Hey,

After noticing a small exhaust leak, I removed the DownPipe on my (freshly bought) FD and was astonished to find not one, but 2 gaskets inbetween the DP and the catalytic converter. So I swore a little at the previous owner/mechanic
The leak was obviously coming from a small space between the 2 gaskets, towards the bottom.
I changed my Downpipe (for a bigger 80mm one) with a new bigger gasket, the problem is I can't tighten the 2 nuts enough to assemble the 2 flanges correctly. I struggled during one hour tightening them (way too much, more than 100nm) but there is still a small space in the bottom, like 1mm... It's like the catalytic converter is not really aligned with the Downpipe (and I don't know how to adjust it, if possible), this angle difference creates this space between the 2 flanges, I guess.
And now, I'm thinking about the easy solution... put a second gasket and probably later (when my breaking in will be completed, and my PFC installed) change my cat for a high flow one.
If anyone else had the same problem, lmet me know how you fixed it please.

 

Can you give us some more information:

1) This is a Left hand drive FD with precat or right hand drive import?
2) What brand are the parts? some brand of aftermarket downpipe bolted to stock cat or some other cat?

Sometimes the problem is that the aftermarket downpipe was not manufactured to a good tolerance, and simply doesn't fit well.

 

Yep. It can also be the rest of the exhaust system is tweaked a bit on the hangers or the motor mounts are bad and sagging.

You can also use Remflex gaskets, they are thicker than normal and compress to fill in voids when exhaust pipes don't line up properly. But you are much better off finding out why they aren't lining up right and getting that fixed.

Dale

 

This is a USDM, lhd. With the stock catalytic converter and with the stock 'downpipe', that I just replaced with a HKS/greddy-like 80mm Downpipe (I'm not sure of the make though : https://www.rx7club.com/3rd-gen-gene.../#post12306453 )

Since the problem was already there with the stock pre-cat, and still here with the new downpipe, I assume the problem doesn't come from the downpipe itself.
Maybe the motor mounts or the hangers, as Dale mentionned above. So I'll try to check those asap.
Thank you guys !

 

 

wow !
My bolt patterns were not matching perfectly neither, I had to adjust 2mm on each 'hole' but it took me a while...
And what a gap ! thanks for sharing the info.
Mine is indeed less important :


I'd like to avoid grinding the flange, I don'ty even know how to do it well...
I'll give a look later today to my motor mounts and exhaust hanger

 

 

Just wanna chime in that I also recently put in an HKS downpipe and had 0 issues. It fit perfectly and slapped right in. I did swap out their crappy hardware for some higher grade stuff, but that goes without saying.

If the flanges were warped out of the box, contact HKS and send it back! I'll bet they'll take care of you.

 

I've checked and my flanges were perfectly flat. But I admit on the photos they look warped. I'll double check tomorrow just to be 100% sure
Otherwise, I'll try to find a big belt sander, thanks for the tip !

Natey : thx, but mine is a 15 year old or so second hand DP and I've no clue about its brand... plus I4ve bought it 90$ and sending it from France will cost me a lot more than its price

I've checked my motor mounts and, actually, I don't know how to see if they are correct or not...
I'm just wondering why there's some black gasket paste there...






And the catalytic converter's hanger looks fine as well

 

Ok, well, I think I've found the reason why the 2 flanges do not align :
the catalityc converter has been soldered at the other end, so I guess it has been bent, I guess... what do you think ?






I'll keep using it in order to finish my breaking in, then I'll install a PFC and toss it away for a high flow.

 

Looks like you have original motor mounts. The driver's side one with the aluminum arm is most likely shot, they weren't the best design to start with. I would either track down a steel driver's side arm and get aftermarket mounts (IRP's mounts are a great way to go) or go big baller and get the Full Function Engineering mounts - you wouldn't have to track down a new arm since they include them. New stock mounts are pretty pricey.

It does look like that cat has been hacked up for some reason and booger welded. Also the heat shield is missing on it. That does seem to be the root of the problem with it not lining up right.

Short term a thick crushable gasket like the Remflex would close the gap up enough to seal. Long term, look at new mounts and a highflow cat/midpipe after you go PFC. Most likely going with a good mid pipe will fix the gasket/alignment issues.

Dale

 

There is an inherent problem with 2-bolt flanges that are used to compress a gasket to create a seal.

Ideally, you want even distribution of flange pressure across your entire gasket face.

What happens on a two bolt flange is the exact opposite of ideal:
-your flange surface begins as a relatively flat sealing surface... perfect!
-you put in a flat gasket to mate with the flat surface... sounds good so far
-you torque down the 2 bolts on either side of the pipe to compress the gasket
-the bolt flanges are nowhere near stiff enough and deflect from the clamping force and permanently deform
-instead of the mating flanges looking like they started out: ] [ (parallel) they look like this: ( ) this is your shitty exhaust joint not sealing
-your gasket is now compressed exactly where you don't need it to be and the real sealing area has negligible pressure on it from the flanges

Short of a flange redesign increasing stiffness greatly you will never get around unwanted deflection of a 2-bolt flange. There are 2 ways you can address it:
1. make a tapered gasket that is fat in the middle and thins towards the bolt holes. I've done this with fel-pro exh gasket material.
2. get stronger flanges that are bent in the opposite direction ) (. That way as they deflect they become flat and when they deflect they don't do so permanently.

#2 is obviously the best choice. Fortunately you can just buy a Bonez HF cat that has pre-bent flanges. I fought this for a long time until I ditched my shitty stock cat which was breaking down anyways. I was pleasantly surprised to see Bonez had pre-bent their flanges back to promote better distribution of pressure across the gasket.

 

I've used tapered washers/spacers (thicker at the outer end of the flange and very thin at their inner end) at the bolt holes to force the centers of clamped parts closer together as the bolts are tightened. When the flanges are too weak to stay flat as they are, the above approach has worked for me to restore a profile that will seal properly. Sometimes you can buy these, but I've made them myself starting with a mild steel washer. I'd guess ~1/16" thick at the outer end and 0.01" thick at the inner end would work here.

BTW, I used Remflex gaskets when I installed my used RB cat-back 10 years ago, and it has stayed sealed since then.

 

The gaskets I linked on Amazon have a metal ring around the middle that's thicker than the graphite-type material of the rest of the gasket. I've been using them for years with a wide variety of exhausts (I've owned at least 30 cat-backs) with zero problems with leaking.

If the parts aren't made well and don't line up right you'll have problems. That's true of pretty much any flange design. Also, 2-bolt flanges need to be thick and robust so they don't bend or warp.

Dale

 

Easiest solution: Take it to any decent muffler shop and let them fix it