RE: [alfa] Valve Clearance Tolerance

[White Anthony <anthony.white@xxxxxxxxxxxxxx>]

Some of us run valve clearances tighter than the specs given by Alfa without
experiencing any problems; others among us consider that the Alfa engineers
probably knew more than we do, and that the valve clearance specs were
provided as they are for a reason.  The best guess as to what that reason is,
is that the average owner doesn't check his or her clearances regularly, so
there needs to be some room for movement.  

(Note: everything below is measured in millimetres.  Sort it out for yourself
in imperial units.)

Valves clearance open as the engine warms.  If you're running 0.40mm cold,
your probably running 0.60mm warm, and 0.80mm hot.  

When you measure your valve clearances, your engine should be cold -- as in,
sitting overnight cold, not "I haven't run it for a couple of hours" cold.
You'd be surprised haw far out your clearances will be on a cold morning if
you set them when the engine had anything like residual heat in it.  I once
set my clearances to manufacturers spec late on a summer afternoon, having
not run my car all afternoon.  The head would have been slightly above human
body temperature.  The next morning we had a cold snap, and the car would
barely run for the first few minutes until some heat was back in the engine
and the clearances opened up.  

Given that clearances do open up so much when the engine warms, running
tighter clearances may have advantages.  I run at 0.20mm intake and 0.25
exhaust, mainly because it keeps the valve-train quieter when the engine is
hot.  When my engine is hot, it means I probably have clearances around 0.60
instead of 0.80mm.  

Those who race will often run tighter clearances as it gives that fraction
extra valve lift in classes that require standard camshafts.  If they run as
tight as 0.10mm on the intake, they gain an extra 0.3mm lift, which isn't
much, but might be worth a couple of tenths a lap.  

However, valves do recede into the head over time, so if you are going to run
tight clearances, you MUST check them regularly.  I check mine every time I
change the oil -- but then I always take the cam cover off to change the oil
anyway, in order to siphon old oil out of the camshaft baths (aside -- if
you've never done this, you'll be surprised how much longer you oil stays
clean if it doesn't have this dirty camshaft bath oil to mix with).  As I
check my clearances every 3000 miles, I don't have any concerns with valve
recession causing unreasonably tight clearances.  If you prefer a more
maintenance free approach, stick with the standard clearances.  

Additionally, if you run aftermarket camshafts, you should start with the
valve clearances specified by your camshaft designer.  

Regards,
Anthony White
Wellington, NZ
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