Hi there guys. I have a question. Whats the diameter of the exhaust ports on a 6 bolt head? I would like to fabricate an exhaust header and would like to know the diameter for the exhaust runners. How big of a diameter to go.
thanks.

1.5" runners is the dsm standard, 1.25 is optional thru some companies which will help spool in trade for a little top end. the port shape is oval and I believe most manifolds have a small transition from the ports to runner.

Thanks for responding. Yeah. I noticed that there is a transition of shape from the exhaust flange to the runner pipe. It goes from oval, then it tapers down and transitions into the round shape. I don't like this small taper transition much. I want to keep the same diameter from flange to runner. I was going to have a shop fabricate me a custom design or I will just wait till I'm done with welding school so that I could do it myself.
This is actually for a non turbo application. The only exhaust header that is available for a non turbo is a header that goes 4 to 2 to 1 and its rather restrictive. So I would love to have a 4 to 1 header with equal diameter runners to the exhaust ports. Or at least just a 4 to 1 header.

do some more reading for the exhaust header. your getting into exh tuning to the extreme, and if youre building your own header then you want to look at the difference in how the two designs will make peak power and peak torque.

depending on application you may be better off with a 4-2-1 if its a daily driver or auto-x car.

just a suggestion. also, you can buy 4g63 exhaust flanges from vendors. i believe stm tuning and jnz tuning may carry flanges. might make your venture easier.

good luck.

Thanks. I have been reading quite a bit about exhaust and all. But anyway, I just wanted to know the diameter of the exhuast ports since they are oval instead of round.