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Weird Coupe GT engine

Routine service on the 1985 Coupe GT "JS" engine today, at 150k miles.
The usual niggles.

But something interesting - a routine compression check produced 10, 4, 5, 9, 
13.  This was unexpected - there have been no symptoms.  There's no oil loss 
(and yes - it's been _very_ hot a couple of times recently), no apparent loss 
of power and no exhaust noise or content symptoms.
John Coughtrie (for it was he who sold me this ****heap) says the engine has 
never been opened.
We were perplexed at the lack of symptoms until - at John's suggestion - I 
repeated the measurements with a plug in the other cylinder.

Usually, I whip all the plugs out and then measure each cylinder in turn.  This 
time, I took them all out, measured 2, replaced the plug in 3 and measured 2 
again.  Like a good scientist, I then took the plug out of 3 and measured 2 
There's no doubt - putting a plug in 3 raises the compression in 2 from 4 to 
a hair under 5.
Is this proof of a cylinder-to-cylinder blow?

 Phil Payne
 Committee Member, UK Audi [ur-]quattro Owners Club