A long update:
Some more work today. I took the exhaust cam off and measured the tappet / bore dimensions and in the process started to come to the conclusion that my measuring gear is not quite up to the job of measuring accurately enough.
JTC turned up with another head and we measured that as well. The tappets were coming up as the same diameter with the micrometer, bores similar and this engine had not been knocking from the tappets, so I decided on a different approach.
The buckets and the bores each have a specification, and therefore an acceptable range. When worn, then I suppose it is possible for the bucket to rock slightly, but more importantly the pressure of the oil coming up the gallery and into the tappets will be reduced because of leakage up the wall with the greater clearance.
Specs are as follows:
Bores: 33.000 to 33.016
Tappets 32.959 to 32.975
The minimum clearance is thus 0.025mm and the max 0.057mm
Having calibrated the bore gauge against the micrometer again I then took measurements and compared the clearance.
On the inlet the range for the 8 valves was 0.035mm to 0.055mm so all within spec
On the exhaust the range was 0.020mm to 0.060mm, so just one tappet was out of spec by 0.003mm when my gauge is only accurate to 0.01mm
This left me with the prospect of reassembly, but with no component change the noise would still be there The oil galleries were inspected and they appear to be clear, indeed my head was quite a bit cleaner that the comparative Aero head.
Now, I had heard comments that sometimes a broken stud can sound like a tappet, and as this engine has two broken on number 4 I was encouraged to fix those. Along the way I discovered that the exhaust manifold cannot be removed without the turbo being free to move, so the turbo was fully removed allowing inspection. The gasket was very oily on number 4
The conclusion was it needs a rebuild. While the radial play seemed reasonable, there was a lot of axial play and having heard metal to metal contact when spinning the exhaust side a closer inspection showed a small groove present on the compression side where the blades have been touching. So, it needs attention.
I am being encouraged to do this myself....... having seen seal kits on ebay for Chinese manufactured components is there an alternate supply for higher quality materials as I understand it is possible to DIY provided you are meticulous about ensuring the turbine wheel and nut are put back in the same place, thus preserving the balance
The two studs that need replacement were on number 4 and I had pre-ordered studs from Saabits. I have drilled the old studs out and re-tapped threads for replacements. I am yet to discover if they are correctly aligned, or some tweaking is required
I will have to return to the question of the noisy tappets again.......... they all seemed to be pumped up and hard, yet we had heard the tapping using a stethoscope previously. Where's the confused smiley .... ah that'll do
Words of wisdom and advice appreciated