Yup,,,,dropped a seat,,,,, and you can blame the new ethanol fuel,,,,, I bet you fuel up on land right???.

everyone should bump up a jet or to that fills up on land

I was a little concerned about the marks on the heads and block in between cylinders on a 9 month old rebuild. A little history from the original rebuild;

Block was square decked .015
Heads were milled .020
Gaskets selected were Fel-Pro MLS #1075's. .041 compressed height.

I decided to measure the gasket thickness after I pulled the heads. Here is what I found;









Gasket thickness at the perimeter, top and bottom all measured approx .045. An understandable # as the gasket is not compressed.

The disturbing part was the .035, and .037 #'s taken between cylinders. This was consistent in all cylinders, both gaskets. If the comp height is .041, which I did verify when I originally built the motor by putting .041 worth of feeler guages in between the head and block.

Any Fel-Pro guru's care to comment?

What's a 'sort of HP500' ?

Please give us the following info:

What cid ? What cam + specs ? Compression ? Intake ?
Short or tall Gill Risers ? Idle rpm ? Initial / total timing ? Did engine crank for a long time before it started right before this issue ?

I've been reading and re-reading this thread carefully and I am trying to get my head around everything going on here.
First, you spent $9K rebuilding this puppy last year? Wow that a big piece of change for just a rebuild! Please break that down a little so I can understand all you did costing $9K
Next, you said you decked the block.015", wow thats a lot and how did you restore your piston deck height after that? What did it measure?
Next you also cut .020" off the head and you were using an .041 MLS Fel Pro head gasket. depending on your deck height with that piston and bore you may have had some serious compression there?
Looking at the head gasket area between the cylinders there appears to be evidence of a blown head gasket. Since the deck was cut (Hopefully even front to rear)) and the head was surfaced there should have not been a head gasket sealing problem from that. The valve seat trying to compress in the cylinder probably lifted the head enough to blow out the narrow area of the head gasket there.
You said you where running the engine on the hose at idle or close (no real high rpm pulls?) and all of a sudden the engine started knocking badly? How long was it running before the EVENT?
Looking at the underside of that #7 piston tells me that that cylinder was having problems before this mechanical failure. that burned oil residue means the top of that piston was getting really hot compared to other pistons shown.
I would suspect this engine had some problems developing before you ran it in the driveway, most likely going on for a while during its last major uses. Possibly real high cylinder temps from lean burning, detonating and maybe cracking the head and allow water into the cylinder before this failure.
I would guess at this point that this engine was going south before you ran it on the hose and it just let go on that run. That was good in a way as if it had let go at high rpms under load the damage would be much greater than you saw here. How long did this engine run after letting go, before you could get to the helm and kill the engine or did it just die on its own?
In any event I am sorry for the damage and loss here and I would hope your next engine will run longer than 40 hours without failing.
Make sure you mag the heads and block before resuing them, I would guess you are going to find that head is cracked. Obviously make sure all the seats and guides are checked and repaired as needed.
The real key here is to find the real underlying cause of this failure because these broken and damaged parts may not be the cause but the affect!

Best Regards,
Ray @ Raylar

Gil manifolds and risers (short ones, yeah I know). This was originally a 1996 HP500, carbureted. Here's the "Sort Of"

509 ci GM Block
SRP's 9:1 comp
Tq plate and Plateau honed and Decked .015
GM forged crank, Dimple rods, 7/16arp's
.007 piston/cyl wall clr, .0022 Rods, .0025 mains, .0030 rear
088 heads, Bowl work, SS radius, sleeved for 11/32 valves, full radius exh, 5 angle int. REV valves
Heads milled .020, 121CC, flowed 356@700, 242@700
Hyd roller (Morels), Cam (224/650, 235/603). 112 Lobe sep, 180# on the seat @1.95
Crane Gold rollers
Dart single plane, 830 HP carb.
Idled 900rpm, 12 deg initial, 37deg total

Cranking time was not the issue..

I still can't see $9K with those SORT OF's unless that included buying the HP500 and all?
What was the measured deck height of the pistons after assembly?
Those heads are normally about 119cc and milling them ..020" would reduce that a good bit.
Just trying to help, not trying to be an antagonist.

Best Regards,
Ray @ Raylar

Curious why you say I have a lot of compression?

eliminator28,i know you are not wanting to put a ton of money back in this engine,but i highly recommend that you do not attempt to reuse the bad head,you will be money ahead if you replace the heads,if you can swing it,i recomend the edelbrock rpm marine heads,rect port,next option a cheaper cast iorn head,several good brands out there to choose from ,also,on every bbc engine i have ever put on a dyno,cyl 7 always has the hottest egt,s.good luck with the rebuild.

All good input for sure and I agree. Particularly about going to the edelbrock heads and your heads being cracked, btdt. I also agree with the head being lifted by the shrapnel in the cylinder causing the near blown head gaskets. I still see no smoking gun though and am wondering if maybe your plug wires were cross firing? 5/7 crossfiring would cause pre ignition in #7 and big trouble like you have. Just a thought. Would hate to see a repeat.

That 5/7 combo again..