[Introduction] [Parts Selection] [Parts List] [Specifications] [Preparation] [Assembly] [Installation] [Results] [Updates] [FAQ's]
The increased displacement is achieved by replacing the stock 4.0 crankshaft (3.411" stroke) with an AMC Jeep 258 crankshaft (3.895" stroke) and by overboring the cylinders (stock bore is 3.875"). The AMC 258 crankshaft has the same critical dimensions as the Jeep 4.0 crankshaft so swapping it in is easy. Since the AMC 258's crank throw is 0.242" longer than the 4.0's, correspondingly shorter connecting rods or shorter pistons (stock pin height is 1.601") than the Jeep 4.0 items would be needed to keep the deck clearance the same as stock (0.022"). I opted for the shorter rods so that I could use stock-type 4.0 pistons instead of expensive custom forged items. The AMC 258 rods are 0.250" shorter than the Jeep 4.0 rods (5.875" v 6.125") but they have the same pin bore diameter, same big end bore diameter, and are the same width so they fit the bill very nicely.
The Jeep 4.0 I6 engine block forms the basis of the stroker engine. Blocks came in various castings over the years as shown here:
The main webs of the '96 and later blocks were stiffened with extra ribbing and a main bearing girdle was added to reduce noise, vibration, and harshness (NVH), making these the best blocks to build upon. All Jeep 4.0 blocks have a deck height of 9.450-9.456".
At the heart of any stroker kit is the AMC 258 crankshaft. This appeared in AMC 258 (Jeep 4.2) engines in various guises as shown below:
1971........3199738..........For Borg Warner automatic
1972-80...3214723.........12 counterweights, 66lb, 64mm nose
1981-86...3235477.........4 counterweights, 46lb, 64mm nose
1987-90...3727...............4 counterweights, 46lb, 54mm nose
For comparison, the 4.0 crankshaft has 8 counterweights, weighs 55lb, and has a 54mm nose.
The #3214723 crank is reputed to the the strongest. The extra counterweights dampen vibration-inducing harmonics to produce a smoother running engine. The #3214723 and #3235477 cranks have a longer nose that was originally designed to accomodate a V-belt crank pulley. This needs to be shortened by 1.0cm if the serpentine pulley from the 4.0 is to be used. Alternatively, a custom 1.0cm thick spacer can be made to fit in front of the pulley thereby avoiding the need to shorten the crank nose. The nose of the #3727 crank is already the same length as that of the stock 4.0 crank so it doesn't require any modification.
Pistons & Rods
The pin heights and dish volumes of pistons available for the 4.0 engine are:
Stock 4.0 cast aluminium '87-'93 83500251, '94-'95 4773157, '96-'04 4798329----1.601" 13.1cc
KB Silvolite cast aluminium 2229/2229C----1.581" 11.0cc
KB Silvolite hypereutectic 3241HC/3242HC----1.592" 13.7cc
Sealed Power cast aluminium 677P/677CP----1.585" 15.2cc
Sealed Power hypereutectic H802CP/H825CP----1.592" 15.2cc
KB Silvolite forged IC944 stroker----1.353" 21.0cc NEW
KB Silvolite forged IC945 stroker----1.353" 10.8cc NEW
Since I decided to buy cheap stock-type cast aluminium pistons (Sealed Power 677P), I also needed to buy AMC 258 rods. These come with part #3180444 ('71-'81 707 casting) & part #3237812 ('82-'90 352 casting). The early casting is reputed to be the stronger of the two but the later casting is more widely available. Cost per set of six is typically about 120USD.
If you decide to use the longer Jeep 4.0 rods (casting no. is 53020126 for all years), custom forged pistons with a pin height of 1.380" are needed to produce zero deck clearance. A set of six of these pistons typically cost at least 600USD so they're not cheap. Keith Black 944/945 forged pistons can be used with the 4.0 rods leaving a deck clearance of 0.028". Milling 0.020" from the block deck reduces the deck clearance to 0.008".
You can save a lot of money by reusing the stock Jeep 4.0 camshaft with stock valve lifters/springs/retainers/locks, but it's only suitable for a stroker engine running a ~9.2:1 compression ratio (CR) or less. A higher CR causes excessively high cylinder pressures at low rpm resulting in detonation. The solution is to install a longer duration performance camshaft and there are several aftermarket dual pattern camshafts available on the market. You also get the additional benefit of more HP/TQ, which is the reason for building a stroker in the first place!
The stock cylinder head is cast iron and weighs 60lb. The valve head diameter is 1.91" intake/1.50" exhaust. Casting numbers are as follows:
The early '87-'90 non-HO heads have low intake ports that flow rather poorly. The later HO heads have higher intake ports that flow more air by allowing a straighter shot into the cylinders. The '91-'95 HO heads with casting no. 7120 have the highest intake and exhaust port airflows, especially at lower valve lifts where it is most important, and are the best for performance. The '96-'98 0630 heads are almost identical except that they don't have a port for the coolant temp. gauge sending unit. The '00 and later heads with casting no. 0331 have smaller exhaust ports to produce a faster warm-up of the catalytic converter and improve emissions, but performance also suffers because the ports don't flow as well as those of the 7120 and 0630 castings.
All of the 4.0 heads respond well to a good port job, with gains of up to 20hp possible on a 4.6L stroker engine.
Stock '00 4.0 timing gear was in excellent condition so I reused it. The problem was that the '00 camshaft sprocket and retaining bolt would not fit onto the Crane camshaft, so I had to source these items from a '94-'98 model. Timing marks on each sprocket show the correct orientation for installation. The chain is a "silent" non-roller unit.
The spring and thrust pin must be inserted into the recess in the cam sprocket retaining bolt, and the oil slinger (cupped side facing forwards) must be slid onto the crank nose in front of the sprocket before the timing cover with tensioner are bolted onto the block.
If you ever need to replace the OEM timing gear, go for the Cloyes Dual Roller Timing Set (part no. CLO-9-3127). This timing set will fit on any AMC/Jeep inline six engine including the 4.0L up to '98. The camshaft sprocket in this set will fit on any aftermarket performance camshaft for the 4.0L engine but it will not fit on the stock '99+ camshaft.