after the hassle I've had ordering a manifold that doesn't fit I'm going to get a mate to fabricate one
I'm going to use 1/2 inch for flanges but what size stainless steel pipe is suitable for fabricating a manifold I've got access to 50.8mm (external diameter) in either 2 or 3mm and what would be the ideal thickness for the 3" downpipe?

 

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Any body?

 

First off pipe and tube are two different animals... tubing is sized by its physical OD (2" tubing has a 2" OD), where pipe is nominally sized (2" pipe has an OD of 2.375").
If you are limited to the 2" tube in either wall thickness, I would go with the 3mm as it will hold up better to the heat cycling, as well as retain the heat more effectively. This will however be a slight neck down from the exhaust port opening. Ideally I would go with 2" sch10 pipe and fittings as the ID is just a touch larger than the exhaust ports, which gives you you're anti-reversion.

 

Thank you for your help I will look for that