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I have a lead on a cheap 3.9 engine with ancillaries I was thinking about grabbing as a backup to my 4.2. From what I gather the blocks and heads should be the same. The idea here is to have a machined block and fresh heads ready to go int he case that my 4.2 takes a crap on me. What are the differences between the two engines mechanically, just the crank or rods as well? The engine is out of a '91 SWB and has 190K on the clock. This is being considered for my '93 LWB.
 

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they share the same piston bore, the 4.2 is a stroked 3.9, thus basically they are the same. since you have the time to invest on a good well done rebuild, Crower or RPI cam always awaken this units, port match the heads and cut the intake trumpets to 50%.
you will be glad you did.
 
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