The outlet on the exhaust manifold goes into a 90 degree elbow. The inlet of the the elbow is round, about 2-3/4" diameter, and the outlet is teardrop shaped. The wide end of the teardrop is toward the front of the engine, like a streamlined strut. The construction is the same as the shells for the manifolds.
The form block was made by the same method as the new block for making the manifold shells. The center layer is particle board and the outer layers are made from blocks of ash glued together and to the particle board core.
To figure out the shape of the blank I assumed all the shaping would be done by letting the steel slide in the blocks rather than by stretching the steel. Strips of card stock were used to simulate the pulled metal and the flattened to mark the flat pattern,
With the flat pattern drawn on the block, the bolt hole locations were worked out by using the washers to see where pressure would be applied.
I sheered the blanks and then decided to try cutting them to shape with band saw. It worked great, much faster than snips or the nibbler.
I leave a gap at the ends. If the blank moves a little it won't hang up on the block.
As with the manifold, I used the scraps from the center layer and the plywood clamp block to make a tapered block to limit the depth the metal forms into the block.
The parts come out very well with about 20 minutes work.
One thing I had to work at was, to remember to stay on the leading edge side of the outlet to have the teardrop going the correct direction. I marked the block but found that drawing an arrow along the forward side worked as a better reminder.
I also made a large block from the ash to form the inlet end. It has the shape like the block I made for the outlet of the manifold. I also put a radius on one edge so it wouldn't crease the steel. The other edge is square to fit snug at the ends.
I start with the big block and get the shape started with the bolts tight. Then I loosen bolts as needed to get the steel to slide evenly from the edges.
The steel pivots on the inside flange as you work it. You pound the inlet down and the outlet moves up. To stop that I use a block of wood and a large c-clamp to hold the one end while working the other end.
With a couple of other wooden tools the shape is quickly formed.
Once the shape is correct the outside edge is lightly shaped to form a tight radius .
Occasionally wrinkles form in the inside corner which can not be easily worked out. The trailer hitch ball works well to shrink them back.
The blunt tool with the rivet gun works well to smooth the blend from the radius to the teardrop.
With the shaping done you can see the teardrop.
The inlet end will get rounded better when the 2 halves are welded.
As with the manifolds, any slight bumps are corrected with the planishing hammer and the trailer hitch ball.
The opposite half is made in the other side of the block.
They come out very well.
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