Preferred way to punch a hole in an exhaust pipe

[AllenV]

I want to weld an oxygen sensor bung onto an early 70s Mercedes exhaust. The pipe is 2" diameter and maybe 14g wall steel with a nice patina of rust.

I have the whole exhaust out of the car in one piece with decent access to the surgery point. It looks like getting clamped onto my drill press or mill is a not at all practical.

Here's the question. How to drill a 3/4 inch hole in the 2-inch pipe using a hand power drill? The drill is 1/2" with the two handles.
1) a step drill
2) a hole saw
3) an annular cutter, which I do have in the right size
4) drill a smaller hole and expand that with a die grinder
5) something else??

Accuracy of the hole diameter is not so important.

I am worried that cutting through the curved surface will lead to all sorts of grabbing and twisting of the drill. So I am seeking advice in advance.

A fifth option would be to cart the thing down to a muffler shop and have them do it.

 

[Ulma Doctor]

A step drill would be my first choice, hole saw would be my second choice.

 

[MikeInOr]

A single flute step drill does an excellent job and usually never catches while drilling.

Are you converting the car to EFI?

 

[GrifterGuru]

I would use a step drill. They cut better, step up the size in increments and give you greater control than using a hole saw with a twist drill pilot.

Use a self-acting centre-punch to mark the hole centre, then use the Uni-bit (step drill) to drill it out to size.

You will also find the resultant hole much cleaner than if you used a hole saw, so you will have minimal clean up of the hole to do, which can be done with a Noga-style debur tool.

One word of caution, wear a mask so you do not breathe in the carbon from inside the exhaust as you are drilling through it.

 

[matthewsx]

Yep, step drill. It’ll be done before you know it.

John

 

[AllenV]

Okay, step drill seems the winner. I have a two-flute drill but will see if a single flute is nearby.

Are you converting the car to EFI?

Not converting to EFI. The car is D-Jetronic , so it actually is EFI already but thoroughly analog. I am putting in the O2 sensor to monitor and adjust mixture as much as D-Jet will permit.

 

[Flyinfool]

Depends on your step drill. Some bits have a good distance between steps and some the steps are very close together. The ones with the steps close together do not do as good of a job on tubing. The outer edges of the hole might end up one step smaller.

An annular cutter has few teeth and is most likely to grab.

A hole saw works well because the teeth are fairly fine. I like to predrill the 1/4" hole and then have a solid rod for the pilot in the saw.

 

[markba633csi]

Plasma cutter. Ok I had to be the smarty pants. Never mind.
Analog EFI should be fun to tinker with- does it use a flap for the air flow meter?

 

[woodchucker]

Okay, step drill seems the winner. I have a two-flute drill but will see if a single flute is nearby.

not necessary, 2 will work. I have both, they both work equally well.

 

[AllenV]

Plasma cutter. Ok I had to be the smarty pants. Never mind.
Analog EFI should be fun to tinker with- does it use a flap for the air flow meter?

No flap. D-Jetronic is the original speed-density electronic fuel injection. No air flow meter at all. Injector pulse length is determined by manifold vacuum and rpm, mainly. A few other inputs fine tune the operation. It’s impressive that it works using late 60s tech.

My little project is to get a data logging laptop hooked up to record air-fuel ratio, vacuum, and rpm out on the road. Record the state of health. The car is 52 years old.