Alain,
Do you know anybody with an original spare parts catalogue? Unfortunately, I do not have the spares book but I do have a manual for the ID. See operation 41 in the relevant page from the ID workshop manual ………

This clearly says no “lock washers” and does not mention a plate.
I must admit I am surprised because the “11D” Traction engines definitely had a one piece “amoeba” locking plate for those screws.
Personally, I think I would want to fit one on an engine destined for a Light 15 – it can surely do no harm and could save a lot of long-term grief.
B….
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This reply was modified 5 years ago by Bernie.