Hi guys, I've been searching like a madman, and Im still confused. Everywhere I look says the 4-2-1 USDM ITR header is a 2.0" collector. I have a 4-2-1 I picked up last night and having used a tape measure for many year, I can safely say that the collector is 2.5" OD. It appears to be 2.3" ID. Whats going on? Its a stock US header, whats up? Help please!!! Thanks, Ryan

 

usdm type r - 2.25"
jdm type r - 2.5"
gsr - 2"

I believe thats right. I had a gsr manifold then switched to usdm type r and it was bigger (pretty sure).

 

hmmm, I'd almost beleive 2.25 ID. I think thats pretty close. If it bolted up to you GSR, than it'll fit my LS as well. Strange. I see so many posts saying that its 2.00" and that it botls right up. I guess well see.

 

Ive never heard of the ITR header ID being 2.0 inches. I believe its 2.25 so your 2.3 with your tape measure sounds correct. JDM is 2.5. Jasma is 3.5.

 

Its about 2.3" ID and 2.5" OD. I'm guessing the collector is measured by the ID not the OD, thats what threw me off. Its a double walled pipe, so its probably okay. I'm just hoping this heavy pig bolts right in place with the stock cat (I know, I know)

 

yeah man. who measures OD when you check for the size???

 

LOL, it was in a moment of panick! i think it threw me off too, because EVERYwhere I looked it seemed like the USDM ITR collector was 2". This made no sense to me, but it seemed consistent. Thanks for the help.

 

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LMAO!!!

 

96-97 JDM ITR's had a 2.5 in(maybe 2.3, but it's not 2.0) collector but are the heavy 4-2-1 cast manifold. CTR manifolds are the exact same thing. JDM 4-1 has 2.5in.

 

Yeah it is 2.3" ID. I found out the hard way it DOES NOT bolt in place, I should have known, the flanges are a totally different size, as well as the bolt pattern. Oh well, off the the muffler shop I go. Honda-tech seems to be gernerally great for correct info, I've found so many other sources though that are out to lunch. There are a lot of guys saying the ITR/GSR/LS exhaust are completely interchangeable. The whole system or axlebacks are, but nothing else.

Jasma is 3.5"? LOL, that'd be a sewer pipe.

 

well all i know is stock type R one is 2.25 collector ID.
and JDM one's are 2.5 ID

 

Yeah you're right about it being 2.25 ID for the USDM. Youd be surprised to know hwo many people still instist that its 2" and bolts right up. It seems to be everywhere.