Upgrading your Jeep Cherokee XJ's 4.0L inline‑six with a 4.2L stroker kit is one of the most effective ways to dramatically increase low‑end torque and horsepower. By increasing displacement from 4.0L to 4.2L, you can expect a 25‑30% power increase—roughly 275‑300 lb‑ft of torque and 250‑260 horsepower at the crankshaft—while retaining the legendary durability of the AMC/Jeep six. This guide covers every phase of the installation: from selecting the right kit and performing necessary machine work to final assembly, tuning, and break‑in.

Why a 4.2L Stroker for the XJ 4.0?

The 4.0L (242‑cid) PowerTech six is already a stout engine, but its long‑stroke design limits peak power. A stroker kit increases the crankshaft stroke from 3.413″ to 3.895″ (using a modified 4.2L crankshaft or a custom billet unit), combined with shorter connecting rods and custom pistons. The result is 4.2L (258‑cid) displacement that shifts the torque curve lower, providing instant throttle response on the trail or when towing.

Compared to other common stroker displacements (4.5L, 4.6L, 4.7L), the 4.2L is a “safe” stroker that requires minimal block clearancing and remains compatible with the factory fuel injection system with only minor adjustments. It’s an ideal upgrade for daily‑driven XJs that still see off‑road abuse.

Components of a Typical 4.2L Stroker Kit

Most quality stroker kits include the following parts:

  • Stroker crankshaft – a cast or forged unit with a longer stroke (often a remanufactured 4.2L crankshaft with a smaller rod journal or a custom billet piece).
  • Connecting rods – shorter rods (usually 5.875″ or 5.850″) to maintain proper piston‑to‑deck height.
  • Pistons – hypereutectic or forged aluminum with the correct compression height and valve relief cuts. Optional dish to control static compression ratio.
  • Piston rings – typically 1.5mm, 1.5mm, 3.0mm moly or chrome sets.
  • Bearings – main and rod bearings sized for the crankshaft journals (often 0.010″ or 0.020″ undersized on the mains).
  • Gasket set – including head gasket, oil pan gasket, timing cover gasket, and intake/exhaust manifold gaskets.
  • Optional camshaft – a mild performance cam (e.g., 206‑210° duration at 0.050″) optimizes the stroker’s airflow. Some kits offer a matched cam and lifter set.

Vendors such as Hesco Machine, Titan Engine Parts, and EngineKits.com offer complete 4.2L stroker packages. Always verify that the kit includes a torqueplate‑honed piston set and that the crankshaft has been cross‑drilled and nitrided for durability.

Required Tools and Shop Equipment

Beyond basic hand tools, you’ll need specialized equipment. Assembling the engine out of the vehicle is strongly recommended, so an engine hoist and stand are essential. Here’s a complete list:

Hand Tools

  • Socket set (3/8″ and 1/2″ drive, metric and SAE) – the 4.0L uses mostly SAE fasteners.
  • Torque wrench (0‑150 ft‑lb and 0‑250 in‑lb) – calibration verified.
  • Allen and Torx bits (especially T‑30 and T‑40 for intake bolts).
  • Gasket scraper, razor blades, and non‑abrasive pads.
  • Piston ring compressor and ring expander pliers.
  • Valve spring compressor (if upgrading cam or springs).
  • Micrometer set (0‑4″) and bore gauge for measuring cylinder bores.
  • Plastigauge for bearing clearance checks.

Engine Removal & Stand Tools

  • Engine hoist (2‑ton capacity minimum).
  • Engine stand rated for 600‑800 lb.
  • Cherry picker or load leveler.
  • Floor jack and jack stands.

Machine Shop Services (may be outsourced)

  • Hot tanking and crack inspection.
  • Boring and torqueplate honing to 0.030″ or 0.060″ over stock.
  • Surface decking the block (0.010″ or less) for a flat head gasket surface.
  • Line honing main bearing journals (if main caps are worn).
  • Crankshaft grinding and micro‑polishing (if using a used 4.2L crank).

Preparation: Remove and Disassemble the Engine

Begin by disconnecting the battery and draining all fluids. For an in‑vehicle swap, removing the intake and exhaust manifolds, valve cover, and accessories (alternator, A/C, power steering) is possible on a lift, but pulling the engine is safer for a thorough rebuild. Follow these steps:

  1. Mark all connections – label wiring, vacuum lines, and fuel lines with tape.
  2. Remove the radiator and fan assembly to provide clearance.
  3. Disconnect the exhaust at the manifold flange and the downpipe.
  4. Unbolt the transmission bellhousing (4 bolts on the AW4 or AX‑15) and support the transmission with a jack.
  5. Remove the engine mounts (frame side and block side) and lift the engine out.
  6. Mount the engine on a stand, then disassemble: remove intake/exhaust manifolds, timing cover, timing set, cylinder head, oil pan, and rotating assembly.

Inspect the block for cracks and measure the cylinder bores. If you plan to reuse the stock 4.0L rods, check for straightness and resize the big‑end bores. Most kits require boring the cylinders 0.030″ or 0.040″ oversize to match the pistons.

Machine Work and Clearancing

A 4.2L stroker in a 4.0L block requires several critical clearances:

Crank and Block Clearance

The longer stroke crank’s rod journal travel will approach the bottom of the cylinder bores and the oil pan rail. Machine shops often grind the block’s lower skirt area and file the connecting rod bolts to achieve 0.125″ clearance between the rod bolt heads and the block. Use clay to mock‑up clearance after assembly.

Piston‑to‑Valve Clearance

With a cam upgrade, valve pockets in the pistons must be deep enough. A minimum of 0.100″ clearance for intake and 0.120″ for exhaust valves is recommended. Check with lightweight checking springs.

Piston‑to‑Deck Height

Target a deck height of 0.005″‑0.010″ (piston below the block deck). This creates the proper quench area when using a 0.043″‑0.051″ compressed head gasket. If the deck height is too high (piston protrudes above the deck), the block must be decked further or a thicker head gasket used.

Bearing Clearances

Use Plastigauge to check main and rod bearing clearance. Target 0.002″‑0.003″ for mains and 0.002″‑0.0025″ for rods. Adjust with different bearing shells (standard, 0.001″, 0.002″ undersize).

Assembly Process

Once the machine work is complete and all parts cleaned and lubricated with engine assembly lube, proceed with assembly:

  1. Install main bearings – place the upper bearings in the block and lower bearings in the main caps. Lubricate with assembly lube.
  2. Install the crankshaft – lower it gently onto the bearings. Install the main caps with the correct orientation (arrows point forward). Torque main cap bolts in three stages per the factory spec (usually 80 ft‑lb). Check endplay (0.003″‑0.008″).
  3. Install connecting rods and pistons – position the ring gaps 120° apart, compress rings, and tap the piston/rod assembly into the cylinder. The rod bearing tangs must align with the rod’s locking slots. Torque rod bolts to 45 ft‑lb with thread locker if using factory rod bolts.
  4. Install the timing set – use a double‑roller timing chain and sprockets for durability. Align crank and cam timing marks. Install the camshaft (if replacing) before the pistons, with plenty of cam lube.
  5. Install the rear main seal – replace the seal in the rear main cap and the block. Use a seal installer to avoid leaks.
  6. Install the oil pump – prime the pump with petroleum jelly and install it. Use a new distributor drive gear if the cam was changed.
  7. Install the oil pan – clean the oil pan rail, apply a bead of RTV silicone, and torque the pan bolts to 80‑90 in‑lb.
  8. Install the cylinder head – use a quality head gasket (Fel‑Pro 8993 or Cometic MLS). Torque the head bolts in the specific sequence (four‑step process: 22, 45, 80 ft‑lb, then a final 90‑degree angle). A stroker’s higher cylinder pressures benefit from ARP head studs.
  9. Install the timing cover and front seal – use a timing cover alignment tool to center the seal.
  10. Install manifolds, accessories, and sensors – reinstall the intake manifold (new gasket), the exhaust manifold (use copper or aluminum lock nuts), and all sensors (crank position, oil pressure, etc.).

Ignition and Fuel System Upgrades

A 4.2L stroker with a mild cam will work with stock injectors (22 lb/hr) up to about 250 hp, but to fully realize gains, consider these upgrades:

  • Fuel injectors – 24‑28 lb/hr high‑impedance injectors (Bosch or Delphi) for 20‑30% more flow. The stock fuel pressure regulator can handle these.
  • Adjustable fuel pressure regulator – fine‑tune the fuel curve; target 43‑45 psi base pressure.
  • Ignition – a higher‑output coil (MSD Blaster or Accel) and copper core spark plugs gapped at 0.045″. Retain the factory distributor but set initial timing to 10‑12° BTDC and total advance to 28‑32°.
  • Intake/exhaust – a 62‑mm throttle body, cold air intake, and a 2.5″ exhaust system with a high‑flow catalytic converter reduce restriction.

ECU Tuning and Engine Management

The factory Renix or OBD‑I ECU (1987‑1995) can be reprogrammed using a chip or an aftermarket piggyback system. For OBD‑II XJs (1996‑2001), a custom tune via a performance programmer (e.g., Hptuners or Diablosport) is ideal. If you don’t have access to a tuner, a MAP sensor adjuster and a signal clamp can trick the ECU into delivering more fuel under boost or high load. However, a dedicated standalone ECU (Haltech, MegaSquirt) offers full control over fuel and spark tables.

Break‑In Procedure

Proper break‑in is critical for ring seal and cam longevity. Follow these steps after the first start:

  1. Prime the oil system – disable the ignition, crank the engine with the starter until oil pressure registers.
  2. Start the engine and hold it at 2000‑2500 RPM for 20 minutes. Do not let it idle; the low oil pressure and splash lubrication can damage the cam lifters.
  3. After the first 20 minutes, check for leaks, then allow the engine to cool completely.
  4. Change the oil and filter – use a high‑zinc break‑in oil (Valvoline VR1 conventional 10W‑30).
  5. Drive the vehicle gently for the first 500 miles – vary RPM, avoid heavy loads, and do not exceed 4000 RPM.
  6. After 500 miles, perform a final oil change (same break‑in oil) and re‑torque the head bolts (if using stock bolts).
  7. After 1000 miles, switch to a synthetic 10W‑40 with high ZDDP (e.g., Shell Rotella T6) for daily use.

Common Pitfalls and How to Avoid Them

  • Block interference – even after machining, the rod bolts may still contact the block. Use a die grinder to relieve the area and re‑check with clay.
  • Piston slap – if using hypereutectic pistons, ensure the wall clearance is 0.0025″‑0.0035″. Too tight and the piston can seize; too loose and you’ll hear cold‑start knock.
  • Overheating – a stroker produces more heat. Upgrade to a three‑row aluminum radiator and a 180°F thermostat. Electric fan kits with a thermostatic switch are recommended.
  • Lean condition – the stock ECU will not supply enough fuel under full throttle. Install a wideband O2 sensor and monitor AFR. Target 12.5‑13.0 under load.

Realistic Expectations

With a proper 4.2L stroker, mild cam, and supporting mods, expect 250‑260 hp and 290‑300 lb‑ft of torque at the flywheel. That translates to a noticeable seat‑of‑the‑pants improvement: the XJ will pull strongly from 1500 RPM redline, surpassing 5.2L V8 torque levels at lower RPM. Fuel economy will drop slightly (1‑2 mpg) depending on driving style. The engine remains reliable for daily driving and trail use if built with quality parts and careful tuning. For more information, consult the NAXJA Forums - Stroker Section and the Stroker Engine Resource site.