As a general rule the only time that this engine will ping is when the ignition is too far advanced.
I can see a few possibilities:-
1. You are misidentifying 'pinging', and it is something else, possibly an issue with the timing chain.
2. It is fitted with plugs that are far too hot. I have never had that with this engine, but did years ago with a two litre.
3. You say the head was rebuilt - it is possible the head was skimmed to increase the compression ratio to where it needs premium fuel - try that.
4. Less likely - vacuum advance is not working.
5. Need to take off head to check - something, possibly a result of the work on the head, or damage during fitting, has left a bit of metal in at least one combustion chamber that gets hot enough under load that it will ignite the fuel. Could be either a still attached cutting from the machining to allow fitting of hardened valve seats or a part of the head gasket.
Hope these ideas help
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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