Oil return safety announcment

[Macrobb]

Wonder what the internals of that engine look like after a run away. Had to exceed well beyond 1200 degrees even if it was for a short amount of time

Probably... Just fine.
The combustion temperature exceeded 1200F, but that's just the temperature of the air. It takes time for that heat to migrate into the surrounding metal, as it's being actively cooled(heads to the coolant, pistons by jets of oil). So, even if you get a really high combustion temperature, it takes a while for it to do damage.

These motors are... very well built, as far as I can tell. They handle a *lot* of bad conditions without breaking a sweat, so to speak.

 

[saburai]

Hey Sidewinded_idi, did you get a chance to notice maximum rpm? If so, how long did you hold it? Or were you thinking of other things at the time ; )

 

[Hydro-idi]

Probably... Just fine.
The combustion temperature exceeded 1200F, but that's just the temperature of the air. It takes time for that heat to migrate into the surrounding metal, as it's being actively cooled(heads to the coolant, pistons by jets of oil). So, even if you get a really high combustion temperature, it takes a while for it to do damage.

These motors are... very well built, as far as I can tell. They handle a *lot* of bad conditions without breaking a sweat, so to speak.

IMO the lower end of these engines are sturdy. Upper end not so much. Can't say I agree with this due to my experience with em. Yes these engines can go for many miles, but these cylinder head castings are junk. They are VERY prone to cracking. Can't tell you how many sets I went thru before I got a good rebuildable pair. Maybe 4+ sets to give you an idea.
Both my rebuilt cylinder heads were junk after 30 seconds of 1500 degree heat. Magnafluxing revealed ~30 or so cracks in each head after that fiasco. I was merging onto highway not paying attention to my get gauge when this happened. N/A engine w/ baby moose pump btw.
Ended up throwing them in trash and rebuilding another set. Also had my pump re-calibrated to stock and all was well after that.

 

[Macrobb]

Both my rebuilt cylinder heads were junk after 30 seconds of 1500 degree heat. Magnafluxing revealed ~30 or so cracks in each head after that fiasco.

I hate to ask, but this isn't like the cracks in the precups, is it? Where they aren't really a problem, even though they look scary?

On my '88, after my experimental decompression gaskets started leaking a exhaust gasses into the coolant, I hauled a trailer one last time. Pulled the heads and found a crack right between exhaust and intake valves, probably from lack of cooling in that spot(EGTs were kept 1K or less, appeared within the 5K miles that the decompression gaskets were on the truck)

Decided to just use it anyway and... No problems with coolant loss or anything.

 

[Hydro-idi]

I hate to ask, but this isn't like the cracks in the precups, is it? Where they aren't really a problem, even though they look scary?

On my '88, after my experimental decompression gaskets started leaking a exhaust gasses into the coolant, I hauled a trailer one last time. Pulled the heads and found a crack right between exhaust and intake valves, probably from lack of cooling in that spot(EGTs were kept 1K or less, appeared within the 5K miles that the decompression gaskets were on the truck)

Decided to just use it anyway and... No problems with coolant loss or anything.

Yes & no. I found multiple cracks in the seats that extended into the cast along with cracks in between exhaust & intake valves.
All of the precups were also cracked & some extended past the fire ring. Wasn't too concerned about that, since it's somewhat normal.

 

[Thewespaul]

IMO the lower end of these engines are sturdy. Upper end not so much. Can't say I agree with this due to my experience with em. Yes these engines can go for many miles, but these cylinder head castings are junk

I agree and disagree, yes they are prone to cracking but that is kinda the nature of the beast with an indirect injection engine that has blocked off coolant passages, however I wouldn't say the heads are junk, these heads flow so much better than any other gasser head from the 80-90s (except maybe your gm vortec heads) the design is actually extremely good, these engines just produce a ton of heat that spikes quickly, any cast cylinder head under these conditions would crack

 

[Hydro-idi]

I agree and disagree, yes they are prone to cracking but that is kinda the nature of the beast with an indirect injection engine that has blocked off coolant passages, however I wouldn't say the heads are junk, these heads flow so much better than any other gasser head from the 80-90s (except maybe your gm vortec heads) the design is actually extremely good, these engines just produce a ton of heat that spikes quickly, any cast cylinder head under these conditions would crack

I've talked to multiple engine builders who say the exact same thing about these 6.9/7.3 idi cylinder heads. Sure the flow characteristics are great, but the casting or whatever material they used when making them wasn't the best.
My 6.9 didn't have blocked off coolant passages FYI. From what I understand, all the 7.3 idi engines had this done to help with emission standards at the time.