timing cams

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

Per Jon's question about how I measured valve to piston clearance on my MGA
TC, some clarifications I'd forgotten in the intervening 12 years:

1. How to turn the cam is a good question.  On the MGA TC, there are wrench
flats on the camshafts themselves, and you can unbolt the cam sprocket from
the cam and put it on a little threaded holder just for that purpose at the
front of the timing case (front cover to Alfisti) so you can turn the cam by
itself without messing with the sprockets and the chain.  An Alfa camshaft
doesn't have wrench flats, but it should still spin reasonably freely in the
sprocket once the 22mm nut is loose and lock bolt removed.  You could use
the factory "pegs/scissors" tool, or a homemade substitute, to turn the cam
by the alignment holes in the cam's "distance piece" or spacer (for want of
the right term) behind the sprocket.  It will turn the camshaft through its
woodruff key.  You could also used the time-honored vice grips, but that's
crude.  On my MG I didn't have problems with the valve springs pushing the
camsahft farther than I wanted on any particular valve.  Maybe because
you're fighting four different springs at once, it evens out some?  You
definitely want to do this smoothly and gently for reliable readings and to
avoid valves hitting pistons too hard. If the valve springs are making the
cams lurch farther than you want, maybe this method won't work.

2. I guess it's true that if you have no clearance at all (interference),
you're going to have some contact between valves and piston no matter how
you assemble the engine or test the clearances, assuming the cams are
installed when you assemble it.  I'd think you'd have some idea ahead of
time that you had some clearance in your planned motor, and that the valve
lift, piston dome, head height, etc., all essentially worked together and
you just needed to check and fine-tune.  I started on my MG from the point
of  having a combination that was in the ballpark but that needed careful
checking and adjustment of timing to make sure the clearance was adequate.
This was because of using an obviously nonstock, but recommended Kawasaki,
cam profile in an MG.  The cam grinder knew in advance it was in the
ballpark.  I don't know what to say in this case if you have interference
right off the bat.

3.  In my case I had an optimal timing to shoot for from the cam card for
performance (timing based on measurement of valve events, not factory marks,
which were now irrelevant with a reground cam, and even more imprecise than
Alfa's marks) but adequate clearance was more important so I probably gave
up a bit of performance to have sufficient clearance.

4. I can't remember anymore whether the nearest the valve and piston got to
each other was at TDC or somewhere near it; I should have read my notes
before posting the message.

The main point of my earlier post was to mention the process, not give a
how-to for people to actually work from. If you're doing this stuff, talk to
your cam grinder or machinist and really get to understand it.  I sweated a
lot of blood on setting up my MG engine and checked and rechecked and
rechecked stuff, but it came out fine, at least for an MG.

Andrew Watry
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided