Need advice with drilling aluminum for tapping and heli-coil

**RknRusty** Mon Jan 02, 2012 8:41 pm

I posted this at RCG in the Builders Workshop, but I figured since we have so much skill around here too, I'd cross post it. So here goes...

I only get one chance to get this right.
The stripped bolt hole is on my son's Pontiac V6 aluminum head, one of the lower intake manifold bolts. It's 8x1.25x35. I have a heli-coil and the right sized TiN drill bit. My drill is a 3/8" electric.

My main concern of course is getting the bit in straight without ruining the cylinder head. It's on a slanted rear facing surface, so when I line up, I'll be leaning over the driver side fender with my back to the hood, aiming down and toward the front. Kind of an awkward position to be in while lining up. Here's a picture:

Need advice with drilling aluminum for tapping and heli-coil SAM_1229

Need advice with drilling aluminum for tapping and heli-coil SAM_1229

That's looking down on the engine over the fender.

Will the existing hole help me stay aligned?
Should I drill in short bursts? I'll be using cutting oil.
How fast should the drill be turning?

Thanks for any help,
Rusty

**Cribbs74** Mon Jan 02, 2012 9:26 pm

Rusty,

If you have a small level attach it to the drill, that way you can unure it is straight and level to start the initial hole.

Use a small bit first and drill a pilot hole dead center of the broken bolt. A pin punch will help the bit stay on center. Low speed on the drill if it is a two speed drill.

you can keep going up in bit size until you have 90% of the material removed, then you can pry out the remaining metal with a small cold chisel or screwdriver, that way you can keep the existing threads.

Or you can drill it all out using the same method and use the helicoil, use red threadlocker if it is a hot spot if not blue should be fine. Helicoil in aluminum or steel is the same just be sure the helicoil you use is the correct length of threads.

Hope that helps. Good luck

**RknRusty** Mon Jan 02, 2012 9:37 pm

cribbs74 wrote:Rusty,

If you have a small level attach it to the drill, that way you can unure it is straight and level to start the initial hole.

Use a small bit first and drill a pilot hole dead center of the broken bolt. A pin punch will help the bit stay on center. Low speed on the drill if it is a two speed drill.

you can keep going up in bit size until you have 90% of the material removed, then you can pry out the remaining metal with a small cold chisel or screwdriver, that way you can keep the existing threads.

Or you can drill it all out using the same method and use the helicoil, use red threadlocker if it is a hot spot if not blue should be fine. Helicoil in aluminum or steel is the same just be sure the helicoil you use is the correct length of threads.

Hope that helps. Good luck

Hi Cribbs. The trouble with a level is this facing is at a 60 degree angle plus or minus how the car is resting. There is no broken bolt in it, just galled threads. What I need to do is drill out the old threads so I can tap it for the heli-coil. I was hoping the existing hole would guide my bit if I'm careful and slow drilling. The best idea I have so far is to get a good square 2x4 block and drill a guide hole in it with my drill press. Then lay it against the head and let it guide me. I'm going to search and see if there is such thing as a drill guide attachment. If there is I bet it won't fit my drill though.

Mudhen Mon Jan 02, 2012 10:05 pm

PV Pilot Mon Jan 02, 2012 10:10 pm

You don't need the board Rusty, just try to keep your angle correct as you drill out.

Make sure the bits are sharp and pretty even on the flutes,,or it will hook and grab, making a oval in the alum.

You can move up each drill bit size up to the one you need for the helicoil tap, this will help in getting it on the same plane.

Use your electric drill on slow speed, no air drill. A little dab of tapping fluid but don't over do it, so you can see what you are doing.

Check the depth of that hole and mark it on your drill bits for a stopping point.

I have done this a bunch if times, you have a easy one there. take your time and move with good judgement.

It looks like you are way outside of the water jacket,,so you should be good, even if you have to step up a size.

Mark Boesen Mon Jan 02, 2012 10:17 pm

gotta ask a dumb question...it looks like the bolt is busted off, not stripped?

**RknRusty** Mon Jan 02, 2012 10:22 pm

Thanks guys. Just to be clear, that hole is not plugged with anything, just the old stripped threads. But I like the idea of nibbling off the sides with a smaller bit and working up to the full sized 21/64th bit that the tap fits. I watched a guy on youtube do it and he did it so casually he could have been smoking a cigarette and eating a snickers bar while he held the drill. Of course he had a straight-on angle of attack, and he was a pro, but I think it will turn out to be easier than I'm thinking.

PV Pilot Mon Jan 02, 2012 10:23 pm

RknRusty wrote:Thanks guys. Just to be clear, that hole is not plugged with anything, just the old stripped threads. But I like the idea of nibbling off the sides with a smaller bit and working up to the full sized 21/64th bit that the tap fits. I watched a guy on youtube do it and he did it so casually he could have been smoking a cigarette and eating a snickers bar while he held the drill. Of course he had a straight-on angle of attack, and he was a pro, but I think it will turn out to be easier than I'm thinking.

Yep, your overthinking it, you will be fine.

**RknRusty** Mon Jan 02, 2012 10:36 pm

PV, what are you calling flutes? Is that the lead edge of the bit? I'll get it spinning when I'm lightly on the hole and then start boring in.
My bits are new Titanium Nitride bits, gold colored. I've had them for 10 years and this is the first time I've used them.
I know I'm over thinking it. Y'all know me, I do that sometimes.

PV Pilot Mon Jan 02, 2012 10:50 pm

Sorry, I meant the leading cutting edge, or also known as the margin. The flute is the downward swirl ground into the bit. My apology's.

The Tie nitrides bits work just fine.

Yep, rest the bit on the hole with medium pressure then get it spinning then apply a bit more pressure, use good judgement, your on soft aluminum. Try not to let it dance or wobble around obviously, tough to do, all you can do is try to minimize that..

John Goddard Tue Jan 03, 2012 4:07 pm

Now I've read thread properly Doh

Jaspur_x Tue Jan 03, 2012 4:21 pm

Mark Boesen wrote:gotta ask a dumb question...it looks like the bolt is busted off, not stripped?

It loks like that to me, in which case the drilling may be tougher to do;steel bolts are tough to drill through.

Good luck, these techniques listed are how I do it on a regular basis.

**RknRusty** Tue Jan 03, 2012 5:41 pm

Got it.
It turned out to be pretty quick and simple. I used three bits, the last being the 21/64" for the heli-coil. The tap made some beautiful threads. As we prepped to install the manifold we found there were two more bolt holes with damaged threads so I whipped out the trusty drill and bored away. The only trouble is one of the holes was at such an awkward angle I tapped it slightly crooked. The threads are deeper on one side and shallow on the other. But I think it'll hold a heli-coil plenty tight enough. It threaded in fine. I'll add some maximum threadlocker to that hole and bevel an aluminum washer to make the bolt head sit flush on the manifold. Hopefully we'll get this thing running in another day or two.

Thanks for the help, you guys are great.
Rusty

PV Pilot Tue Jan 03, 2012 7:24 pm

Glad it worked out for you, pretty easy task, MOST of the time. Now you are a professional heli-coil'er'r Very Happy

**RknRusty** Tue Jan 03, 2012 8:21 pm

I don't get my diploma until it has 10k miles on it. The final exam was that third hole and I tapped it crooked. My son is worried about it, but I think if we slather it with max threadlocker and the coil holds the torque, we'll have a good seal. And I plan to bush the bolt head with a tapered spacer to distribute the load in the right direction. It'll work.
I appreciate y'all's help.

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