Introduction   Research   Engine Assembly   Installation & Driving   Epilogue   Thank You's

Research

One source stated that custom rods and pistons must be used with the 258 crankshaft but refused to say why. In fact, I was told I could quote them on this! But finally after many phone calls, and faced with the research outlined below, they admitted it was possible to build a stroker using the right combination of factory parts. However, that was qualified with the statement that by using factory parts, the resulting engine would be a revver, not a torquer. This company will not be named nor mentioned in this article.

Other sources suggested that the right combination of factory components should work but couldn't produce details, or say for sure whether it was possible. So this project began by determining what combination of factory, and possibly custom parts, could be used - in theory. I state "in theory" because the actual assembly of components is needed to finally determine whether a combination of parts found on paper will actually work.

Component Specifications and Clearances

The 4.0L and 4.2L Jeep engines share many specifications
crankshaft end play	connecting rod piston pin bore diameter
crankshaft main bearing journal diameter	connecting rod bore
crankshaft main bearing journal widths	connecting rod bearing clearance
crankshaft main bearing clearance	connecting rod side clearance
crankshaft con-rod journal diameter	connecting rod maximum twist and bend
crankshaft con-rod journal width	piston weight (less pin)
crankshaft max. out-of-round and taper	piston pin bore diameter
cylinder head combustion chamber volume	piston pin diameter
piston - to - pin clearance	piston pin bore centerline - to - piston top

However, another Mopar book gave different piston weights. This is just an example of the difficulty in obtaining accurate information. Nonetheless, these similarities told us that:

• The 4.2L crankshaft should fit the 4.0L block (at least in terms of journal sizes and clearances).
• Either connecting rod can be mated to either crankshaft and piston (but not necessarily work due to piston heights and compression ratios).

By using the 4.2L crankshaft, the stroke would change from 87.4mm to 98.93mm, almost a half-inch increase! With the stock 4.0L bore (98.4mm), the resulting engine displacement becomes 4.51L, or 275ci!

Piston Height and Deck Clearance

The highest point that the piston can in the block reach (called TDC, or top dead center) is calculated as:

For the stock 4.0L, the value is:

For the 4.0L using a 4.2L crankshaft and rods the value is:

The first value can be verified against the block's specified deck height and clearance of pistons below the block deck; it checked out. The second value shows us that there is not a clearance problem when using the 4.2L crank and rods with the 4.0L pistons. However, use of the longer 4.0L rod would require a custom piston. Note that it does not tell us whether the piston will go too low in the cylinder, or if the piston will clear the crankshaft counterweights throughout the stroke. This must wait until actual trial assembly.

4.0L piston/4.2L rod at top dead center.

The 4.0L rod is longer (but lighter) than the 4.2L rod. Ideally, a longer rod should be used. The rod length affects the piston acceleration and deceleration and thrust angles during the stroke, which does affect power. Generally, a longer rod does produce more power, and according to the Mopar Performance book, the 4.0L stroke/rod length ratio is ideal. However, tests on Chevy V-8's show that the power gains are not huge and for this application any increase in power should be negligible.

Static Compression

The change in final piston height (from above) changes the total combustion chamber volume, so that has to be accounted for during these calculations. Starting from a stock 4.0L compression ratio of 8.8:1, the new compression ratio will become approximately 9.4:1 before any resurfacing to either the block or the cylinder head. 9.4:1 is starting to get a little high, but is still within the "acceptable" range for pump gas. Various other articles, both from offroad and from street car magazines, tend to draw the line at 9.5:1.

Other Concerns The 4.2L engine received some changes over its lifespan. For example, in 1981 the crankshaft reportedly became about 12 pounds lighter due the removal of some counterweights. For street performance, the newer crank is more appealing. For idling over rocks, the older, heavier crank is definitely the choice to make! In either case, the 4.2L crankshaft must be still be compared against the 4.0L crankshaft in many respects:

• Bolt pattern of flywheel flange on the crankshaft, so that we can use a late-model flywheel with the required fuel injection timing/trigger notches.
• The flywheel flange protrusion from the back of the block, to ensure a.) clearance between the flywheel and the block and b.) correct engagement by the starter.
• The diameter and depth of the pilot bearing hole in the crankshaft, to determine that a compatible pilot bearing (for the transmission input shaft) exists.
• The "snout" of the crankshafts must be the same, so that no problems with regards to the timing chain or harmonic balancer or serpentine belt will appear.
• Clearance between the counterweights (and rods) and the inside of the block.
• Clearance between the 4.2L crankshaft counterweights and the 4.0L pistons, when attached to the 4.2L rods.

  Also needed is a suitable camshaft to handle the increase in displacement. Likewise, the stock fuel injection was designed to handle an engine displacement of 4.0L. Mopar Performance offers a kit to install the fuel injection onto older 4.2L's, but a 4.5L displacement may be beyond the capacity of the injectors and the throttle body. Specifications on the injectors are difficult to find.