Andrew's piston to valve clearance method

[Jonathan Coates <jon@domain.elided>]

Andrew wrote:-
You can also measure valve to piston clearance with a dial indicator, which
is more work, but also more accurate, especially in non-stock engines where
clearances can be all over the map.  The method recommended by MegaCycles, 
a
Bay Area performance cam place, for my MGA Twin Cam with non-stock cams and
surfaced head was to assemble the engine completely and set up the cam
timing, bring the piston to TDC on the stroke where the particular valve is
open a bit, then release the cam from the chain so it can be turned
independent of the crankshaft (on a 4-cyl Alfa, loosen the 22 mm nut and
remove the small lock bolt), and turn the cam slowly and carefully more
open, measuring the displacement of the tappet (also the displacement of 
the
valve) until the valve hits the top of the piston. That's your clearance.
Do this gently.  Ideally you'd want to do this for each valve, because they
may all sit at different depths in the head, especially if you've had a
valve job done by other than an Alfa expert.  And pistons may vary 
slightly.
You need to reconnect the cam to the chain to turn the engine in between
doing each separate valve, and when you're done.

As well as more good stuff

My questions are these:-
If you have an interference problem, wouldn't you have done the damage by 
timing the cams at TDC of the pistons already?
How does one turn the camshaft gently, feeling for the touch of valve to 
piston? I've struggled to turn camshafts "gently" because of the big strong 
valve springs working against me.
Would a tuner start with the cams timed a bit off and _then_ turn them past 
the point of maximum dollar lossage?
Is the minimum clearance at TDC, even with fast opening cams, or a fraction 
after (valve catching up with piston)?

Curious in UK
Jonathan Coates
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