I saw something interesting today, so I thought I'd post about it. If y'all are sick of 603 head posts, tell me and I won't do it anymore.

In my search to salvage my (cracked head) '87 SDL, I came across a '91 350SDL (300K mi. supposedly) with a #17 head and a huge pressure leak into the coolant system. The leak was so big that 3-4 cranks made tiny bubbles appear at a slightly loose hose connection at the top of the radiator. About 15 cranks put so much pressure in the coolant that taking off the cap popped like the engine was almost at running temp.(It never started or caught at all, even with 40 sec. pre-glow.) My '87 with a cracked #14 head was bad enough to hydrolock every 20-30 starts, and it used a pint or so coolant in less than an hour running, but it took awhile and was nothing as dramatic as this.

The guy was selling it as "blown head gasket," which seems to be an understatement. When I took off the head from my '87, even though I could see some small cracks near the valves, and the machine shop found the biggest crack up inside the #5 prechamber, I couldn't really identify a spot on the gasket or mating surfaces that showed where coolant was getting into the cylinder. If small cracks in a #14 head leak a pint in 40-50 miles, would you think that what I describe in a #17 head would be from a much bigger crack, or a much bigger breach in the gasket, or ??? Someone must have seen this effect before, but I hadn't.

I suppose that, depending on the location of any cracks, they may be obvious or not in the "free" state, but very different when torqued down.

I haven't been here for awhile, so I don't know about your head threads, I assume you are looking for a good, un-cracked head?

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Chief

1991 350SD

Yes, on a different thread I've been discussing. Need, but can't afford the retail price. Just thought this was an interesting thing about the pressure.