15
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The sleeves are whatever size you buy. They come in thinwall and thickwall sizes. Is that definite enough?
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Old 09-16-2002, 01:31 AM   #16
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Geeze, Fast, 14 posts, and no-one offered an answer! Me neither! Good Luck!, and call a machine shop., bet they have the answer right in front of them.
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Old 09-16-2002, 03:36 PM   #17
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One more try. The drill bit is a plain ol run of the mill 25/64" (.3906) high speed drill bit. The kind everyone has in their set. The tap is a special one use only tap called a HeliCoil tap. As the coil itself is 3/8" x 16 on the inside, it is special on the outside and the tap is NOT a regular tap size. HeliCoil is a brand name and the kit you need is a # 5521-6. This contains the tap, coils and the insertion tool. For extra depth you may insert more than one coil.
Does this get it covered yet ?
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Old 09-16-2002, 04:02 PM   #18
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Mikep gave you the best answer in the 3rd post. You must not have followed the link. The inserts from McMaster-Carr cost 1.26 a piece and you need a 1/2" 13 tap to install them.
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Old 09-16-2002, 04:47 PM   #19
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If you have room, drill and tap to 7/16-14
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