I just replaced the donut gasket on my USDM Mugen 4-1 header
with OEM Part #18229-S04-X12 GASKET, EX. FLEXIBLE (51MM-64MM)

After I bolted the header to the cat, it looks like I still have space around the donut.
I made sure I bolted the bolt/spring/nut combo until it hit the stops. So I can't tighten it down anymore.

The OEM donut fit around the header collector just fine. It just looks like its not thick enough to fill the gap..

My setup is USDM Mugen 4-1 header -- Stock USDM OEM Cat -- USDM Mugen Sport Exhaust w/Twin Loop.

Did I use the correct gasket? If not, what's the correct part# of the gasket I should use ?

 

Ok.. I confirmed with King Motorsports that the USDM gasket should be the one to use.
When I took off the header brace near the cat, the header dropped a tiny tiny bit and fit a tiny more snug.
I took it for a test drive and it still sounds like I have an exhaust leak. You can't hear the leak at idle.. I can only hear it when I'm moving and giving it enough gas to put some back pressure on the system.

So I guess the next thing to check is if the cat has a hole in it or something..
The sound is pretty loud and seems to come from the header/cat area where they connect.

The noise I'm hearing sounds like a muffler with a hole in it, but coming from the middle of the car. I didn't replace the gasket on the other side of the cat because I didn't open that end up.

 

18229-PCX-003 GASKET, EX. FLEXIBLE (00-07 S2000)
18229-SCA-T11 GASKET, EX. FLEXIBLE (02-06 CRV)
18229-S7C-003 GASKET, EX. FLEXIBLE (02-06 CRV)

 

Those gaskets are 2.5" arent they? The USDM Mugen he is talking about needs stock ITR size gasket.

I had this same setup back in the day and IIRC I just re-used the stock ITR OEM header gasket.

 

Yep. After looking at it again yesterday, it looks like the correct gasket is the OEM ITR one, but I wish it was a smidge thicker so it would compress and make a better seal. The header seems to be tilted up a tiny tiny bit, so the bottom of the gasket is in the cat, but more of the top of the gaskset is showing.

I tried pulling the header down, but it wouldn't tilt down anymore. I might try to unbolt/rebolt the manifold to see if that helps a little bit.
As of now, I think the OEM gasket is creating a good seal, but I'm not certain. It still sounds like a have a leak around the general area. The only other place it may be leaking is at the cat itself, or right after the cat. I've never replaced the donut after the cat, so I guess I'll inspect it to see if it has deteriorated.. I don't think I'd be throwing a code 45 if the leak was after the cat though.

Oh yeah.. my car is throwing a code 45 after two consecutive 30 mile trips.
I don't have a scanner, so I can't tell if the code is for a system too rich or too lean.
The helms says that the too lean scenario can be attributed to an exhaust leak.
I may just bring my car to the dealership if I can't figure this out by the weekend. I'm just tired of working on my car right now.

 

Dont yank on the header too much, you can crack the welds. The point of the donut gasket is to allow the cat to pivot, or move if need be, taking stress of the header. I dont tighten those bolts extremely tight for that reason.

 

Put posting this for reference in case someone else reads this and needs it in the future.

The TypeR OEM donut gasket is correct.

My "exhaust leak" noise was due to my timing belt being off 1 notch at the crank and it advanced my timing to 30 degrees. It created that "open header" sound when point load on the engine. I fixed the timing belt error and it sounds beautiful again.