350 heads on sb 400

[JACKIE FOSTER]

how do you go about putting a set of 350 heads on a sb400 w/out drilling out the steam holes are can this be done w/out any problems down the build and will it run hot?

[Restobutcher]

Jackie,
The deal is that the 400CID small block has siamesed cylinder bores (where the cylinder partition is very very narrow). These blocks require special steam vents that pass through to the cylinder head to avoid the possibility of hot pockets forming at the top of the block in the junction area between cylinders where it meets the head. It's actually about flow...as coolant is scavenged through the heads at a fairly high rate, this flow tends to draw the coolant into the stream as it enters from the top of the [siamesed] cylinders.

My feeling is that many of the folks that build 400's do so for drag and circle track applications where they destroke it to around 377 and operate it at sustained high RPM. I believe that these are the primary applications where cooling would be an issue. Either way, I wouldn't risk it by ignoring the steam holes that are easily machined into the 350 heads.

If I were attacking the steam hole task, I would approach it by first installing 4 locator studs in the block at the approximate 4 corners of the head (these can be 8" hardware store bolts with the heads zipped off). Now, swipe some Play Doh from your favorite kid and make some butter patty sized pieces. Place these pieces on the head in approximately the area of the steam holes in the block (there will be 3 on either side). Lightly oil the exposed clay to prevent sticking. With the clay in place, gently lower the head down over the studs and press just hard enough to put "tell-tales" in the clay where the steam holes are located. Remove the head and your exact machining locations will be copied onto the clay. Press the tip of a pointed punch through the clay in the center of each hole and tap with a hammer to make your drill centers.

Next, take one of your new gaskets and hold it up to the head to verify that the steam hole locations are correct. Support the head on a drill press and machine each of the holes, punching through just into the water passage. Finish up with a die grinder to make a nice little .015" chamfer on each bolt hole...problem solved.

The Butcher

[TANGO]

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 1/8 inch 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 at a 40 degree angle towards the exhaust side of the head. Drill the 3 remaining holes straight down towards the top of the head.

PROCEDURE

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.

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.

3. For the angled holes, level the head at 40 degrees so that you will be drilling from the intake side of the head down toward the exhaust side

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 .156 or 5/32 inch.


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.

[junkcollector]

You have to have the steam holes because of the siameese cyliners, they are not optional.