67/427/400 Factory Side Pipe Heat Riser Question

[Thomas S.]

When I replaced the heat riser valve in my 67 BB (The weight had shearded off), the through-hole measured just under 2" (1.92). It was replaced with a correct NOS (3887034), but I never measured the through-hole diameter.

Hopefully this is not a dumb question, but is the flange, casting and bolt pattern of the 2" and 2-1/2" heat riser the same with the only difference being the through-hole and flapper?

[Joe L.]

When I replaced the heat riser valve in my 67 BB (The weight had shearded off), the through-hole measured just under 2" (1.92). It was replaced with a correct NOS (3887034), but I never measured the through-hole diameter.

Hopefully this is not a dumb question, but is the flange, casting and bolt pattern of the 2" and 2-1/2" heat riser the same with the only difference being the through-hole and flapper?

Thomas-----


No, the flange, casting, and bolt pattern of the 2" and 2-1/2" are not the same.

[Thomas S.]

Thanks for the reply. That's what I thought. The only other question is why is the thru-hole in the heat riser flange only 2" in diameter instead of 2-1/2"?

[Mark E.]

Making sense why GM does something can be exasperating. But I believe the diameter of your sidepipe mufflers is less than 2" (1 7/8" ?). So the heat riser isn’t the only restriction.

[Thomas S.]

That’s what I figured. A 2-1/2 “ exhaust system heat riser has a 2” thru-hole.