Exhaust Manifold Flanges - Drilling Holes

To cutout the center opening I decided to drill the 4 corners with a 3/16" drill.  To locate these holes I made a small fixture for center punching all the holes.  It's made from 3/8" bar stock 2" wide.  It has 2 3/16" bolts on the edge to locate it along the edge of the 1-1/2" bar stock.  By turning the bolt heads slightly you can align it very precisely with the bar stock.  I only center punch 2 of the corner holes and the flip the bar stock over and mark the other 2 holes.  More on that later.

The mounting holes are oblong to fit on 5/16" studs.  One hole in the fixture is centered on the outer end of this hole.  The other is just touching the edge of the inside of the hole.  The idea is to drill the 2 holes in the flange with a 3/16" drill, then drill the outer one with a 5/16" drill.  To finish the hole I plan to use a 5/16" rat tail file and just file down to the bottom of the inside hole.
The inside hole also serves as a pivot for belt sanding the radius and flats on each end of the flange.  This shape should simplify making the ends and give them a little more steel to stiffen them when bolted to the cylinder.

The first step is to clamp the fixture to the strip of 3/16" x 1-1/2" steel.

Using a 3/16" transfer punch the centers of the 6 holes are marked on the steel.

The end of the flange is scribed on the steel.  Before I clamped the fixture on I used a blue felt marker to highlight this area.  It's easier to see the scribe line when cutting the ends.

I don't want to wear out my transfer punch so I only make a light make on the steel and then hit the marks with a center punch after removing the fixture.  I've gotten in the habit of using the center punch after marking holes with a prick punch.  The center punch has a shape the same angle as the drill so the drill centers in it better.

I used the transfer punch to locate the fixture, in one of the hole punches, to scribe the opposite end for cutting.

With the first part ready to drill the fixture is positioned to mark the next piece.  The flanges are 3-1/2" long and I'm using a 48" piece of steel so I spaced them every 3-5/8" from each end.  You get 13 pieces from the strip of steel.

With the parts all marked the next step is to drill the 3/16" holes for the mounting holes.  I like drilling thin stock like this with a center drill.  It's to short to flex, so you can control the location better, and it drills it's own pilot hole.

To keep the piece flat on the table I de-burr each hole after drilling.

I need to locate the pivot hole over a 3/16" bolt so after drilling the holes I opened them up slightly with a #11 drill.  The hole fits snugly over AN-3 bolts.

Having the holes enlarged to #11 also allows the transfer punch to to be used to locate the fixture to mark the other 2 corner holes for the inlet opening.  I've got one punch in a hole and am using another to mark the holes.

Because the corner holes are drilled at an 18 degree angle the center mark was located farther from the edge to allow for the angle.

I made a wooden fixture to hold the steel while drilling at the 18 degree angle.  At first I only made it wide enough to hold a part.  It was easier to hold a strip with a few parts on it so I widened the area with the angle on it and drilled a clearance hole for drill chips to fall through.
I still had trouble with chips sticking to the wood.  It was however a lot easier to make this from wood than steel.

To locate it on the drill press I positioned a piece of steel and the used the center punch mark to locate it and the fixture.  Then I held the drill down snug while I tightened the clamps holding the fixture.

Then it's just a matter of drill a hole, clean up the chips, de-burr the hole, and drill the next hole.

With all the holes drilled I cut off the parts with the band saw.  I have an idea for how to do this with the torch but I need to do some cutting of the inlet holes before I build a fixture for cutting the ends.

I used my block of plywood with a bolt sticking up as a fixture for cleaning up the ends and getting a nice radius.

A light touch to the belt in the opposite direction and you have a nice finished end ready for a quick de-burring with a mill file.

The next step will be to drill the 5/16" hole at the ends.










 The next step was to increase the size of the bolt holes.  I made a fixture to hold the flange while drilling.  I've grown tired of parts whirling around the drill bit.
To locate the fixture I put the 3/16" bit in the drill and lowered it to the outer hole, then clamped the fixture to the table.

 Change the bit to 5/16", clamp the part and re-drill the hole.

 I then lowered the table and re-positioned the fixture. so I could drill the hole with a 21/64" drill.

 To reduce the filing needed to finish shaping the hole  I then did the same process for the inboard hole.

The difference is I want the edge of the 5/16" drill to line up with the edge of the 3/16" hole.

I started by using the 3/16" bit to locate the hole like with the outboard hole.

With the bit in the hole I clamped a guide fence along the back of the fixture.  The pocket in the fixture is reversed to hold the flange from moving away from the drill.  You really need the short center bit to do this.  A long bit bends too much.  It might even break.

With the fence holding the fixture from moving fore and aft I measured the position of the fixture along the fence. I added 1/16" to this measurement and locked the vernier as a gauge to position the fixture 1/16" to the left so the 5/16" bit would now drill in the correct location.

Slide the flange to the left in the fixture, clamp it and drill.

After cutting out the inlet hole I'll finish this oval in the vise with a 5/16" rat tail file.