measuring valve clearance

["Watry, Andrew (LNG-SFR)" <Andrew.Watry@domain.elided>]

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.

This method has the advantage of showing you real direct numbers, and not
having to disassemble the engine if your clearances are OK.  And even if
they are off a bit, you can create more clearance by adjusting the cam a
bit, though that might not be how you ultimately want to fix the problem;
you can also go with a thicker head gasket, or machine the piston tops a
bit, or sink the valves more into the head.  This method is most easily done
with the engine out of the car, but I had the thrill of doing this several
times on my MG, once with the engine installed.

I last did this 12 years ago, so I may have messed up one or two details,
but this is the general idea.  I have notes at home I could consult if
someone is more interested.  Alternatively, if none of this makes sense to
you, either I'm messed up, or you're in over your head and shouldn't be
doing something this delicate and important anyway.

All that said, for stock Alfa engines I wouldn't even worry about this,
unless you know the head has been cut a lot.  The stock cam timing marks are
notoriously approximate, but even with that lack of precision I've never
heard of valve/piston collisions in normal engines.

Andrew Watry
Berlina Register
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