Porting Exhaust Flanges

[32GTRB]

I recently acquired a 3.5" cat back and some new front pipes. My front pipe outlet however is 3" and so is the inlet on the catback (flange is 3" and pipe diameter is 3.5"). Guessing this is because most cats come in 3"?

As I have a 3.5" cat on the way, wanting to know if its ok to port the flanges of the front pipes/catback out to 3.5"?

Alternative is to weld new flanges on but thinking porting is much easier and I can do at home with a die grinder.

[scotty nm35]

The tube steps down on most of the flanges I have seen. Not much chance you can diegrind it out, you will need to cut and weld.

I assume this is a Jap exhaust, as they love using step-downs. If that's the case the tube is most likely metric, not imperial. Makes it very hard to match.

[32GTRB]

The tube steps down on most of the flanges I have seen. Not much chance you can diegrind it out, you will need to cut and weld.

I assume this is a Jap exhaust, as they love using step-downs. If that's the case the tube is most likely metric, not imperial. Makes it very hard to match.

Yep its a jap exaust (blitz front pipes, unknown catback). I've got some 3.5" flanges on the way but if i cant grind it out looks like cut and weld is the way to go.

[scotty nm35]

Well the flanges won't suit either seeing the tube is metric.

[zebra]

Well the flanges won't suit either seeing the tube is metric.

just wrap some mig wire around the tube to fill the gap. she'll be right mate...

- every exhaust shop ever

[32GTRB]

Well the flanges won't suit either seeing the tube is metric.

Well thats a pain!

[XRATED]

. so this is what I would do.

Buy 2x long radius stainless reducing cones 80mm to 90mm.

Buy 2x 3.5" stainless exhaust flanges with gaskets and bolts.

Cut the step down and flanges off the exhaust and front pipes.

Cut the reducing cones at the right diameters to adapt the metric exhaust tube to the 3.5" flanges... the size difference will probably be a few mm.

Weld (remember to clock the flanges correctly).

Cheers

J.