Hi guys,
I have ought a new exhaust and the J tubes are about1-2mm smaller in diameter than the mating tube- so big that it doesnt seal and sucks air in off throttle (causing abackfire!)
How do I seal it?
Will exhasut cement work?
RTV silicon during assembly?
or do I try the silicon coulers used on intercoolers and the like.

I tried lagging it ip and it worked a bit but the lagging got shagged.

you can buy a product called muffler putty to do the job, its always a good insurance policy i rekon when i slide j tubes in place and use the clamps because usually i find the clamps end up squishing the j tubes into a slight egg shape (ever so small) leaving two open holes for exhaust to leak. Otherwise that metal filler epoxy putty will do the trick.

Keep an eye on it over time, as it is brittle. Vibrations don't seem to knock mine off, but i check just in case. to 'uninstall' the exhaust with the putty in place is a matter of giving the joint a whack with a spanner to break the metal putty bond, and it will crack off.

If you just bought the exhaust and it doesn't fit together then take it back! That is unacceptable. J-tubes should seal using only the gaskets and clamps. If you didn't get new gaskets and clamps then that is the issue. If you bought a new engine and it didn't run, would you be asking on the forums how to fix it? Of course not, you should take it straight back to the supplier.

Who did u buy it from yes send it back

If you have origional j pipes / exhaust heater boxes then use the locktite exhaust sealant blue stuff

If it's a muffler you plan on sticking with for a while it's well worth the effort to cut the stupid donut/clamp joint off and weld flange joints on instead.

That really was a derp on VWs part and those new wire mesh type donuts make a hopeless situation even worse.

Guys it's one of. The sidewinder exhausts from classic
Only got around to installing it on the weekend and hat to peen one side closed to get it to fit the pulley driving the supercharger
So I can't take it back!
It is disappointing though. The tube joining the main extractor had over 6mm clearance!
I had to heat it with an oxy and shrink it!
Really quite a poor fit, but @ 1/3 the price of the a1 or csp ones thought I'd make it work.

Vw54
Others reported these as greAt!
I think I remember Craig fitted one and said it fit fine.
This was the second batch he got in...

I have two of them and the only thing i had to do was cut and weld the bend on the muffler so it cleared the rear caliper. both the j tubes were a good fit.