XJ6 Fuel filler caps interchangability

Are series 2 and 3 fuel cap assemblies interchangeable between tanks? This will dictate whether I go to series 3 tanks or use modified original series 1 tanks. Thanks.

Maybe I have answered my own question. :upside_down_face: Apparently series 3 are a bit taller… Oh well.


I suspect that the Series III fuel caps sit higher because they each have a small nipple at the front leading to a fuel vapor pipe located in the C pillars. I don’t know much about the Series 1 or 2 fuel systems but I doubt that the early Series 1 cars had the complex fuel evaporative recovery system with an assortment of valves, hoses and pipes and a carbon canister.



Thanks Paul, the caps that are (were) on the car do have that little take-off pipe. I know because they make it a pain getting the filler assembly on and off without breaking the pipe.

I’m surmising stuff here but the series 3 tanks have 2 o-rings in each tank and the series 2 only one. Maybe that has something to do with it.

FWIW, I bought a second hand pair of S3 caps for my S1. The original caps were pitted and I was able to remove the much prettier S3 caps from their lower fittings (roll pin etc) and fit them to my S1 fittings. The S1 does not, of course, have the FI return circuit hence the surgery.

Do you mean the vent tube? I don’t think the return compromises the filler cap architecture in any way. Interestingly, Barratt and that other mob in the UK list Series 3 caps will fit 1 and 2 series but not the other way around, apparently.

I believe the pot metal caps (sealing aside) are identical, the unleaded only caps are much deeper for cars with catalysts and they look ugly.

Nope the S3 lower parts have a fuel injection return circuit.

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Yes, identical.

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