I'll have a look tomorrow and take some pics if necessary.

I hope this helps John, I have a 2016 Morgan and on mine it measures 78mm to the outside of the seal.

Thanks guys. My old seal you could move out of the groove very easily by pushing one end using two fingers to grip the bottom third, the other end would then balloon out. Are yours a tight snap fit that you must pull over the groove. My original & the replacement are a loose fit that easily goes into the groove.

It looks like a tight fit John, but to be honest I haven't tried to remove it.

Thanks. I measured the ring it is 73mm on the inside diameter. The groove is 71 mm. I have cut out 3 mm to give a tighter seal.

After I wash the car this afternoon I will take it for a spin to test the result.

Well after washing the car I tested the reduced diameter seal.

The fault when starting cold has gone completely.
The fault when hot & simulating a refuelling is intermittent as is it starting a hot engine after a stop of 1-2 mins.
This is a far better result than last night when it was every time.

I expect to get 2 more seals in Viton next week so will then make this current one an even smaller diameter to see if that works.

John

There is something odd about your cap (the fuel cap). I now have a small collection of caps, new and old. My original 2007 cap, a new MMC cap on the car, and a CLA300 cap from Newton (the manufacturers).

All the caps have the same structure for the secondary seal

The primary seal is the locking insert, the secondary seal is by way of an o-ring around the sprung loaded disc in the cap, o/d 78mm as mentioned by Dave. There is no other seal on the cap.

Your picture shows what looks to be a rubber sheet gasket in addition to the sprung loaded disc

Once the sealing disc is removed the cap looks like



The edge of the sealing disc showing the o-ring



The disc and o-ring is fairly well spring loaded and appears to make a good seal with the filler cap body. Although on my old cap the o-ring is showing signs of age.

I have to add that on my Plus 4 the seal on the locking insert definitely seals the tank and there is an intake of air when I remove the insert, so I don't know to what level this secondary seal is important if the primary seal is good.

Just wonder at what point the flat gasket (third level of seal) was added to your cap.

Mike,
there is no third gasket it is just the photo showing the dirt on the inside of the cap.

I have the large o ring & also a smaller one that seals the shaft to the disc of the cover.

John

Won't win concours with a dirty cap !

These fuel caps are made by Newton Automotive, tracking down the locking inserts on the Car Builder Solutions web site, they are available as non-venting, and as venting.

In the case of emission control they need to be non venting inserts so the carbon filter stores the fumes.

For older vehicles they would normally be vented inserts. In that case the large disc and o-ring would become the primary seal and release fumes once the pressure is high enough to lift the disc.

The Newton locking inserts come in two styles, the older type has a sort of rubber band style of gasket

The un-vented https://www.carbuildersolutions.com/uk/un-vented-lockable-insert-for-aston-necks-excl-2

The vented https://www.carbuildersolutions.com/uk/vented-lockable-insert-for-aston-necks-excl-2

I have purchased the non-venting and it did fit my old filler cap.

The later (current) style of locking insert has an o-ring seal, the part number on the insert here on my desk, for the non-vented is #Z45LT. I haven't yet found these on a re-sellers site, There was a thread some time ago on Talkmorgan where someone got hold of Newton and purchased an insert.

Assuming your cap is in good shape, it may be worth getting a new locking insert as this should be the primary seal for a non-vented system.

Another thing to check would be the fuel hoses to see if there are any cracks. My old hose looked OK but I did find some fractures although they were under the hose clips.