Baffling big block

[Chicken lights]

This thing is cursed.

Long story short my friend started working at a GM dealership in the 60’s, worked at various GM dealerships until early 2000s, has built race car motors in his spare time, built many many engines in the last 50 years for projects and other people.

He’s been around, in other words

So, he’s building this for one of his cars. I’ve NEVER seen him take apart a motor he’s just finished putting together.

First time around the oil pump was junk, wiped the camshaft, total tear down and rebuild. Second time around everything was good except apparently it was overheating.

It’s not overheating. Coolant is coming out the radiator on the test stand, from where the radiator cap goes

We got it fired up today, yet within 30 seconds of running coolant was coming out of the rad. It’s not heat related

 

[Chicken lights]

So, around 1987-1988 Chevy went to a serpentine belt setup. I’m not sure of the exact year but around that time they reversed the flow of the impeller on the water pump

I’m betting the water pump is wrong and forcing coolant to flow the wrong way.

We tore the whole top end off today, there was nothing glaringly wrong. We’re talking large volume of coolant coming out the radiator the wrong way.

We pressure tested each cylinder. We did compression tests on each cylinder. With the motor together we used a heat gun on each exhaust port. One cylinder stood out as being cold yet the other seven were hot, but, that shouldn’t cause coolant to back flow

We even went as far as to pour water in the top rad hose and watch it come out the lower rad hose, to make sure of no blockages there

There is NO cracks or broken parts allowing compression to interact with coolant to pressurize the rad. All the gaskets looked fine.

My friend is gonna order new head gaskets tomorrow. The plan is put it back together, with a different water pump on Saturday

It’s completely baffling

We’re out of ideas and grasping at straws

 

[Chicken lights]

Well, maybe I found something. I grabbed a straightedge and feeler gauges from my shop today. There’s at least 0.003” clearance in one spot on one head, and 0.005” clearance on the other head. That’s quite a bit, I’d say he needs to have the heads machined flat.
As far as I can remember you place the straight edge corner to corner, then straight up and down, then the other corner to corner. If the feeler gauge goes under the straightedge you’ve got issues

 

[Hruul]

From what I recall how you have measured the heads sounds correct. A quick call to a engine machine shop would make sure.

 

[Chicken lights]

Apparently 0.006” is the tolerance on an iron head for a big block. He had the heads on before I could get there. Finished putting it all back together Saturday afternoon. No coolant and no belt we started it up. Oh and we put on a used GM water pump from the ‘70s.

Ran good, rad felt like it was pulling a vacuum instead of building pressure. We put coolant in it, ran it, still no problems. Put the belt on. Still ran good. Put the rad cap on and let it run

We’re blaming the new water pump turning the wrong way.