Thread: Coolant in cylinders; fresh 400 sbc build

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Bolts go into water jackets, so that is normal. thats why you always put sealer on all head bolts on a sbc

462624 1976-1979 350 with 75.5 cc chambers

Unfortunately, the 624 casting are probably the worst of the GM castings for cracking. They are prone to cracking across the exhaust seats and thru to the heat crossover. They’re not good candidates for any decking either.

Mortec says they are from a 75-86 350 or 400, 76cc chamber. They were manufactured with 1.72/1.5, 1.94/1.5 or 2.02/1.6 valves

If they have the 1.72/1.5 or the 2.02/1.6 valves and they are perfect – they may be worth drilling steam holes. If they are the small valves or (as is very likely) show any cracks, I’d start looking for new heads.

If you have any budget, the Vortecs are a good way to go. This means a new intake manifold as well, but will definitely bring the 400 alive. GM Part Number 25534421 ($1100 a set or less on ebay) is a great head, 185cc runners and 66cc chambers. They’ll need steam holes drilled.

Good Luck!
Glenn

We really need to know more information about the block and pistons before you jump and buy different heads. What is the piston crown configuration? Can you post a pic? Is there a part number stamped on the top of the piston? With the piston at TDC, how far is it down in the bore? You can measure this with a set of feeler gauges and a straightedge ruler. What I'm getting at is that you have a fairly short cam in the motor now and with the large chambers of the heads you have now, the static compression ratio works out ok on pump gas. But if you go with smaller chambers, you may exceed the match-up of static compression ratio with the cam timing you have now and you'll be tearing back into the motor to change the cam. Let's try to make this work the first time. Post the info I asked for.
You mentioned going with 305 heads. I can't think of a worse combination than 58cc heads on a 406 with a short cam and unknown squish. Detonation City.

With the short block on a stand, get a buddy to stand in front of the motor and turn the crank with a socket and long bar on the harmonic damper retaining bolt head. This bolt is torqued to 65 ft/lbs, so it will not unwind if you turn the crank backwards easily. Roll the crank around so that the #1 piston (driver's side front) is at approximately top dead center. You will be standing at the side of the block at #1. Stand your ruler up on edge and span across the cylinder bore about a half inch from the edge of the bore. You will place the ruler at either 3 oclock or 9 oclock position. Placing it at noon or 6 oclock will allow the piston to rock on the wrist pin and give you an erroneous reading. Start with 0.025" or so and slide the gauge blades under the ruler and all the way to the cylinder wall. Have your buddy rock the crank back and forth so that the piston is coming up to TDC and back down in the bore just ever so slightly. If the piston doesn't push the ruler off the deck, then use more feeler gauge blades and try again. Maybe go 0.005" at a time until you find how far the piston is down in the hole at TDC. This is called the piston deck height. We must know this dimension before going any further. And don't forget the pic of the piston crown.

Originally Posted by glennsexton

Unfortunately, the 624 casting are probably the worst of the GM castings for cracking. They are prone to cracking across the exhaust seats and thru to the heat crossover. They’re not good candidates for any decking either.

Mortec says they are from a 75-86 350 or 400, 76cc chamber. They were manufactured with 1.72/1.5, 1.94/1.5 or 2.02/1.6 valves

If they have the 1.72/1.5 or the 2.02/1.6 valves and they are perfect – they may be worth drilling steam holes. If they are the small valves or (as is very likely) show any cracks, I’d start looking for new heads.

If you have any budget, the Vortecs are a good way to go. This means a new intake manifold as well, but will definitely bring the 400 alive. GM Part Number 25534421 ($1100 a set or less on ebay) is a great head, 185cc runners and 66cc chambers. They’ll need steam holes drilled.

Good Luck!
Glenn

I made another post but I guess it evaporated. Anyway, all I know about the heads is that they have been machined and have new valves so there should not be any cracks. I'll measure the valves tomorrow when I drill the holes. Thankx

Originally Posted by Bigbzc

I made another post but I guess it evaporated. Anyway, all I know about the heads is that they have been machined and have new valves so there should not be any cracks. I'll measure the valves tomorrow when I drill the holes. Thankx

STOP! Don't do anything else until you have read and understood post #45.

Yup - what Tech said - I was a bit premature in a head recommendation. It was my mistake to "assume" stock pistons and deck height. We might still get to the same place (i.e., Vortec heads) but it is really important to get the details of the PDH and squish.

I don't plan on using different heads as I am out of money for this project. I will drill the 350 heads I have tomorrow and hope to have them in by the weekend depending on the weather

make sure you use the 400 gaskets

Originally Posted by cffisher

make sure you use the 400 gaskets

Yes sir! lol Gottem loaded in the truck to take to work tomorrow. I was hoping to see "paths" on the head or gasket from where the water leaking but there were none. I guess this could be because they were not on there very long. I do see the point that it CAN leak because there is not much of a seal in the steam holes of the gasket

after looking at the gaskets carefully I noticed that there were no traces of water from the steam holes. What I did discover is that the cylinder seal of my gaskets are overlapping the coolant ports that are just beneath the spark plugs. So, now what?

I think I figured it out. Measured the diameter of the bore on the gaskets and it measures 4.180" This block has NOT been bored and it's diameter is still 4.125" Looks like they gave me the wrong gaskets. Let me know if I am correct

No--gaskets are correct---the heads are wrong

Can you take a couple close ups of the head and post them? Is either a bad picture or something nasty happened in the machine shop.

Originally Posted by glennsexton

Can you take a couple close ups of the head and post them? Is either a bad picture or something nasty happened in the machine shop.

I was thinking that they shaved to much off the heads. The pics I posted are not bad, what yo see is what it is. See the notch in the coolant port in the 1st pic? That is where the biggest leak is but ports on other cylinders over lap the compression ring on the gasket and leak into the cylinder.