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Going to swap my old 258 w/ 4.0 head for a 99 model 4.0 in my 84 CJ-8. Already have the mpfi setup, just going to use it along with the TF999 trans. Question is will the  flexplate and starter both work on the 99 4.0 engine? I've been told that I need the flexplate that fits the engine and to drill it to fit the trans. torque converter. Just need to know which is correct. Thanks

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Hi, Jeep49...Unless you have a Mopar EFI conversion with a crankshaft damper and front mounted crankshaft position sensor (CPS), you need a crank position signal for the MPI system.  In 1999, the 4.0L engine uses a bellhousing (manual) or converter housing (automatic) that has a CPS opening around 11 o'clock at driver's side.  The flywheel or flex plate must match the housing in order to provide a TDC crankshaft signal from a CPS.

So, you have a 1984 999 Torqueflite flex plate.  You need a signal for the CPS, either front of the engine (HESCO remote CPS and matching crankshaft damper) or OE rear:  1991-up Mopar MPI type.  If you don't have a front crankshaft position sensor (CPS) and matching damper (see HESCO), you cannot use your 999 TF transmission case with the Mopar MPI system.  

Unless you buy a crankshaft damper and aftermarket CPS, you need either a 1991-95 version of the 999 transmission or a change to a 32RH transmission.  This may be problematic from the Dana 300 versus NP231 transfer case standpoint.  You'd need an Advance Adapters clock ring to use a later NP231 pattern transmission adapter with your Dana 300 (gear drive) transfer case.  The 32RH or an AW4 conversion would involve electrical and electronic controller needs.

If you use a remote front CPS and damper, you can use your 999 with the 4.0L engine as long as 1) the 4.2L flex plate fits the 4.0L crankshaft properly, or 2) you use a 1991-95 YJ Wrangler 4.0L 999 flex plate and converter with your 999 transmission.  In any case, make sure the stick-out of the 1999 4.0L crankshaft is correct for properly positioning the flex plate, torque converter and 999 transmission housing.  1991-95 parts should meet this need, but make sure by taking measurements.

Also make sure that the 1991-95 CPS and flex plate will work with the 1999 MPI (single rail) PCM system.  You want the TDC signal from the CPS to be the same between a 1999 MPI engine and a 1991-95 999 transmission's CPS signal.  Simply put, you want matching clock positions and CPS signals for the PCM.  You have the 1999 4.0L PCM?

The only other approach would be a conversion to the later 32RH or AW4 transmission, and this means electronic/electrical hookups and controllers.  In other words, much more involved, including clocking the Dana 300 (see Advance Adapters' clock ring).  Your 1984 999 transmission case to transfer case adapter must already be "clocked".  See whether your OE transmission to transfer case adapter will fit the later 1991-95 999 and also whether the output shafts between 1984 and 1991-95 999 transmissions are the same spline count and stick- out lengths.

How are you addressing the CPS issue?  We'll go from there...


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after reading my original post, it was pretty vague. Here is the total scoop. I have the complete MPI setup currently in operation on the engine described in my post. It has the damper pulley cps, V-belts, a/c and is the kit that HESCO sells. I intend to swap all the MPI parts to the 99 engine. It's basically a long block with all accessories missing. I'll use my 91-95 distributor and coil, etc from the 4.2L engine. The 99 engine still has the dist. in place. Is there anyting special I need to know before swapping the distributors out. I'll replace the waterpump with the correct one for v-belts. The biggest issue is the flexplate. I've been told to use the same model year flexplate as engine(99)and to bolt the 4.2 flexplate to the 99 4.0 flexplate and redrill the tc mounting holes to match the 84 model 999 tc. Since then I've been told by someone at HESCO to use the original 999 flexplate that it will bolt up to the 4.0 and I can use the OEM 4.2 starter instead of the very expensive late model starter. He also said my 4.2L damper pulley for the CPS would bolt to the 4.0 crank snout w/o any problems. If everything Hesco told me is correct then this will be a pretty simple conversion, if not, it still will be pretty simple just more expensive, If I've left any info out you need just let me know. I just need another opinion before starting. I'm also going to replace the rear main seal and oil pump before I start along w/ new freeze plugs. Thanks for your help.

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jeep49...By listings, the 1991-97 distributors have the same part replacement number.  1998-99 differ.  I'd use the distributor from your Mopar EFI kit currently in the 4.2L engine.  It should fit without an issue.  Make sure you use the correct OEM sealing method for the distributor.  

Note: As a precautionary, you can compare the two distributors; push the shaft upward and measure the lengths from the bottom of each distributor driveshaft to the base of the distributor housing flange where it sets on the engine block.  You need matching measurements for the distributor gear to engage the camshaft properly, the rotor to align right with the distributor cap, and the oil pump drive tang to engage the pump drive correctly.

The only loose end was whether to use a 1991-95 flex plate for a 4.0L with the 999 transmission.  That flex plate is nearly the same diameter as your original one but has a different torque converter bolt circle diameter than the 1984 flex plate and torque converter.  You would need to use a later torque converter if you use a later flex plate, and there's no point to that approach.  As long as your 1984-style 904/999 flex plate fits the 4.0L crankshaft correctly, and the crankshaft flange stickout from the block of the 4.2L and 4.0L engines match, you've got it.

Note:  Compare the crankshaft back ends.  You want the flex plate's center hole to index with the flange.  You want the flex plate's crankshaft bolt holes to match, and the bolt part numbers are the same for 1984 and 1991-92 flex plates:  J3151407.  The number changes in 1993, so compare threads at both of the crankshaft flanges.  There is also a flex plate "REINFORCEMENT" ring that fits outboard of the flex plate and sandwiches the flex plate to the crankshaft flange.  This is part number J3214094 for both 1984 and 1991-up 4.0L 999 applications, you should have this ring on the 1984 engine...Basically, the crankshaft flanges need to match.  If so, this should be a bolt-up without any problem; check these details and compare the two crankshaft ends.  If a match for fit and stickout from the block, you can use your '84 flex plate, reinforcement ring, new bolts (OE replacement type or equivalent only), the 1984 torque converter and the '84 starter.  I use Loctite 271 Red on flywheel bolt threads unless the bolts come with thread locker.

Once you have the parts separated, you can take measurements for the 1984 flex plate fit to the 4.0L crankshaft and also confirm the torque converter fit to the flex plate.  Check the stack height as I suggested in my last reply.  Use that last reply and this reply as a reference for your check-off list.

I did your homework.  Here are details on the 904/999 in general from Advance Adapters, including the flex plates for early versus later 999 applications.  I've included links to the ATP catalog listings:


Below are details on the 999 flex plates used in 1984 (Jeep CJ7 4.2L) versus 1992 (a year I picked, basically 1991-up YJ Wrangler 4.0L with the 999).  Note that your 1984 converter has a different bolt circle diameter than the later 4.0L 999 converter.  This makes the flex plates different.  I actually like the wider spacing like your 1984 type.  You can use your 1984 flex plate (if in good condition), otherwise buy a new Z-101 type to replace it.  This will, as HESCO shared, allow use of your early starter.  The flex plate diameters are only slightly different, but your 1984 converter does show a 0.020" bigger diameter (for whatever reason, maybe starter drive types, maybe just a manufacturing standard, etc.) than the later 4.0L flex plate:


Part Number Z-101 Flex plate for 1984 Jeep CJ 999  OE AMC Part #: 3232138

Fits:  A904, A999 Trans. Torque converter mounting hole spread is 11.25 inches.

Flywheel Pitch:  12 TO 14

Outer Diameter (in inches):  13.83

Teeth (in inches):  164

Balanced N/A  [4.2L crankshaft internally balanced]


Part Number Z-338 Flexplate

6 bolt holes; Torque converter mounting hole spread is 10 inches. OE #  CHRYSLER: 52117760; CHRYSLER: 53006328

Flywheel Pitch (in inches):  12

Outer Diameter (in inches):  13.81

Teeth (in inches):  164

Balanced N/A  [4.0L crankshaft internally balanced]


Let us know your findings when you measure and compare the crankshafts flanges, distributors and other details...


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The Mopar EFI Kit damper (HESCO type) should be for a short snout crankshaft.  Your '99 4.0L engine has a short snout crankshaft for a serpentine belt.  So, the Mopar EFI Kit damper on your 4.2L will fit the 1999 engine without an issue.  

Your current 4.2L engine should have the long snout crank for V-belts if it is not a serpentine belt crankshaft.  You may have the special HESCO adapter cap washer to compensate for the longer 4.2L snout.  In order to secure the damper all the way into position, some Mopar EFI kit installations have needed the HESCO cupped washer when the long snout extends slightly beyond the damper face.

For the 4.0L engine and crankshaft, use the correct crankshaft bolt and an OEM type damper washer.  On 4.0L (short snout) installations where I used earlier V-belts and pulley, the crankshaft bolt needed to be longer than the serpentine belt bolt.  Make sure you use the right length damper bolt, one long enough to penetrate deeply enough into the 4.0L snout's threads.

This should be your current Mopar/HESCO damper:


This is HESCO's universal damper bolt and washer for Jeep inline sixes.  Ask them if it is a long enough bolt for the 4.0L crankshaft snout when used with your Mopar/HESCO type V-belt damper:





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Thanks, jeep49!  Across the 'net, there is a flood of information and misinformation on this swap, and the 904/999 Jeep CJs get marginalized.  I happen to be an automatic transmission fan, so your project is of real interest to me!

Attention to detail and some photos, your cell phone would work great, is valuable to others...You know the key issues and focus.  Should be a straightforward swap with some footnote details.

Did you consider installing your 4.2L crankshaft in the 4.0L block for a 4.6L stroker?  There is more involved, it's not as simple as a crankshaft swap but has many gains.  That engine build and viewpoint have been covered extensively at the magazine.

Check out the Tony Hewes/Hewes Performance Machine interview video series at the magazine and my articles on tuning, injector choices and so forth.  We have discussed the 4.6L stroker here at the forums as well.  Here are some video and article links on the 4.6L stroker six build: http://www.4wdmechanix.com/?s=4.6L, a place to get oriented.

I've considered producing an HD video how-to (Vimeo On Demand streaming rental) for building a 232, 258/4.2L, 4.0L and the 4.6L variant of the Jeep inline sixes.  This is a topic that could stand some clarification and detail, with no steps left out.  

How-to instruction on that level typically becomes one hour or more of finished edit/product.  If there is sufficient demand, the subject is certainly significant.  The AX15 transmission build (click here to see the trailer, the video is one-hour and 34 minutes long) has been popular.  We'll see...


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