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The flexi joint has failed on my 2003 TF. I have acquired a “new” downpipe which I was assured was original MG but looking at it the pipes at the manifold end are of slightly smaller diameter than the downpipe fitted to the car (which I assume was original). The original is on the top in the photo.
Firstly will the slight reduction in diameter make a difference to performance?
Secondly are there better downpipes that I could fit (or would I be better off just replacing the flexi?)

I suspect that the part at the bottom is for the 1.6 K series engine, and yes it probably will affect performance (IMHO).

I’ve got hold of a micrometer and measured the tubing. Both the exhausts have OD of 41.5mm. On closer inspection, the pipes of the older exhaust have been flared out slightly where they join the flange to give an inner diameter of 39.9mm compared to 37.7mm of the newer exhaust. However, lower down, it seems that the inner diameter are the same (although difficult to access for measurement).
It would thus seem that both the new and old downpipes are actually very similar apart form the flare at the top of the older exhaust. There does seem to be a little “excess” weld on the inner face of the new exhaust which I think I could trim to achieve a better flow.
Do my observations make sense?

The same exhaust system is used on the 1.6 and 1.8, I would think the new one is aftermarket. Rimmer's list the new-old-stock down pipe at £240 where as the aftermarket is £120.

If it's possible to remove some metal with a rotary file, you can use the joint gasket as a pattern to mark the new flange in order to give you something to work around. Good luck.

Any step in the exhaust outflow will cause turbulent flow and limit exhaust extraction (probably multiple caveats to that statement, but you get the drift).

I'd check the alignment of the two pipe holes in the flange to the gasket - assuming both align perfectly (i.e. no obvious intrusion within the gasket line), then all will be fine. If the internal diameter (ID) of the pipes is smaller than the gasket ID, then you could take a die grinder and enlarge to match the gasket to resolve the issue.

On another car I just had a new flexi welded in to an existing pipe.

What sort of rotary file is best to use for this sort of work ?

JeremyC wrote: ↑

Fri Feb 02, 2024 10:50 am

On another car I just had a new flexi welded in to an existing pipe.

I've had this done too.

Notanumber wrote: ↑

Fri Feb 02, 2024 3:05 pm

What sort of rotary file is best to use for this sort of work ?

A carbide rotary burr will do the job

Thanks Rob.