At 11:02 AM 8/28/03 -0400, Steve Hubbard (Otis15@aol.com) wrote:
>.... (71GT). .... #1-175/ #2-175/ #3-"O"/ #4-175. ....
>.... found #3-----Valve Stuck Open because it was bent. ....
>.... there are no Marks or Damage on the Head Surface, Piston or the
>Valve, ....
When you install bronze valve guides in the MG head it can be suseptible to
valve stem binding do to the greater thermal expansion of bronze vs iron
guides. For bronze guides you need a skosh more running clearance. If the
valve does hang open in the guide, then the rocker arm can stop in the
"actuated" position, and then the crown edge of pushrod top socket may hit
the ball of the adjusted screw and force the rocker arm and valve beyond
the normal maximum lift point. This could cause the valve head to hit the
top of the block and bend the valve strem. Iron valve guides generally do
not present this problem.
Start by measuring the current thickness of the cylinder head.
With the stock 1800 engine the intake valve lies completely inside of the
cylinder bore, so it would not hit the top of the block. Stock camshaft
the valve timing is such that the piston never gets close to the valve
heads. Due to the geometry of the cylinder head the exhaust valve head is
positioned to overlap the side of the cylinder bore, so it can hit the top
of the block if it moves that far. The intake valve has a similar problem
with the smaller bore 1500 engine.
If you have a high lift camshaft, or high ratio rocker arms, or a shaved
cylinder head, or any combination of these, then the exhaust valve can get
close or hit the block at full lift. High engine speed and thermal
expansion at high throttle settings can aggrevate the situation.
The 1972-1974 North American market MGB used the "L" head which was thinner
for higher compression, and the valves would reach down closer to the
engine block. The 1972 and later engines had eyebrows cut in the top of
the block for increased running clearance for the exhaust valves. If you
put the "L" head on an earlier engine you run a high risk of valve
interference and should add the eyebrows to the block. Ditto for a heavily
shaved head or high lift valves. You should allow at least .080" clearance
for the exhaust valve head from the block at full lift.
Original thickness for the B-Series cylinder head was 3.187" (except for
the "L" head). I commonly cut a "16" or "18" head down to 3.140" thickness
to reduce combustion chamber volume form 43cc to 38cc for use on the
1500/1600 engines. I think the original thickness of the "L" head is
similar to this 3.140" thickness. The .047" reduction in height is barely
functional with a flat top block. A "Fast Street Camshaft" with .064"
increased lift for the exhaust valves is marginally unacceptable with the
stock height head, and when combined with the shave head definitely
requires the eyebrows in the block about .100" deep.
For the "how to" on adding eyebrows, check here:
http://mgaguru.com/mgtech/power/cm202a.htm
Barney Gaylord
1958 MGA with an attitude
http://mgaguru.com
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