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Discussion Starter · #1 · (Edited)
This is my second crack, wish I knew which brand, as these went in 200,000 miles ago, it could be any, but suspect they were purchased from GM.

I have seen as high as 21 psi at the manifold, but never higher than 15 psi since I cracked the first piston 35,000 miles ago and installed the IC. I'm now of the opinion that anything higher than 12 psi on a stock 21.3:1 compression engine is trouble.
 

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So much for cutting the tops down to lower compression. Piston just does cut it as is. 200K ain't that bad for a piston. that's a lot of miles.
 

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In this case what happens to the block, is it salvageable. Do you have to bore it out one size? Just curious....
 

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Discussion Starter · #4 · (Edited)
Nothing happened to the block, I'll find one piston of the same bore and hone it for new rings.

Long story short I'm not putting my 599 std. bore back in, I've already bolted in a .030" overbore 599 with the heads modified for lower compression.
 

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One thing to consider in the case of cracked pistons is high EGT exposure at some point in the engines life. Usually the bottom end lets go in high boost cases.
 

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Discussion Starter · #6 ·
highest EGT ever is 950*F. Bearings look good for 200K.
 

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Great. That makes me feel good.:D I hauled about #4500 to Phoenix last night and brought about #8500 pounds back with me. On the flats I was seeing 10psi with about 700-750Deg. But in the Superior mountains, which has a long unforgiving steep grade, which lasts about 25 miles, I was runnung 15-18psi and would back off the throttle just a hair when I hit 1250deg. She was most happy at 15psi and 1100deg.....but all she could do was only 45-50MPH. However, that speed is acceptable for 8500# plus.

QM, are you running a IC? I would think that one would be running on the line with these #'s I posted and maybe, just maybe, the IC would be the deciding factor? If you are running a IC, I may be asking for trouble pushing my truck to the limit while I pull. The temps were in the 60's....so there would be no way I would pull this kinda load in the summer without an IC. Good post QM....I was just contemplating these things on my trip last night.

Or, am I missing the whole point here? Doesnt matter if its a cooler 15psi...its just straight 15psi is what these pistons cant handle?
 

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My bad...you are running a IC. I missed that at first.
 

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:eek: :eek: :eek: Censored

I guess I will be doing some adjusting to the turbo master after work. That really sucks.
 

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:eek: :eek: :eek: Censored

I guess I will be doing some adjusting to the turbo master after work. That really sucks.
Thats the thing though....If I adjust my TM any for less boost, my egts Hit the roof.

I guess time will tell for me.
 

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:eek: :eek: :eek: Censored

I guess I will be doing some adjusting to the turbo master after work. That really sucks.
me too. Mine's set up at 15 psi and if I hold will go higher. Have held under heavy load on the way back from Alabama at 15psi for several minutes up long grades. I don't allow it to exceed 15, though. I think I'll back it off to 12-13 max just to be on the safe side until someone can say for sure.

My EGTs stayed below 1000 deg, though (no I/C)
 

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Long story short I'm not putting my 599 std. bore back in, I've already bolted in a .030" overbore 599 with the heads modified for lower compression.
What did you do to the heads to lower compression?
 

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Discussion Starter · #14 · (Edited)
I removed material from the precup and combustion chamber toward the valves. Nothing too serious, maybe a point of compression lower.
 

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This is my second crack, wish I knew which brand, as these went in 200,000 miles ago, it could be any, but suspect they were purchased from GM.

I have seen as high as 21 psi at the manifold, but never higher than 15 psi since I cracked the first piston 35,000 miles ago and installed the IC. I'm now of the opinion that anything higher than 12 psi on a stock 21.3:1 compression engine is trouble.
I assume this came as no real surprise, did it? Were you experimenting with boost or simply seeing what a stocker can handle?
 

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Discussion Starter · #16 ·
Not a big surprise, but I didn't suspect a piston crack 'till I saw it.
 

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QM, did you have long sustained pulls @ 15 psi.? What were your EGTs @ the time?
 

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Discussion Starter · #18 ·
Last time I pulled a trailer with a tractor on it and implements, I was seeing what EGT's would do holding a sustained 15psi. Never went over 950*F. This might have been the beginning of the crack. Last piston crack was obious, this one smaller and subtle.
 

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That's a little scary. I don't think 950 is that bad.
 

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Pre or Post?

Guys, do we have a convention here on this list? EGT probe location is PRE unless otherwise stated? Just curious, several folks mentioning the famous EGT's on this string and I cannot tell who's measuring where. Thanks!
 
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