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4.2 crankshaft

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idrambler View Drop Down
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    Posted: Sep/06/2018 at 1:29pm
I have an 87 4.2/258 from a 87 Eagle...may have to replace the crankshaft....what years will work????...
thanks........
Jim....AMCRC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/06/2018 at 2:52pm
So far as I'm aware, any 82 and up crank will work, as you have the lightweight crank, which I think 82 was the 1st year for.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote matty 401 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/06/2018 at 3:20pm
i believe any 258 crank 72 up should work  they have different counter weights but i think they all inter change 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Sep/06/2018 at 3:44pm
I believe all 4.2 cranks interchange, and do not need to be re-balanced when they are replaced. 

The earlier cranks with 12 counterweights run smoother, are stronger, and result in longer bearing life, but they are heavy. The later cranks with 4 counterweights run rougher, are weaker, and reduce bearing life, but they are lighter. 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/09/2018 at 7:58am
All 199/232/258/4.0L cranks will physically interchange. You need to match the rods and pistons with the stroke of the crank though. In your case, any 72+ crank will work. The crankshaft flange was changed in 72 along with the change from Borg-Warner to Chrysler automatic transmissions. In mid 1980 (late 1980 models, all 81+) the 258 was lightened and a new lighter crank was introduced. I'd match the balancer with the crank (72-80, 81-90), but I'm not certain that is necessary. All 258 rods are the same, I think, but the earlier set might be a bit heavier than later. Check total weight -- you don't want to mix weights of rods and pistons. If you use your old set or the set from the crank donor you should be fine.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/09/2018 at 9:07am
One more thing... a simple balance job can be performed on the six because the crank is internally balanced. Check the weight of all pistons (with pins and rings) on an accurate scale. A digital postal scale is good enough. Weigh all six, then grind a little off the flats of the pin bosses under the piston untill all are close to the lightest one. Good cast pistons might be within a tenth or two already, in which case I'd leave them alone.

Next weight EACH END of the rod.You do that by supporting one end and setting the other end on the scale. Do this with bearing ans nuts on the rod. and of course the cap. Run the nuts up finger tight and the cap about where it should be when installed. Little end is easy -- scales are usually short enough that putting the rod on it's side with little end on scale will work great. Weigh all, find lightest, grind a little at a time off the top flat until all are close. For the big end take a 2x4 block and drive a nail in it that will hole the little end high enough the rod is about parallel with the bench top when on the scale. Take a little off the flat on the rod cap.

A machine shop can get the crank a bit closer, but for a typical mild street build this is "close enough". Doing a V-8 like that is helpful, but the crank itself isn't naturally balanced like a straight six. The only engine more balanced is any horizontally opposed engine with an even number of cylinders. They have pistons directly opposite each other, even a four will run smooth. V engines are the hardest to balance... along with I4s. Well, odd numbers of cylinders are hardest, but few of those made. Two strokes don't count -- they fire every time the piston is up, so a two stroke three is almost as balanced as a four stroke six.
Frank Swygert
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