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Just Using a Jeep 258 Crankshaft with a 4.0L Jeep Inline Six Rebuild?

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Been talking about stroking the 242 block I got last year for a while now, but I got tied up with building my XJ and fixing some suspension "over build" (it had 9" of lift) issues with the TJ I bought last summer. Also, got on the idea of a SBC or SBF swap for a while. Now that I've pretty much tapped my "Jeep savings", I'm back to building the extra 4.0L with money being tight for the project.


I read in Moses' articles that just adding the 258 crankshaft with a 242 rebuild will also do wonders. Will I really see any reasonable gains or should I just put the money elsewhere instead of getting the 258 crank? I want to keep the build below $2k. That should be enough for machining, master rebuild kit, valvetrain, etc. Also, I would like to throw a clutch in there too.


I scraped pretty much everything from the 242 tear down, but the block, crank, and the head is bare. The motor was in a salvage yard and the valves were rusty.




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Hi, Wayman, great to hear your plans...I'm not sure what machining and parts cost in your neighborhood, but let me emphasize this: It will cost almost the same to rebuild a Jeep 4.0L into a 4.6L stroker as it will to rebuild a stock 4.0L inline six thoroughly.


That said, the added cost, at bare bones budget, will be the pistons and 258 crankshaft core or casting.  You need the right pistons for the stroker, or you will not achieve proper piston height in the cylinders.  On the piston side, if you use a hypereutectic type at 8.7:1 static compression with a zero-deck, the cost difference will be minimal.  It will include block decking as part of the machining process to meet piston/deck height match.  In my Hewes Performance video interviews, you will hear Tony Hewes talk about the rod and crankshaft matches and which pistons the different approaches require: http://www.4wdmechanix.com/HD-Videos-Building-a-4.6L-Jeep-Inline-Six-Stroker-Motor.html.


On a very low budget, conceivably, you could get by with a 258 crankshaft (serpentine belt short snout version is easier and popular, saves cost here) and connecting rods, bearings, a timing chain set with sprockets, a new camshaft and lifters, pistons/pins and rings, valves and valve springs with retainers and keepers, a complete overhaul gasket set (Felpro simplifies here), and a new oil pump and screen (Melling high volume).  Hot tanking is a must, and this means new camshaft bearings, too.  Freeze plugs, a water pump and oil filter round this out.  This is a minimal parts list.


The minimal machining list after the hot tanking and camshaft bearing installation would be the cylinder head valve seat grinding, head decking, valve guide work (silicone bronze liners, minimally), block decking as needed for the stroker pistons, connecting rod and crankshaft reconditioning (as required) and piston fitting.  Boring and honing the cylinders and line boring the crankshaft centerline is typical fare for a quality rebuild.  Balancing reciprocating parts, if affordable, is a desirable add-on to the rebuild. 


You mention a new clutch, and you should also have the flywheel resurfaced (if acceptable for this flywheel's design and condition, provide me with details on the year and application flywheel, I'll comment back) or replace it with a new one.  As a reciprocally moving part, the flywheel would be among the balancing pieces...A crankshaft pilot bearing is required, one that will work with your later transmission and the 258 crankshaft.


Price machining locally, shop online for best buys on rebuild kits and pistons, etc.  Have your block and head casting assessed by a reputable machine shop before plunging.  Do you have a quality 258 crankshaft core already?  Make sure the crankshaft will turn and polish at 0.010" or 0.020" undersize on the rods and mains, optimally 0.010"/0.010".  If in a real pinch, and with a rougher crank core, 0.030" undersize is not terrible for an engine that will see reasonable use and not be desert racing.  Block wise, a 0.030" oversize is acceptable, 0.040" would be on the edge, and 0.060" oversize for a Jeep 4.0L is more than I would want for proper cooling.


Share your findings.  I'll comment objectively, I've been rebuilding engines professionally for 45 years and know what costs are reasonable.  I am happy to comment and encourage other members to jump in here with helpful suggestions and experiences with a low-dollar stroker motor rebuild!



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  • 4 months later...

I had the same deliberation as WMCCALL this summer. I decided to go with the stroker build so that I wouldn't hate myself later for not spending the few extra bucks later on. I only drove mine for a few months before I did some other work to my Jeep which has left it with limited driveability for awhile. Here was my impression of it in that time. 


The first drive of it is pretty disappointing. Driving around town in the lower RPMs it is essentially the same as the stock 4.0. Don't expect a beast of an engine. I opted to stay with the stock injectors, but I was surprised that my mileage stayed the same.  For some reason I was thinking more displacement = more gas consumption.  Talked with the machinist that did my work and he said putting a larger injectors, a big bore throttle body, and re-sizing your valves adds another bit of power across the RPM band. I just didn't want to add another $700-1000 for that at the time. It ran fine on 87 fuel. I was advised to use 91 once it got cold out though.


When I noticed the difference is when I got my engine up in the 4-5K RPM range. Given the only times I get there is hopping over an object on a trail, a hill climb, or trying to spin the tires, I notice quite a bit of difference in the higher RPMs. I could run up and through things that before would bog my engine down to the point of almost stalling before. 


My machinist did my stroker for about $700 more than a standard rebuild with a bore out would have cost. That extra money was for the crank, a little extra machine work, and rebuilding my heads with stronger springs. If you do decide to go this route, make sure your machinist shortens the nose of your crank or machines you a spacer. This caused me problems when my machinist forgot to mention that and I noticed that the harmonic balancer wasn't lining up. The 258 crank nose is about 1cm too long.


I've had second thoughts about doing the stroker build, but with some more upgrades I will get some extra power to play with and have all my stock equipment (including the a/c I installed before the rebuild) work with the engine I built. If you don't care about having your stock equipment work and are just on the hunt for more power I would go for the SBC conversion. I look forward to doing one of those one day, but the one I built spends a lot of miles in town too.



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Umm, interesting...The most notable difference, for me, with the stroker is the bottom end, low speed response.  I did a lot of research with Tony Hewes on the tune for these engines, and ultimately narrowed the injector change for your 1998 (pre-coil-on-plug, you have the distributor) to a simple Ford 302 5.0L injector that can be found at a reasonable cost in rebuilt form.  See my article on the 4.6L inline Jeep six-cylinder stroker motor tuning for more details, your performance gains would shift with the injector change, though you likely would not see any difference at lower speeds.


Thanks for bringing up the crankshaft snout length issue.  We mention the crankshaft snout length needs in the Tony Hewes video interviews, HESCO offers a special step-washer and longer hub bolt to bridge the length issue on longer snout crankshafts.  Some opt for machining the snout to correct length for the serpentine belt pulley/damper. 


Very important is your compression ratio and camshaft choice.  With the right choices here, the engine can develop more bottom end while saving fuel.  What camshaft and compression ratio did your builder use?  Many strive for mid-range to high rpm horsepower gains—frankly, I don't.  The target with our recommended CompCams 252 grind camshaft is great power from an idle to 4200 rpm (will spin to 5000 rpm, just not making any more power), good fuel economy and strong bottom end torque...We hold to 8.7:1 compression and can readily run 87-octane fuel at our elevation (4400 feet).


We can talk about this more...I'm curious about your camshaft choice.



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