De-cat/cat gasket woes

Post by **indigolemon** » Wed Apr 17, 2013 4:10 pm

Yup - BB4 chassis code was never sold in the UK, so gets a pre-cat test

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Wed Apr 17, 2013 4:14 pm

Is it leaking from somewhere mid-point to the bolts?
Could be the bolts have actually been over-tightened and the flanges have warped slightly, so they now aren't flat mating faces?

I hate exhaust paste personally, always avoid using it except last resort (which is nearly always avoided), but in this case I reckon that's the way to go...

Post by **Pushki** » Wed Apr 17, 2013 4:19 pm

wurlycorner wrote:Is it leaking from somewhere mid-point to the bolts?
Could be the bolts have actually been over-tightened and the flanges have warped slightly, so they now aren't flat mating faces?

Yes, it's leaking mid-way between the bolts.
I would have thought the gasket would take up any warping of the flanges.

Post by **vanzep** » Wed Apr 17, 2013 4:31 pm

looks like a solid fab decat - horrible things

i never could get mine to seal or bolt up correctly.....got rid of it in the end

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Wed Apr 17, 2013 4:44 pm

Yep, the gasket should do pushki, but it'll only do it so far.

The gasket is only likely to crush, not re-expand. So it will initially crush reasonably evenly as you tighten, then if you go beyond that point to where the bolt area of the flanges start to warp/bend inwards, towards the very centre of the flange the pressure can actually start to decrease because the flange at that point starts to go slightly concave in reaction to the bending that's going on in the opposite direction at the outer ends of the flange (if that makes sense?).
If that does happen, the gasket won't re-expand to fill that gap.

Well that's the theory, anyway.

It might not be that complicated here though. Do the flanges look as if they've bent at all?

@vanzep what do you think was causing the problem with your de-cat pipe, that you could never solve?

Post by **vanzep** » Wed Apr 17, 2013 4:50 pm

massive flanges

that dont seal
to be fair the solid fab decat probably works fine with a solid fab exhaust system

Post by **Pushki** » Wed Apr 17, 2013 5:04 pm

Yes it's a Solid Fab AFAIK.

I've just been out to a local friendly garage and asked their advice.
One of the customers has suggested using RTV silicone because it's flexible and won't fall out.
Has anyone heard of this? I do have some that can withstand temperatures up to 600ºF.

Post by **vanzep** » Wed Apr 17, 2013 5:39 pm

one of my pals who builds/restores classic prewar motorbikes says this is all he uses on exhausts - high temp rtv

Post by **Pushki** » Wed Apr 17, 2013 5:43 pm

Thanks vanzep. I think this is what I'll use.
I'll get some high temp' copper silicone tomorrow and give it a go.

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Wed Apr 17, 2013 6:14 pm

Best of luck pushki!