At 12:10 AM 3/6/2000 +1100, Neil Cotty wrote:
>
>About to get some work done on the 1600's motor .... I am fitting a high
lift cam and flat top +040 pistons ....
>
>.... is it necessary to put reliefs in the block when the bore is at
3.01"? I am using an Early MGB head with the 1.56" Inlets and the 1.344"
Exhausts. .... I hear tell via Barney's Website that with the 1600 Mk2 at
1622cc the cutouts may no longer be necessary as the 1.56" valve will
travel inside the bore without contact. ....
YEEOOWW! Wait a minute. That's not exactly what it says. You might like
to go back and read it again. With the 3.01 bore the intake valves may
clear inside of the cylinder bore, but they will still be damn close and
bear checking. The exhaust valves will definitely still overlap the side
of the bore above the block by a considerable amount. This is easy to
check where the #2 and #3 exhaust valves are close together in the middle
of the head. The cylinder and valve arrangements are mirror images or each
other, so the same an each cylinder. Measure the distance between the
edges of the heads of the two center exhaust valves, and then measure the
distance between the walls of the two center cylinders. The difference
there will be the amount of overlap.
The original combustion chamber height for the early MGB head is about
.430" (inches +/-), to which you can add about .030" for the thickness of
the head gasket (but it varies). From this .460" (nominal) you subtract
the maximum lift of your valve, measured with zero clearane at the rocker
arm (.350"-.356" for stock 1500/1600 cams), and also subtract any amount
that may have been removed from the bottom of the head via prior decking
(original head thickness was 3.187"). The amount you have left is the
clearance for the valve (or interference if negative number).
Clearance of valves from the moving piston is no problem in these engines
(as long as you don't use a crowned top piston), as the valves are partly
closed as the piston approaches the top of stroke, and the piston never
rises above the top of the block. Otherewise Crane recommends a minimum
valve/piston clearance of .060" for intake valves and .100" for the exhaust
valves (more thermal expansion there). As the block is not moving and
expanding like the pistons and rods, clearance for the valves from the top
of the block should be a minimum of .060" for the exhaust valves. A little
less clearance would be okay for the intake valves, but they seldom have
more lift than the exhaust valves. If you don't have at least .060"
clearance between the wide open exhaust valve and the top of the block you
will need the eyebrows cut in the block.
With a stock cam and valve gear you can deck the "18" head by .045" to
bring the chamber size down to the proper 38cc for the 1500/1600 engine,
and you will still have the necessary .060" clearance for the valves. For
a stock "15" or "16" or "18" head (but not the "18-L" head), if it is
decked only a minimal amount for cleanup (not to exceed .020" material
removal), then you can use valve lift up to about .400" and still have
adequate clearance for the valves. Note, most "high performance" street
use cams will not be greater than .410" lift at the valves. With any
higher valve lift or any abnormal decking of the head you will be needing
the eyebrows in the block (at least at the exhaust side) to maintain the
minimum .060" clearance for the valves.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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