exhaust leak at finned nut

[SLASHSIXER]

I recently, with the help of a long time pal and this forum, re-assembled my '74 R90/6. We re-built or renewed every non-engine or gearbox related component without known complication. I'm happy to report that after ten years in boxes in the shed behind my house, it still runs pretty well and looks pretty good and I am having much fun.
However, I still have a leak at the left finned nut. I tried tightening the nut as much as possible and then renewing the seal washer pair between the finned nut and the cylinder. No dice, still leaking. When I fully compressed the split washer around the header with finger pressure, it didn't seal around the exhaust pipe, so there was the source of the leak. So I measured the O.D. of the header at the sealing point>>37.45 mm per my digital micrometer. I think that this dimension is supposed to be 38 mm. I have thought of two possible solutions:
-make the split bigger on the split seal washer ring, by filing away the edges of the split (don't favor this method)
-go the the Autozone, borrow their tailpipe expander and try to make the pipe bigger.
-buy a new exhaust header system (really don't like this method). I purchased this cheap aftermarket exhaust system (brand unknown) ten years ago.
I figure that someone has encountered this before. Thanks very much.

 

[beemerPhil]

Be sure the split ring is against the head, then the solid ring, with the concave side towards the taper on the split ring, then the nut. NEVER put these parts on without a good dose of anti-sieze; this is aluminum on aluminum, and the nut will often sieze, tear off the threads, and get unpleasantly expensive down the road....if the nut gets stuck, cut it or split it off, they're 'only' $30.

Expanding the header will crack the chrome, and won't fix anything- assuming the pipe actually fits the exhaust spigot.

Making the split bigger is counter-intuitive; from your description, you suspect that the split is already too big.

I think you probably have the parts out of order....or the sealing rings are damaged.