Drilling steam holes in smallblock Chevy heads
The SBC 400 block was designed with steam holes in the deck and heads. The steam holes were designed to rid the cooling system of steam caused by localized heating under some conditions due to the 400 block having siamesed cylinders (meaning the cylinder's OD's are physically connected to each other due to the large bore size combined with the block having the same bore spacing as the 1955 SBC 265 cid engine).
SBC heads with steam holes can be used on other blocks without problem by using a 350 head gasket.
 How to drill a head for steam holes
Heads without steam holes can be drilled for them by using the correct 400 head gasket with steam holes as a template.
- 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.
- 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.
- 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 or sliding. The maximum size for these holes is 0.187" or 3/16".
- Use a sharp bit and take your time.
- Clean off the heads when done.
The steam holes can be done by hand if attention is paid to angling the drill bit on the three angled holes on the intake side, and a starter bit is used to to start the hole straight down to make a small indentation to use to keep the bit from sliding as you lay the drill over to the correct angle, as mentioned above. The three holes that are drilled straight down are not a problem. Steam holes can also be made using a drill press equipped with an adjustable table.
 Cooling system mod
If the steam holes are drilled or not, a modification to the cooling system that can help the efficiency overall is to connect the port at the top of the water pump to a rear water port on the intake manifold. Not all intakes have a water port in that location. But on many intake manifolds a pipe threaded port can be added.
 Are steam holes needed?
There are quite a few builders who claim the holes are not needed on an engine that will see "normal" duty, i.e. not idled for extended periods like a delivery truck, for example. Among them is noted engine builder Joe Sherman (of Joe Sherman Racing) who says, "I have built hundreds of 400 engines, WITHOUT the steam holes, and never had a problem. Guys used to send me their heads for me to drill the holes, because I only charged 80 dollars to do the job, but I never did it for my own stuff."