The outer casting of the block is only 6mm thick on the original #53 block.This block seemed to work fine in 12V trucks,but for some reason ,the 24Vs resonance,and harmonics,combined with heat/cooling of towing heavy tend to crack some of them.The later blocks were thickened to 12MM,to eliminate the potential problem. Not all 53 blocks will crack,mine in my 2000 had 100K hard miles,most over 500Hp,som over 650RWHP,with N20,and very heavy towing,and it is perfect,no problems at all.The Killer of 53 blocks is not big HP,but getting the temp up (usually )towing heavy,topping a hill,then cooling to 180,on the way down,repeat that a few thousand times,and that is what seems to aggravate the cracking problem.
Some 53 blocks are alright, the good ones have a very smooth casting. The bad ones have a rougher look to them. To see this you need to look at the area below the freeze plugs where the block has a radius. If your block looks very smooth in this area you may be safe. My 53 cracked on my 99 truck. There are some pics of what I'm trying to explain on the TDR or DTR you could do a search to find this. Stamey has posted quite abit about the 53 block issue on one of those sites. Piers posted pics of the way the casting look good vs bad. Just my .02cents. Thanks Steve B
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