I have a '91, pre-facelift, XJS coupe. The fuel tank was rusting out and I had my mechanic change out tanks. The inlet for the sump tank return hose on the new tank is slightly larger than the inlet on the original tank. The original inlet looks to be 12mm while the new inlet is 1/2" OD. I understand Jaguar frequently uses a 24 thread per inch pattern on the inlet thread and it is hard finding a conversion. The old hose connector is too small for the new inlet.

How do I find a sump tank return hose (breather hose) that will fit the inlet on the new tank?

 

Where did you get the new tank? I replaced the tank in my '87 with one from SNG Barratt in May. I had a bit of trouble with one of those fittings, but in the end it turned out to be me cross threading it. A hole with 12mm ID and threads with 1/2"(12.7mm) OD sound like the right combination to me. The only line I had to change on my tank was the main feed line from the main tank to the sump tank. This got replaced with 1/2" fuel hose because the connection on the tank was very different.

 

The inlet on the new tank is visibly larger than on the old tank. Not much but enough the threads on the return hose from the sump tank won't engage. The connector easily rattles around in the new inlet. I initially thought there had to be an adapter to to solve the issue but if it is available Google can't find it.

I bought the tank from Welsh Enterprises in Ohio and as best as I can tell both Barratt and Welsh buy their tanks from the same supplier. Multiple phones calls to Welsh has finally discovered they sent the wrong fitting kit with the tank. Can't tell until the new kit arrives if the new kit will fix the problem or not.

 

For anyone stuck trying to install a new fuel tank while the fitting kit delivery is delayed, Welsh says the shape of the xjs tank didn't change over many years. Jaguar used multiple inlets over the years and the manufacturer doesn't attempt to replicate all the different configurations. They make one tank and numerous fitting kits to match the generic tank to different models. The inlet on the tank changed from 12mm to 14mm. The inlet takes a m14x1 adapter. m14x1.5 is more common but the threads won't match.