Re: 258 for AMC Spirit

[adh@xxxxxxxxxxxxxxxxx (Sandwich Maker)]

" From: farna@xxxxxxx
" 
" As Matt posted, only a 1971 258 (or any pre 72 AMC six) would be a problem. All others, including the 4.0L, have the same mounting points whether in a Jeep or car. There are four bolt bosses on each side of the block in a trapeziod pattern. Cars use the forward three, jeep the rear three. Just save the metal plates that bolt to the side of the block.

i was under the impression that the pre-'80 'heavyweight' sixes did
not have the mount points for the eagle front diff, which attaches
directly to the engine.

" Since you're building an engine, you might want to get a 4.0L block. It's a bigger bore than the 258, and a 258 crank can be used in it. With a 0.030" overbore and 258 crank it makes a 280 cubic inch six. You can use a 258 or 4.0L head, but the 4.0L head is the better one. When you stroke a 4.0L like this, you end up with about 9.7:1 compression using stock replacement 4.0L pistons. That's not a problem with a carburetor, but the EFI system doesn't like it much. If you plan on converting to EFI the easy way is to choose a high overlap cam that bleeds off some pressure. You can use the carb intake from a 1981 or later 258, but will need the 4.0L exhaust manifold. The older ones crack easy, but are stainless and easily welded. Keep it from cracking again by installing a muffler hange near the manifold -- the tubular manifold doesn't support the weight of the system as well as a cast iron manifold. 

speed pro replacement slugs for the 4.0 [677p] have much bigger dishes
than stock and even in a 'stroker' boost compression to only about 9
1/4 to 1.  you can run a stock 4.0 or even milder 258 cam if you want,
on pump gas.

" The only special thing needed is that the front of the 258 crank needs to be trimmed 0.30" or a thick spacer (washer) made to go between the balancer bolt and balancer to push it back further.

this is true if you have a v-belt crank.  serp belt 258s are not as
common as v-belt but do exist.

you'd also want a centered fan setup from a wrangler or grand
cherokee, not the oddball offset fan of the cherokee or comanche.

if your eagle is an automatic, you could grab both engine and tranny
from a wrangler, as even the post-'95 ones are basically electronic
tf900-series torqueflites.  other jeeps use longer 4sp autos which
wouldn't just drop in.

" 4.0L rebuilders are pretty plentiful now, and cost no more than a 258. If you don't want to use a 4.0L block, consider the head anyway. It flows much better than a 258 and is a popular Jeep upgrade. Manifold caveats listed above would apply, and there are some holes that need to be filled, but that's easy. 

and while never used in eagles, all 4.0 blocks do have the mounting
points for the eagle diff.
________________________________________________________________________
Andrew Hay                                  the genius nature
internet rambler                            is to see what all have seen
adh@xxxxxxxxxxxxxxxxx                       and think what none thought