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Are all 258's equal. YJ 258 into a 84 CJ7

853 views 3 replies 2 participants last post by  dirtdudeaz 
#1 ·
I have a 84 CJ7 with the original 258 with 2150 carb, nuttered and automatic transmission running 33’s. I have 3.73 gears and would like to get a bit more oomph out of this situation – not off the charts but some more giddy up. I prefer to stay with the 258. Additionally, I have some blue smoke at startup and I think is a valve seal which made me think of doing the 4.0 head upgrade to gain the additional HP. Then, I picked up a rebuilt YJ 258 that has about 10-15K miles on it and believe has all faulty valve seals as the plugs are all fouled with ash. I have not heard it run as I got this engine for free and have little information on it as to what the rebuilt consisted of. My thinking is while I am running on my original 258, I take the YJ 258 to a machine shop, have them rip it apart and inspect and give me a report on what was done and put a 4.0 head on it then. Possible some additional machining to get some more HP if it hasn’t already been done. When the engine is done drop the engine into the CJ7. So, are all 258’s the same? I would hate to spend all of this time and money to find out I have opened a can of worms. So is there anything I need to look for to make sure this is a 100% drop in candidate?


Block Numbers:
Passenger Side #’s 801C29
Drivers Side: EF3235444
324. 87

Thanks!
 
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#2 ·
There were some slight differences between early 258's and the later ones, but yours being later with an 84 and the yj, they would be the same as far as bolting one in place of the other. If you plan on swapping accessories, I would ensure the belt rotation is the same (both v-belt or both serpentine), or get a different water pump since the rotation would be different. Also your head bolt diameters are different. Late Cj models were 7/16 and the YJ's switched back to 1/2", but this would be good for the 4.0 head swap since those are 1/2" as well.

Just curious, what kind of driving do you normally do? I am curious if 4.10 gearing could help give you more oomph as well? 3.73 maybe perfect for highway and streetability, but the 4.10 could be better suited off road.

I am running a 4 speed manual with 4.10 and 32's, and I love the way it takes off. I am also currently doing a 4.0 head swap and EFI, and I hope the little bump in power will help!
 
#3 ·
Most of our driving is around town driving with an occasional highway thrown in. Some dirt roads, but very rarely off road. She is off the road in the winter. Thanks for the info on the 258 rotation. That rings a bell with me because my original 258 has factory air and although it sounds crazy to have in a jeep with the top off I have used it when its 90+ and in stop and go traffic and it was a godsend. So I need to see if all of the a/c brackets etc will bolt up. The YJ engine is missing a few pieces like the fan and pulley so is there an easy way to figure out the rotation?
 
#4 ·
So it sounds like your gearing is probably best for what driving you do then; otherwise, you might be screaming down the highway doing 60 with any lower gears.

I think your best would be to do a 4.0 head and add fuel injection, maybe a nice healthy cam. Your other option would be to build a stroker motor.

As far as rotation goes, I am not sure the best way. I think the pulleys would be the only thing I can think of, of if you have any belts on it already. Otherwise, maybe pull the water pump and see what direction the vanes are going.
 
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