The cylinder heads on the Small Block Chevy 400 were prone to crack and
the cores for this engine are increasingly more difficult to find. Machining
the 350ci cylinder heads to match the 400ci heads is an excellent way
to keep your 400ci engine alive.
To use the Small Block Chevy 350 head on a 400 block, it will be necessary
to drill six holes in the cylinder heads. Use a 400 head gasket as a template
to mark the heads and Drill the 3 holes on the intake side using a 3/16 drill
bit at a 10 degree angle towards the exhaust side of the head. Drill the 3
remaining holes using a 1/4 inch drill bit straight down towards the top of the head.
Start by marking the approximate steam hole locations with a marker
or die. Next insert 2 dowel pins into the heads dowel pin holes and lay
the 400 head gasket onto the head over the dowel pins. This will keep
the gasket aligned precisely as you mark the holes. See picture below.
Level the head with the deck side facing up and drill the 3 holes that
run parallel to the exhaust side of the head using a 1/8 inch drill bit. The
maximum size for these holes are .250 or 1/4 inch.
For the angled holes, level the head at 10 degrees so that you will be
drilling from the intake side of the head down toward the exhaust side as
shown in the picture below.
We drill these holes on angle to reach the water jacket. If
you simply drill straight down you won't find the water jacket and the holes
will be useless.
To drill the remaining 3 holes in the head use a starter bit first
then switch to a 1/8 inch drill bit. This will make it easier to get the
holes started on angle without drill deflection. The maximum size for
these holes are .187 or 3/16 inch.
Below is a picture of the finished Chevy 350 heads with the 6 SBC 400 steam holes
drilled and a 3 angle valve job. Please keep in mind that in most cases the small block
chevy 350 heads will have a smaller combustion chamber than the 400 heads. This will
give you a higher compression ratio.
The SBC 400 engine is not a good choice for use in marine applications
unless you have a closed water system. The steam holes tend to get plugged up rather
quickly followed by a blown head gasket. If you do choose the 400 engine for marine use
then the maximum diameter steam holes may help. A better choice for use in a marine
application would be the 350 engine or 383 stroker engine.