So elementary I'm afraid to ask

I am almost embarrassed to ask this (ok, I AM embarrassed)

I am replacing my side pipes on my '66 s/b and it seems the existing pipes are 2 1/2" which is not stock ID, (should be 2") or at least this is what appears to be the case. This won't be the first time I'm finding more inaccuracies about the car than I was told by the PO.

In shopping for replacement pipes I found these listings for side exhaust amongst the various suppliers:
1965 and 1966 s/b: 2" ID
1965 only in s/b: 2 1/2"
1965 -1967 b/b: 2 1/2"

With that said, I got to the 2 1/2" ID when I measured the ID of the heat riser gasket. I imagine this is a no-brainer way of determining an accurate ID of the entire exhaust system. Am I correct? Thanks.

Well, maybe, but not always. On my '69 BB the original exhaust ID is 2", but the exhaust manifold is 2 1/2". The stock exhaust pipe that connects to the exhaust manifold (and on the passenger side, the heat riser), starts out as 2 1/2" but then narrows down to 2" before it gets to the pass through holes in the frame. In '68 the BBs were 2 1/2" all the way back to the mufflers. I never liked the narrowing from 2 1/2' to 2" on the '69, so when I replaced my exhaust along time ago, I just bought the system for a '68 BB.

My point is that sometimes things are different on these cars and not always for obvious reasons. If you are trying to keep the car as close to original as possible, I would recommend buying a judging manual for you car through NCRS. I am not suggesting that you become an NCRS nut, because there are tons of those people out there, just that you can learn a lot about what your car is supposed to look like through this manual and then you can make an educated decision on how you want to maintain your car. Here is a link to there web site. http://www.ncrs.org/

I hope this helps!

Rich

^^ Thanks.

I was a member of NCRS, went to one of their judging shows, got 3rd Flight, and will never do that again. Good organization and all that but their thing is not my thing.

On to the side pipes issue ...

I heard rattling at idle so I dropped the side pipe and when I shook it I heard something inside shifting back and forth. I couldn't get to it no matter what I tried and it's probably better I couldn't. In my searching around I found out that if whatever was inside the pipe ended up lodging itself sideways and restricted exhaust flow too much, my little 327 would be in deep $hit. So I decided to replace the side pipes and in making sure of what I needed to order, I noticed that 2" is stock for my '66 but everything I was measuring was 2 1/2" so I was left with how the heck do I determine with certainty the "existing" ID of what I have regardless if it is "correct". I don't want to replace the exhaust manifolds. I only need a way of knowing for sure what their ID is. I assumed that the ID of the heat riser gasket would measure the same as the pipe itself. I tried to measure at the flange end but we know that the flare makes that less than an optimum location to measure.

I will probably drag the side pipe to a muffler shop tomorrow and have them tell me what I have. I feel like a putz for not knowing for sure, but I would rather do something and make sure before I order the parts than to have to ship them back with my tail between my legs.

I appreciate your input.

Nomatter which size you go with keep it the same for the whole system. Remember the smallest size determines the volume of the exhaust. Lets say you have a 2 1/2 inch manifold going into a 2 inch exhaust then all the system can flow is what 2 inch's volume is. Even if you then go to 2 1/2 Inch pipe further back into the mufflers the flow is still only what the 2" allows. The only reason I can think of for GM to have mixed size's as stated by another poster is to change the noise harmonics to get rid of a resonance without using a resonator.