Back in the 1960s and 1970s some T bucket builders used a 1-1/4" pressure cap inside of a conventional threaded Model T brass era neck. As time has gone on, those pressure caps have begun to fail.  This is your replacement. 

We generally recommend a 4psi (or less) cap to preserve three part tank integrity but the lowest available cap of this size is a 7psi.  

The standard height we have found is a filler neck is positioned 5/16" above the top of the tank.  MEASURE YOURS BEFORE ORDERING and communicate if it is different! Why...because, if yours is higher, they'll be a gap, if its lower, the cap's tab won't be able to grab.

The top of the neck is a standard Model T e.g. threaded 2-18tpi and can receive hexagons, dogbones, hi fin, low fin etc.