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We don't see a crack in blocks all that often but the #4 cylinder would be one area where it can happen. That cylinder gets the hottest. Sometimes we see a failed piston cooling nozzles there which often results in a rod sticking out the side of the block. That crack appears quite long and not sure how deep it goes.
 

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Your engine is a bit older than you think. It is a very early one produced March 14 1985 at the Consolidated Diesel plant in Rocky Mt. NC here in the USA. It is a commercial model BT3.9-C100. Now, as for the engine block, this is going to be a guessing game because Cummins no longer shows a replacement block for that engine. If you look at the turbo side of the block, do you see 2 large freeze plugs to the left of the oil filter? That is normal for the early style blocks. That changed in the1990's to a new block called STORM. On those blocks you will find 1 large freeze plug and 1 smaller screw in type plug where the block heater would be fitted. I looked up an engine built in 1987 which should have the same block as yours and it shows the following part numbers. Starting with the newest which is the one that is current they are 5449532, 3934565, 3929045, 3928794, and 3916251. The part numbers for the STORM blocks are 3933224, 5405080, 3938366, and 3932012. This notes this is for all 4bt engines 1991 and newer. The block number 4991816 you list shows to be a non sellable item in the Cummins system. Now what does all this mean. Well, first, if you planned on a block heater and used the newer style block you'd need an updated version. Second, STORM blocks handle the oil filtering differently so you would need a different oil filter housing, oil filter gasket, and possibly a new oil pump. STORM blocks circulate oil at a higher rate than most older styles. All this being said, this is an I don't know for sure situation. You need to contact Cummins. Give them your engine serial number (ESN) and tell them the situation. They will tell you the correct replacement block. You can ask them about the block part number you found. That may be a foundry casting number and not a part number. Also ask them about the oil filter mount, gasket, and oil pump as to whether they have to be changed. If you can't work it out I'll go through my Quick Serve and ask the questions. I don't mind doing that.
 
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