The JEEP 4.0 motors have great power to torque ratio (190hp/220lbs.of 
torque) direct from the factory, but when you start modifying the vehicle 
with lift kits and bigger tires you start degrade the performance of the 
in-line six. I ran into this problem after lifting my XJ, and my ol' six 
started to lug a little trying to move the extra masses. I went looking for 
some bolt-on performance upgrades. I looked at the typical things that the 
street racers do to modify their "rides". Things I looked at: underdrive 
pulleys, airfilter/induction system, larger/bored throttle body, after 
market computer, etc. Underdrive pulleys these things work well for street 
vehicles running decent rpm's; but for my application (idling around all 
day) this would cause two possible problems 1. overheating engine due to the 
water pump low rpm's 2. a charging system that might not be capable of 
keeping up with my demands. Airfilter/Induction systems are great for road 
cars to pull in cool air; but, off-roading this could cause a problem while 
making deep water crossings (possible hydro-locking issue). Larger/ bored 
throttle body this is a great option it makes the engine a little more 
responsive but only really shows minor power gains at higher rpm levels. 
Aftermarket computers the only one available that doesn't make you break 
open the waterproof casing is the JET chip. I have heard people relate this 
chip to computer failures. The other option is to send the computer out, 
have the waterproof silicone sealant removed, have the chip custom 
programmed, and then have the sealant replaced, big $$$.

The engine had a lot of miles was showing some wear any of these bolt on 
items probably would have only masked the root problem. The Cherokee needed 
a fresh motor. Due to this being my daily driver I set out to find a 4.0 
engine that I could rebuild and drop in, keeping my transportation "down 
time" to a minimum. I started looking for options (internal) modifications 
I could do to the new engine to increase the torque output and keep the 
horsepower the same or increase it a little also. After searching for 
months from every source I'd known, I found a article about stroking a 4.0L 
to anything from a 4.5-4.9L this immediately caught my attention (more cubic 
inches means more power). There are many options as to how to do this and 
none have proven to be a better combination than the other. To put it as 
basic as possible one method is to take the crank and rods out of a 258 
(4.2L) older jeep engine and put it into the newer 242 (4.0L) block. This 
gives you almost a ½" more piston travel therefore creating more cubic inch 
displacement. There are other combinations that work also and I will give 
you the benefit of each to the best of my knowledge. Before I go into 
detail about the different combinations I will also let you know the 258 
engine had two style crankshafts. One crankshaft (older version) has 12 
counter weights and the other (newer version) has 8 counter weights. Either 
of these crankshafts is capable of doing the job for a stroker; but the 12 
counter weight shaft is more desirable if the engine rpm's will be above 
5,000 (drag racing applications).

Stroker Combo's

Combo 1: 258 (4.2L) crankshaft and rods pressed on 242 (4.0L) pistons and 
assemble engine using cam of choice, etc. The down side of this is that the 
258 crank and rods on the 242 pistons make the piston to deck clearance out 
of factory spec. The piston sits about .020" to low in the cylinder making 
the optimum "quench" ratio to large. This combo will work and a few engines 
have been built like this; the possible problem of bad emissions, 
spark-knock (possibly needing higher-octane fuel to reduce), and carbon 
build up, could occur. The good side to this type of build up is that it is 
the least expensive of all options.

Combo 2: Same set-up as combo 1, but "decking the block" shaving the 
block-to-head contact patch down to get the "quench" within factory specs. 
The down side to this is that the closer you get the piston to the head, the 
higher your compression ratio goes and your already increasing the stroke 
which is also increasing your compression. This is a popular combo that has 
also been built in to a few stroker motors. The possible side effects are 
high compression, spark-knock (possibly needing higher oct. fuel), could 
also occur. The good side is a lower cost build up that should run a 
little cleaner than combo 1.

Combo 3: 258 crankshaft and rods pressed on to custom pistons that have 
been "dished" out to keep the compression ratio down to a range that will 
allow low octane gas. The down side to this is mostly the cost to have 
custom pistons made and figuring out what piston top (unproven) design will 
suit the engine best. The good side is that the pistons can be made to any 
dimension needed keeping the compression ratio down and block decking could 
be eliminated.

Combo 4: 258 crankshaft and longer 242 rods pressed onto custom pistons 
that have been "dished" out to keep the compression ratio down and have the 
wrist pin relocated to allow for the longer rod. This combo allows for a 
better "rod ratio" keeping the piston at TDC longer to allow optimum 
combustion. The down side of this is again the price of custom pistons and 
having to choose an unproven piston top design to use. The upside is 
compression ratio of choice, better rod ratio (possibly making for a cleaner 
running engine), and block decking optional.

Combo 5: This one is not for the timid. 258 crankshaft that has an off set 
grind making the stroke even longer, custom rods and custom pistons. 
Assembled in a 242 block. I don't have any true details on this set-up so 
I can't give you any more information at this time.

The displacement is as follows.
4.0L is a stock newer straight 6 jeep engine
4.2L is a stock older straight 6 AMC/JEEP engine
4.5L is combo's 1-4 with a stock bore 4.0L block.
4.6L is combo's 1-4 with a .030" over bore 4.0L block.
4.7L is combo's 1-4 with a .060" over bore 4.0L block.
4.9L is combo 5 with a .090" over bore 4.0L block.

These are some options that are available for anyone wanting to built a 
Jeep/AMC "straight 6" stroker motor. There are also kits available that make 
a build up a little more "user friendly". If you don't already have the 
parts needed for the build up then some of these kits are very close to the 
cost of getting the pieces yourself and you don't have the added time 
involved.
Some kit sources are:
Clifford Performance
Accurate Power


I'll review the options above and decide which direction to go with my 
stroker project by next story. Also in the next writing I will be looking 
into cam shaft and fuel system modification options to accommodate the added 
CID.