406 Steam Hole question.

[C20-67_N_MO]

*** MODS **** I know this should be in the Engine forum but more folks look here!. Thanks.

Okay I got a friend of mine that we both bought this 406 SBC engine this past weekend. Well we were curious about the pistons if they were dome topped and what not.

After we found that out we saw that there was not steam hole drilled in the nice port 461 heads. And since he want to run Aluminum heads instead of the iron ones he decided to drill them by himself.

The deal is they do not go into a water jacket.

So my question is are they called steam holes for releasing steam only or are they suppose to have some kind of water flow on this 400 SBC engines?

Thanks for any help you can give.

[Kerry P]

I'm guessing that if you drilled into the head using a 400 head gasket as a template one half of the holes hit a water jacket and the other half did not. Some of the holes need to be drilled at a steep angle with respect to the head's surface. If you drill too deep going "normal' to the deck you can ruin the head and still will not hit a water passage.


Yes water should pass thorough the holes to eliminate steam pockets that can form in the block due to compromised water flow. This is a by product of the siamesed bores.

I have used aftermarket blocks and been told that with their extra thick decks to not add steam holes but to run them with Chevy blocks and heads.


If you search the internet you should be able to find a diagram illustrating the way to drill the holes. It has been published in a couple of performance car magazines over the years.


Kerry P

[C20-67_N_MO]

Kerry P - Thanks for your input.

I found this so hoping this will help my situtation.

http://www.gregsengine.com/convertin...ds-to-400.html

[Kerry P]

That link is better than the articles I referenced.

If you already have started holes straight down it may be difficult to now drill them at an angle. I would use a two flute end mill to create a small flat pad at the proper angle before trying to re-drill any holes with a twist drill bit.


Kerry P

[C20-67_N_MO]

Had to use the cast iron heads.

The Aluminum heads we tried to drill did not reach into the water jackets. But then again we didn't drill at the 10-30* angle either.

Thanks for all that was looking and Kerry P for trying to help. (From my home state of Arkieland!)

[Kerry P]

You should be able to make a lot of torque and decent HP with the ported 461s.

How big is the cam?

Kerry P

[C20-67_N_MO]

With the 1.6 ratio rocker arms the machine shop is saying .580/.580 on a .525 true lift cam.

Which I am not beleiving in those numbers. From .525 to .580 on just .5 additional lift?

They were saying this engine should be making 500hp easy but we'll see. Going to fire it up this evening.

[Kerry P]

Switching from 1.5 to 1.6 rockers should change the .525" lift to .560".

You might want to check your coil bind and seal to guide clearance. Also big cams with high ratio rockers can get you into valve float a lot earlier than you might expect. If your on a dyno you should be able to see the curve make a sudden transition indicating float.

Sounds like fun. I'm jealous. My '70 long bed is getting a low mileage (and low power) L31. About half your HP.

Kerry P

[capev86]

the steam holes are what is often referred to as "cross drilling." the 400 small block shares the same bore center to center spacing as the rest of the old school (55-95) small block chevy v8's. the bore is so big that there is no room for the water jacket on the middle two cylinders on each side. gm realized that this could result in quick to overheat conditions. many engines with Siamese cylinders employ cross drilling to help draw heat away from the bores.

in 1995, my dad picked up a '77 400 that had 10,000 miles on it since a rebuild that included a bore out to a 406. it had a few other minor performance mods. we put it in place of a 350 in an 84 burb c10. with a couple local test drives hauling our 6000 pound camper we cracked the cylinder heads. turned out the block was cross drilled but the heads were not (likely the factory had been using leftover stock) and the extra load/pressure generated by towing did them in. with the correct iron heads on we towed our camper out to yellowstone and back on a month long trip and that engine with the stock 350 tune q-jet did not miss a beat.

obviously the 383 (or 377 w/ the stock 350 cid bore) is a better bang for the buck as it gives you the torque potential of that stroker crank but with the fuel economy of smaller pistons and better cylinder cooling. but the 400 is definitely a hot piece if you can get one. the mpg only dropped from like 17/18 to 15/16. not too bad for big block like muscle. there are aftermarket companies now making super big bore super long stroke small blocks based on the old school and LS series engines. but they are super pricy as you'd expect.

[C20-67_N_MO]

Well we didn't quite get it fired up tonight.

This is going in a 1967 RS Camaro, and not my truck. :-(

As my truck is about 2 years from being completed.

And I made a boo boo.....the 1.5 to 1.6 would only be .1 increase in lift not .5 as I stated. And .560 lift sounds closer than the .580 that the machine shop had stated. And with the 13:1 compression .100 dome top pistons this thing will either blow or go.

Only time will tell at this point.

Yeah I love the 400 small blocks but never had to deal with these steam holes. So we are just hoping that the block is not cracked from not having them drilled in the head from the PO.

More later......

[Kerry P]

Originally Posted by C20-67_N_MO

Well we didn't quite get it fired up tonight.

This is going in a 1967 RS Camaro, and not my truck. :-(

As my truck is about 2 years from being completed.

And I made a boo boo.....the 1.5 to 1.6 would only be .1 increase in lift not .5 as I stated. And .560 lift sounds closer than the .580 that the machine shop had stated. And with the 13:1 compression .100 dome top pistons this thing will either blow or go.

Only time will tell at this point.

Yeah I love the 400 small blocks but never had to deal with these steam holes. So we are just hoping that the block is not cracked from not having them drilled in the head from the PO.

More later......

'67 Camaro, that's funny. I'm building a 406 for my '68 Camaro.

If your 406 was primarily from a high performance application you're probably OK from a crack standpoint. One of the aftermarket block makers told me that the whole steam hole issue is all about low rpm problems. They said that over 3000 rpm there's no issue even with a GM 400.

Good luck,

Kerry P

[C20-67_N_MO]

My buddy installed a nice solid lift flat tappet camshaft and switch to the 3/8" studs and the 1.5 ratio rocker arms. Nothing too radical but keeps breaking the street tires loose all thru 3rd gear. After we discovered the previous cam had a flatten lobe or two and we initially thought we have a bad carburator.

Now are quest is to find a very GOOD TIG welder to fix the aluminum heads so we can just sell them and recope some of my money.

LESSON LEARNED.

Take it to a professional and let them have the liability issue!

[Alex Hayley]

What are the specs on the new cam?

[C20-67_N_MO]

Originally Posted by Alex Hayley

What are the specs on the new cam?

Elgin Pro Stock E-1090-P

.050 256 intake 264 exhaust

Valve Lift
.538 intake .556 exhaust

Lobe Centers

99* Int. 111* Exh.