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Bad Vibes -Pilot Bearing? Release Bearing? Other Bearing?


NAXJA Forum User
Got an 89 MJ that was converted into 4WD, but not by me so it's a bit of a Frankentruck. Started getting really bad vibrations when under power. Checked all the U-joints, and they're all good. Took out the front drive shaft to make sure that wasn't it, and it's still the same. Now it has started to make a screech that is definitely coming from what sounds like right under the shifter. The noise lasts for about 5-10 seconds after start up and then goes away. Not sure what transmission it is, but the slave cylinder is on the outside, if that helps. Wondering what you guys think. Pilot bearing? Release Bearing? Some other bearing? The vibes only happen under load. When the clutch is pressed in when driving, the vibes stop.
Is it were the pilot bushing / bearing, it would likely change as the clutch was depressed. If it only happens on first first up when cold, it may also be the fan/serpentine belt squealing.

External slave cylinder means an AX-15. Since an '88 MJ (or XJ) would not have likely come with an AX-15, it's possible the pilot bearing has the wrong inside diameter. This happens particularly when doing a stroker conversion, as the earlier crankshafts have a different pilot hole diameter.

When I had this issue, here is the info I have from my research:

Pilot bushing #PB-122
ID: 0.580"
OD: 1.055"

Pilot bushing #PB-117
ID: 0.755"
OD: 1.055"

Pilot bushing #PB-207
ID: 0.755"
OD: 1.001"

I got this info from Phoenix Friction Products, from whom I bought my kevlar clutch kit and flywheel. They can supply those bushings, though they should be able to be cross-referenced at a local parts store. This was the clutch kit I used: http://www.phoenixfriction.com/p-407...-10-12-in.aspx

The 4.2L crank that is used on a stroker has a hole of 1.056"
The 4.0L crank that is in Renix and HO motors has a hole of 1.001"

The early model AX-15 trans (before 1991-ish?) has a input shaft of 0.580"
The later model AX-15 trans has an input shaft of 0.755"

Note that these sizes aren't exact. For example, the trans input shaft is slightly smaller. My trans mic'ed at 0.752".

I think drilling out the bushing is likely to not be all that accurate.

Mine would squeal when the clutch was engaged, and be quiet when disengaged.

David Bricker / SYR
The release bearing could indeed be the culprit. Most clutch kits come with a cheap release bearing, and most folks go ahead and install that bearing.

If I were you I would try to locally source pilot and release bearings. That way it should be easier to return the parts you don't need.

Count yourself lucky that you are at least getting a warning from whatever is going south.
Thanks for the answers guys, much appreciate your time. Turns out I was a dumbass on this one. I pulled the truck into the garage and gave it one more check over before dropping the tranny. The rear u-joint was now obviously shot. Maybe it was under load before and appeared tight, I don't know. Anyway, a new u-joint and a good greasing of the drive shaft slip joint, and the vibes were gone. Oh, the screeching sound? It's the blower motor! I'm beginning to think my Youtube mechanics' certification means s**t!