Morris Minor Owners Club

With the tappets set at 12/15 thou what would you expect the rocker and valve top to expand too when at full operating temperature??

I take it you mean: -
'How much does the clearance between the rocker and the top of the valve stem reduce by when the engine is up to operating temp'.
It may close up by a couple of thou but not much more as the expansion of the valve train and engine components is in all directions.
If the rocker to valve stem clearance is set to 0.012" minimum that is more than sufficient clearance for expansion.
The 'A' Series in my opinion runs much better with 0.015" clearances but creates more noise.
With my own 'A' Series the valve clearances are set at 0.019" but that is mainly because of the 1.5 ratio roller rockers and the camshaft profile.
The 'B' Series engine has 0.015" clearances which does not impact on the reliability or efficiently or creates more valve train noise.

Do you have a set of feeler gauges? If so, it surely cannot be too much of a hardship to measure the change yourself - at some opportune time? The clearances are there for several reasons - one is to avoid valves not seating correctly and another is to provide the correct valve timing according to the specification. Excessive valve clearances will cause premature wear at contact points in the valve train.

Do remember that exhaust valves run at higher temperature than inlet valves and inlet valves can be cooled by the incoming charge. Extra noise is a rough measure of extra wear, of course - try tapping something gently (quietly) and compare with hitting the same item much harder (noisier) with that same hammer.

Do remember, too that some parts of the engine can heat up more quickly than other parts. The design engineers sorted out the necessary/most appropriate clearances a very long time ago!