Thread: Coolant in cylinders; fresh 400 sbc build

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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.