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Old 08-11-2018, 11:32 PM   #1
rlew21
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Default procedure for drilling and tapping for 1/2" head studs

I picked up a set of IAG ARP 1/2" headstuds and am planning to drill/tap the block myself (it isn't exactly rocket science). I'll be doing it on a buddy's bridgeport and probably locking the quill and feeding the knee up while doing the drilling to keep the bit from auto-feeding into the existing hole. After that I'll be running a 1/2" x 13 plug tap through all of the holes and then finishing up with a new (nice and sharp) bottoming tap to clean up the threads and get that last bit.

I tried calling IAG to ask if the holes need to be "counter-bored" with a slightly bigger bit, and if so to what depth, but wasn't able to speak to anyone at the machine shop, only the general support staff which is in another location. Common sense leads me to believe it would be "counter bored" to leave a few threads on the head-stud not engaged in the block that way the majority of the threads are engaged and the stud can be threaded all the way down and bottom out.

If anyone who's done this could tell me that I'm doing this the right/wrong way it'd be highly appreciated!!
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Old 08-12-2018, 09:47 AM   #2
KillerBMotorsport
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I wouldn't think you'd want any stud threads not engaged. That would create a significant stress riser.

Definitely reach out to IAG, and be careful for info you get off the internet. Consider going old school and buy a book on fastener technology and design. Machinist handbook is a good primer too.
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Old 08-12-2018, 11:26 AM   #3
viper_crazy
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Why wouldn't you want all the threads engaged?
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Old 08-13-2018, 03:40 AM   #4
rlew21
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viper_crazy: When I said a few I literally mean 2 or 3 threads (at 13tpi that would be less than a quarter of an inch) to allow the head stud to tighten all the way into the block (more meat down there) as opposed to bottoming out on the shank of the stud.

KillerB: I can appreciate the comment on checking the machinery's handbook (it's a good read!) but the application here is a bit particular considering the threads are being tapped into a hole that will be slightly too large at the "top" of the block if I'm not mistaken.

Also I had called IAG again after posting and was told that since the drilling and tapping operation was done on the CNC the guys at the shop wouldn't be able to tell me if the holes are counterbored out above the threads to allow clearance for the stud shank! I spoke to a rep again and not one of the guys at the shop (apparently that number is off limits to customers). I sure hope that the guys down there running the cnc machine can see if it performs 2 or 3 operations/tool changes and know how to read a caliper depth indicator ;-)
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Old 08-13-2018, 07:51 AM   #5
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You're right on the engagement at the top, it's not ideal, but if you loot at the numbers neither is using 1/2" studs as it does not completely remove the OEM threads. Semantics as many argue that it has no effect on the integrity of the clamping force or integrity of the interface. Plus there's lots of 1/2 applications out there, and the process has been done for many years. If I HAD to use 1/2 studs I would thread form instead of a traditional tap as it produces a stronger thread. I would still think sinking the studs completely would be a better option. I'm sure someone at IAG can give you an answer, you've just not reached the right person yet.
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Old 08-13-2018, 05:46 PM   #6
fwillyj
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---here's some old notes of one Subaru where I did the 1/2-13 studs.

-I copied this info from an old file---you might figure out something from it to help you.

-I did some depth checks and machined some light witness marks on the tooling shanks to correspond with the top of the block deck.
---------------------------------------------------------
1-to tap the head bolt holes requires a 6" tap, a long .422" and a 5" long .515" reamer to do the 1/2-13 threads.

2-clamp the case to the mill.

3-install the drill chuck.


4-set rpm to 150 rpm

5-double check depths of hole to see if they correspond to witness mark rings machined onto the shanks of tooling.

6-eyeball center the .422" reamer and dry ream to bottom of hole to near the witness top ring mark machined on reamer shank.

7-install .515" reamer, dry ream to length of flutes plus about .090" to the machined witness ring.

8-remove the reamer and blow out the chips real good using a 6" long 90 degree air nozzle tip before installing the tap.

9-install the 6" long tap, lube with dark threading oil, normal plunge & prepare to stop---coast to a stop and reverse at first machined witness mark on shank.

10-charge $8 per hole.

Last edited by fwillyj; 08-13-2018 at 11:41 PM.
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Old 08-13-2018, 10:03 PM   #7
rlew21
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Thanks KillerB and fwillyj! BTW, here is a more readable version of your notes:

1 - to tap the head bolt holes requires a 6" tap, a 5" long 0.422" and 5" long 0.515" reamer to do 1/2 - 13 threads.
2 - clamp the case on to the mill.
3 - install the drill chuck
4 - set rpm at 150 rpm
5 - double check depths of hole to see if they correspond to witness marks rings machined on shanks of tooling.
6 - eyeball center .422" reamer and dry ream to bottom of hole to near the witness top ring machined on reamer shank.
7 - install .515" reamer, dry ream to length of flutes plus about .090" to the machined witness ring.
8 - remove the reamer and blow out the chips real good using 6" long 90 air nozzle before installing the tap.
9 - install 6" long tap, lube with dark threading oil, normal plunge and prepare to stop, coast to stop and reverse at first witness ring machined on shank.
10- charge $8 per hole.

I just couldn't follow it with the odd spacing and my slight OCD wanted to see it all typed with proper spacing.

Also I just placed an order with MSD (another $130 towards stuff to make fancy holes) and picked up a 0.46" reamer and 1/2 x 13tpi forming tap. The different charts online say that to get a 75% thread you want to use a 0.4608" to 0.462" hole, and this is aluminum so I figure I can get away with loading up the tool a little bit more to get a more fully formed thread.

fwillyj: from your procedure it appears that you don't counterbore the holes. I did notice that there was an incredible amount of threads on the studs compared to oem so I guess no need to drill the top and allow clearance for the shoulder of the bolt

Also, I found an interesting article while looking into the clamping loads exerted by the individual studs and the pullout strength of the different percentages of threads created by a forming tool. It addresses exactly the concern that I had when I started this thread as you can see in the diagram of the stud stopping in the "dog point" and one stopping on the "shoulder" of the stud shank. Also it's an article specific to subaru head studs! https://www.maracing.net/choosing-the-right-head-stud/
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Old 08-14-2018, 12:55 PM   #8
TurboJJ10
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Quote:
Originally Posted by rlew21 View Post
I picked up a set of IAG ARP 1/2" headstuds and am planning to drill/tap the block myself (it isn't exactly rocket science). I'll be doing it on a buddy's bridgeport and probably locking the quill and feeding the knee up while doing the drilling to keep the bit from auto-feeding into the existing hole. After that I'll be running a 1/2" x 13 plug tap through all of the holes and then finishing up with a new (nice and sharp) bottoming tap to clean up the threads and get that last bit.

I tried calling IAG to ask if the holes need to be "counter-bored" with a slightly bigger bit, and if so to what depth, but wasn't able to speak to anyone at the machine shop, only the general support staff which is in another location. Common sense leads me to believe it would be "counter bored" to leave a few threads on the head-stud not engaged in the block that way the majority of the threads are engaged and the stud can be threaded all the way down and bottom out.

If anyone who's done this could tell me that I'm doing this the right/wrong way it'd be highly appreciated!!
You don't need or want a counter bore. The drill size should be 15/32" DO NOT follow a drill/tap chart for size as that will result in too much parent material being removed and the threads not being sound. Also, I am unclear as to what the .522" reamer is for....but it should not go down to the existing threaded area. Again, that would remove way too much parent material.
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Old 08-14-2018, 07:10 PM   #9
rlew21
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Turbo: I'm going with killerB's advice and going to use a form tap which calls for a 0.46" reamer to get a bit over 75% thread form from it. This is going off of charts that gave drill sizes in decimal, not fractional and then the decimal conversion of that fraction. Fwillyj mentioned using the 0.422" reamer to pre-drill for a cutting tap and a 0.515" reamer to allow clearance for the head-stud shank so it can seat all the way. Why wouldn't you want to remove all of the old threaded section and have it threaded to the full depth of the normal hole?
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