Engine Trivia Questions

[AZJeff]

I am building a new short block for my MJ, and have found a couple of variations between what engine is in the vehicle now, and the one I am assembling, and this made me wonder about these small differences.   I wonder if any of you AMC gurus would know the reasons for these:

 

1.   Cylinder Head Dowels   my existing engine that I am replacing is from a 2000 XJ.   The cylinder head is located to the block with two dowel pins.   My replacement engine block is from 1992, and does not have the dowels nor the holes for dowels.   Does any one know why they added the dowels on later engines (aside from the obvious convenience when installing the head)?

 

2.  Timing Chain Types   my existing engine has a "silent type" timing chain and sprockets.  The new chain my engine machine shop recommended is a traditional "roller" type.   I know that AMC used roller types in the past, but I don't know when Chrysler switched to the silent type.  Does anyone have any comments?

 

3.  Camshaft Thrust Control  my new camshaft comes with a spring loaded plunger that fits into a new special bolt that retains the camshaft timing chain sprocket.  This plunger pushes against the inside of the timing chain cover, and controls end movement of the camshaft.   The old engine from the 2000XJ lacks this feature, and obviously has some other way to control thrust.  Does anyone know when and why they made the change?

[Strokermjcomanche]

I believe the dowel pins started in 96 , which would explain why your 92 doesn’t have them . I haven’t come across any other reason except for better alignment. I’m sure you know cutting a couple old head bolts and sliding the head over them will net the same result. Depending on the head that you are using , you may want to consider using a different year head gasket that won’t have the dowel holes in it . 
 

I  believe that when the 4.0 started using the ignition coil rail 99/00 Is when the 4.0  stopped using the cam thrust pin . I believe they swapped to the 2 retaining bolts as it better prevented the cam from walking better than the spring pin did . 
 

The later timing cover doesn’t have a place for the cam bolt spring pin  to push against just as an FYI . You would need to use an older cover  .
 

If you decide to use a double roller timing chain sometimes you need to clearance the new timing cover inside to prevent the chain/cover from rubbing/grinding . 

[75sv1]

I have 'swapped' an HO head onto a WJ block. I'm probably the only person that had issues with the dowel pins having interference. Yes, the '96 motor and up.  The '99+ camshaft has a thrust plate. It more accurately keeps the camshaft in position. It might help make a few HP more. I know that there is a double thrust washer ones for other motors. A guy I know makes them for Pontiacs. Claims +8 hp. There is a double roller made for the 99+ by SB. ??? I read an article that below 5K RPM the silent chain is stronger. Above  $5K RPM then a roller is more stable.