Anir

I need some advice from someone who has tapped threads before.

After having a mechanic incorrectly attempt to retap a stripped 7/16" hole, I now have a hole that is too large and will not securely hold a 7/16" bolt.

Therefore, I am planning to enlarge the current hole to securely accept a 1/2" Grade 8 bolt. I have the following potential Snap-On tools:

(1) A 1/2-20NF hex rethreader tap (specifically recommended only for retapping the same size thread)

(2) A 1/2-20NF threading bit stamped "Use 29/64 drill". It has a square end, and is designed for making a new hole (i.e. stronger, I believe, than a rethreading tap).

My question: What is a 29/64 drill? I understand that a rethreading tap is turned with a ratchet, but how do you properly use a threading bit? Can I put it in a regular drill that can run at slow speed, and simply slowly advance it into the current 7/16" hole.

I don't want to screw this up, because it will get difficult to work with if the 1/2" hole proves too large. Thanks for the help.

ZCAT3

Anir - I believe they are suggesting you drill the hole out first with a 29/64th drill bit and then use the tap bit. Given that 7/16ths is 28/64ths you probably can just go ahead and thread the tap without doing any drilling. I had a similar problem in my car with one of the bolt holes for the rear seat back. The threads got stripped from removing and installing a DAS bar. It is really hard to get the correct angle on those bolts once the threads have been tweak even just a bit.

JimBob Jumpback

I dun thoughted dat dose taps was fer shoes.

ed devinney

You *might* be able to tap without opening up w/a 29/64 drill. However, depending on how much metal the tap is pushing against this might just get you a snapped-off tap in the hole. Taps are very hard but not tough. They're also a royal PITA to get out once they break.

Since this is a 7/16th bolt I'll presume it's for a seatbelt? If so, next question is do you have the same profile bolt as the original, with the unthreaded shank, etc. It may make a difference...

Do you have enough material there to helicoil? I'm not sure if that's a good solution for a seat belt bolt, but can't see why not.

Anir

Bill and Ed (and JB),

Thanks for the replies. Yes, this is for a seatbelt, so I need to do it correctly.

The good thing is that both the NF 7/16" and NF 1/2" have 20 threads per inch. So, maybe, the tap will go in OK without drilling first. However, maybe pre-drilling lightly with a 29/64 bit would be a more intelligent approach.

Unfortunately, this is the rear seat floorwell outboard hole (originally for the 3-point seatbelt floor mount and now for the outboard lap belt), and there is no way to access its backside from under the car, even with the rocker panel off. So, no way to put a big washer on the back of the bolt.

Ed, what is "helicoil"? A metal version of teflon thread tape or something?

Thanks!!

BTW, I received this helpful e-mail from a board member, too.

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"You should not use the re-threading tap for a different size. Use the standard tap. I would suggest you use the drill size they spec, otherwise you are going to have a much harder time taping the threads.

Where ever you buy the tap, should have the tap handle to drive it. You MUST do this slowly. A drill is too fast and may lack the torque to do it as well. Get some WD-40 and keep shooting it into the hole as you run the tap through it. Tap in until it feels tight then back out a bit. Keep this back and forth (adding WD-40) and you will get a clean set of threads.

It is pretty easy to do, though you might want to practice once by drilling the correct hole size in a scrap piece of metal.

As an alternative, if it is important to try to keep the original thread size, you can get a thread repair kit. It is a special tap and then an insert. Again, a good hardware store will have these."

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ed devinney

Hi Anir -

There are a number of thread repair kits as mentioned above - Helicoil and Timesert are two. *Kind* of like teflon tape, but these really repair. Timeserts are often used on 911 engine cases when the studs go - <a href="http://www.timesert.com" target="_blank">www.timesert.com</a> .

Given that it's a seat belt bolt on a car as desireable as a turbo, you might consider having a machine shop repair back to 7/16ths - mismatched bolts seem so declasse ;-)

Good luck!

PS - if you decide to tap, strongly consider drilling first. Taps are really hard, and if they break you can't drill them out. Ask me how I know... Practice is another good suggestion. Again, ask me how I know...

Anir

Ed,

You sound like you've learned about this the hard way. <img border="0" title="" alt="[Wink]" src="wink.gif" /> Thanks for trying to spare me the same.

The timesert looks pretty strong, but I'd be happy to pay a machine shop to do it correctly. How would they repair it back to 7/16ths? Weld material into the hole and then re-tap it?

ed devinney

The timesert is basically a threaded sleeve, threaded inside and out. They open the hole up to a larger size, and thread for the outside of the timesert. The timesert takes up the space between the larger new hole and the desired original.

Helicoils are similar, but use what looks like a small slinky instead of a solid sleeve. I've heard timeserts are stronger but am no expert in this area.

mojorizing

Hi Anir,
If this regards a p-car, shouldn't this bolt be metric?

Anyway, a Helicoil insert looks like a spring that replaces the stripped threads. You'll have to drill out the hole to the recommended size, then tape out with the proper Helicoil tap, then screw in the insert. You'll have a threaded hole with the original thread size.

If you do go to 1/2"NF, use drill to 29/64". Taps also come in three different variations: taper,plug,and bottom. The difference between them is the amount of taper that the tap has to cut the threads, start with the taper tap, then plug,then bottom out the threads using bottom tap if it is a blind hole(as opposed to an open hole). Using cutting oil, turn 1/4 turn then back off to break the metal chip from the cutter on the tap.

Good luck!
Kevin

Anir

Ed,

I agree that the Timesert looks like a solid design.

Kevin,

Thanks for the help. Interestingly, the 993 does use 7/16th, not metric, for the seatbelt holes. I was surprised, as well. Maybe, it's some sort of DOT thing, or an international standard??

I have a Grade 8, 1/2" bolt, so I think tapping out to a bigger bolt is a valid approach. However, I'd love to use the factory 7/16" hardware, to keep things stock.

I'll do some further investigation into the Timesert and Helicoil. Thanks for steering me in the right direction.

Robert Henriksen

The holes *are* 7/16-20, rather than metric; I had the same problem, with exactly the same hole, as Anir . Fortunately, I was able to clean up the existing threads with a 7/16-20 tap (well, actually I chickened out & got Mike C to help me) and get the original bolt back in.

Good luck!

Anir

Ed,

Your Timesert suggestion worked beautifully!

I ordered the Timesert kit #0762 (7/16"-20) from the company itself for $61. The kit came with five inserts, and all the necessary tools except the tap handles:

- A drill bit for removing the stripped threads
- A bit for countersinking the hole
- A tap bit for making new threads
- An insert driver

<img src="http://boards.rennlist.com/upload/image002.gif" alt=" - " />

The first two drill bits require a larger tap handle, which would not fit in the available space. Therefore, I jerry-rigged a ratchet to slowly turn the drill bits. Worked well.

A typical T-shaped tap handle (below) worked great with the smaller tap bit and insert driver. I used one of the shorter Timesert inserts. According to the directions, you want to use a Timesert that is no longer than the hole depth.

<img src="http://boards.rennlist.com/upload/00952557000-dv.jpg" alt=" - " />

The repair took all of 10 minutes, and the resulting hole feels as strong as stock, and holds an eyebolt very securely. And best of all, the hole is still the factory 7/16" size. Thank you for steering me in the right direction. <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

BTW, I purchased a Helicoil repair kit, but returned it after examining the Timesert - a much better design than a simple slinky coil.

Robert Henriksen

Anir, thanks for the follow-up. Great to hear it was so straightforward!

(Oh, and you DID buy that from a domestic dealer, right? <img border="0" title="" alt="[Wink]" src="wink.gif" /> )

Anir

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Robert Henriksen:
<strong>Oh, and you DID buy that from a domestic dealer, right? <img border="0" title="" alt="[Wink]" src="wink.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica"><img border="0" alt="[hiha]" title="" src="graemlins/roflmao.gif" /> <img border="0" alt="[hiha]" title="" src="graemlins/roflmao.gif" />

Actually, I bought it from my favorite Iraqi auto parts distributor. <img border="0" title="" alt="[Wink]" src="wink.gif" />

E. J. - 993 Alumni

Glad you got it fixed Anir. I hate to think that that 993 TT would be driving around with mismatched bolts. <img border="0" title="" alt="[Wink]" src="wink.gif" />

As far as the sizes go, every car imported and sold into the us has the same size seat belt bolts. We can thank the DOT for that.

E. J.