slant six exhaust manifolds
This bit of info is going to help.
Many people take their manifolds off and just put them back on or replace them with new ones with no headaches. After I tried it and checked everything first I don’t know how most people get away with it. I have to assume most fit better than the new manifold -or the manifold that was replaced before it - but my intake was off also - original from the factory. The first exhaust replacement (that broke and I replaced) was Chrysler - not an aftermarket and was way off.
It is best to make a cardboard template of your head to use to make sure your ports line up. (especially if you have the hotbox milled) I used thin cardboard from a shirt/gift box. Make a center hole to fit over the center stud, end holes for the last two outboard exhaust studs and cut-outs for all other studs. Place it on the head and use a ball-peen hammer around each port to cut it out of the template. Then place it on the manifolds and you might be surprised and your manifolds fits perfect - you might be surprised that it’s nothing like the head.
The center hole on the intake which lines up everything may not be in the correct place. I’ve tried two different manifolds and neither was right. Your intake manifold ports should line up exact or a little smaller than the head ports. Your exhaust manifold ports should be a lot larger than the head ports - mostly toward the center of the head. As you figured out, the manifold has to expand.
It can expand about 0.170 inches at 1100 degrees. If all is perfect they expand equally from the center in two directions - so half that for each side. Point is - if they fit perfect cold, they will be covering the head port a little or a lot when hot. The intake ports don’t expand.
I figured .200 inch expansion to be safe -.100 each side / 12 = .0084 per inch. The port closest to center won’t move much. Each port from there out moves a little more. The last (#1 ) ports move the entire .100”. Measure how many inches each port is from center then cut that much out of the inboard side of your template. I added a little more to each one just to make sure they never block the head port.
It will be very hard to find someone to mill the hotbox mounting surface in my experience. 0.003 off at the hotbox moves your ports out of whack about 1/32 inch. Depending on which side of the square they cut wrong it can be up and down or side to side (exhaust port is 1/32 higher than intake port on one end and lower by the same amount on other end.) Make sure you check this or make the machine shop do it.
If all else fails it’s pretty easy to file .003 off of one side to make them line up. If it’s off more than that you may have to return to the machine shop. Hopefully your surfaces are smooth but that isn’t a guarantee they are right either.
You can (And I think most do) just “port” or grind out any material that is in the way to fit your template but sometimes there isn’t enough metal to waste. You will most likely have to port them some. Do this with the hotbox gasket in place...
I just did mine and it made a world of difference but they were pretty “fritzed” to start with. I have an un-synchronized first gear. I’d have to come to a complete stop just to get it in gear when sometimes I just wanted to make a “rolling stop”. NOW, my truck will take off from a complete stop in second gear with no problem when it wouldn’t before. It made a huge difference to make sure the ports are lined up correctly. It takes a little time, a good exacto knife and a good ruler or calipers that measure in .001”s but it’s worth the extra work in my opinion.
Replacing the studs is a good idea. Also get brass nuts so that it doesn’t rust together next time and coat everything with high temp Anti-Sieze compound.
Then hope there never is a next time.
Explaining this to most mechanics would probably be a waste of time. They'd think you were a nut and I'm sure many here think I am - they might be right.
Explaining it to a machinist would be easy. S/he may already know.