Thread is probably 2BA if a normal M5 thread is too tight in the hole. 2BA is about 4.7mm outside diameter, but with a pitch of 0.8mm, same as normal M5.
If the M5 is really loose in the hole but binds up eventually, the threads are most likely 1BA, so around 5.3mm OD, but this has a 0.9mm equivalent pitch.
All the best,