Compression readings...

[Gordon Hunter]

Hi folks,

Seasons greetings to you all.
I recently acquired a '67 P1800 (the one with the electrical problems in a previous thread) The engine ran fine but with a LOT of smoke.
I assumed that the rings had gone (correct in the long run), but on running a compression test, the results (checked twice) 170 165 170 165 seemed to suggest to me that there could just have been some play in the valves and that the smoking was in fact due to the valve seals having given up (I've seen this in some more modern engines).
On removing the head for inspection though, it was obvious that the rings had gone on two of the pistons like I originally suspected.
My question is this, I had understood that as a rough guide, you could have at least a 10% deviation on the compression results for each piston before assuming that the rings needed replacing. So do I conclude that the valve seals have gone as well, but that the oil entering the bores from the leaking seals has had a sealing effect around the broken piston rings resulting in a very optimistic compression reading?
This is hardly a life or death question, but I'm keen to learn from other folk’s experiences so I can becoming better at diagnosing such issues.
Does what I say make sense, or can anyone enlighten me to the cause of the optimistic readings?

Any comments appreciated, thanks,

Gordon