Well im looking into getting some headers for my truck but i would like to know how the flanges are measured so that i can try bolting it right up to the pipes already there. Online i see flanges that are 2.5" and flanges that are 3" but when i measured inside to inside of my current pipe it is 2.25" and outside to outside is more than 2.5" but then i measured the distance from the middle of one bolt on the flange to the other and it came out to be 3" exactly, would that be what they are actually measuring?
My truck is a 1993 GMC sierra with a 5.7L engine.
Please all i am asking about is how the flanges are measured on cars, not what will fit on my truck. So i would just like to know if the flanges on the header collecters are measured from bolt to bolt or some other way (they have 3 bolts total)
OK, here's the deal. The standard 5.7/350 header flange is 3". All "Collector's" are like reducers to connect the small exhaust pipe to the large header flange. The collector is very short in length but since 3" is the standard header flange, most all bolt up to the header flange with the 3 bolts and gasket. (Use the aluminum gasket at Autozone)
What your missing/asking is simpler than it seems so bear with me.
First of all, when you install headers, you cut off the original exhaust pipe flanges like 24" down since the header is much longer, then the exhaust pipe get's welded to the collector which bolts to the header with 3 bolts.
On the outlet side of the short collector that's now bolted to the header is where different sizes now come into play.
The collector's are like reducers that go from 3" to your current exhaust pipe size. So to get your right size, you need to determine the current size of your exhaust pipe that will be welded to the collector, and simply buy that size collector. You always want the collector to slip inside the pipe. So if your exhaust pipe is 2.25" inside diameter then you buy collectors that are 3" x 2.25" outside diameter.
Most exhaust shops will weld the exhaust to the collecter for like $40 bucks so just figure that into the cost and is the normal and final procedure once the headers are in. I know I've had many welded up at Car-X or wherever.
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