Exhaust Valve Cutouts
for MGB blocks

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When 18v cylinder heads or modified cylinder heads are fitted
to non-18v blocks there is a danger of the exhaust valves hitting the
top of the block when the valve is at full lift. This applies to both
standard and modified lift camshafts.
To overcome this problem cutouts are needed in the block. The easiest
way of machining the cutouts is to use a half-round hand file ( not too
rough a cut on the file please!).
Prepare a cylinder head for marking out the position of the exhaust valves
by stripping all the valves from the head, leaving the exhaust valves (held in
place with an elastic band on each valve). Apply some dark coloured felt-tip
pen or engineers blue to the top of the block around the bores.With the head
studs fitted to the block, lower the cylinder head into place (you don't need
to fit a head gasket for this job). Push an exhaust valve down till it contacts
the block, rotate the valve to mark the top of the block. Repeat this
operation for each of the exhaust valves.

Remove the cylinder head and inspect
the cutouts, if you can't see them......
do them again!
Using the half-round file carefully file
the block to a depth of around 3mm
or 1/8th" and to the edge of the valve
plus 1mm or 40 thou.
Recheck the valve to block clearance
again by lifting the exhaust valve to
cam lift plus 2mm / 80 thou.
The accompanying photographs show
the shape and dimensions of the standard
cutout in an 18v block.