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Greg's Engine & Machine

Convert SBC 350 Head for use on a 400


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.

Chevy 400 steam hole locations


1.) 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.

Marking the SBC 400 steam hole locations

2.) 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.

Drilling the straight Chevy 400 steam holes

3.) 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.

Note: 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.

Leveling the Chevy 350 head at 10 degrees

4.) 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.

Drilling the angled Chevy 400 steam holes

5.) 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.

Chevy 350 heads converted for use on a 400 block

Marine Use: 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.