94 X Cab SR5 4x4. Curious as to how much up/down & side to side flex there should be in the driveshaft center support bearing before it should be replaced. Bearing itself appears to be good but I have a approx 3/8+ inch of upward play, no downward play and just a tad of side to side play. From what research I've gathered there should be some flex. Bearing is no longer centered in the rubber surround, more towards the ground. Still the original bearing at 250000 miles.

 

You're well past the mileage of when that carrier bearing should be replaced. They usually last 80K to 150K, depending on driving habits/conditions/modifications. It's showing it's age with that 3/8" + play & the bearing no longer being centered in the rubber. The rubber always gives up on these long before the bearing goes bad. Any clunking noises or vibrations noticed? You'll usually get one or both of those at around 25 & 45 MPH if it's going bad.
Also, if you're planning on keeping the truck & the u-joints & cardan joint are original, you should seriously consider replacing those with new as well.

 

Tore into it today and it looks like this after 28+ years and 255000+ miles. She is spinning a little lopsided. I hear just a tad of bearing noise. Have new one ready to go on just need to order new flange nut. Hopefully that nut is not to much of a bear to break loose.

 

I'd like to pick up on this thread, since I just replaced my Center Bearing with a TIMKEN. After a full week of driving and 200miles, my bearing is already OFF-Center to the low-side. What's with that? Any ideas? ('91 Extra Cab 4x4)

 

I assume you meant the bearing is a "TIMKEN" brand instead of "Timpen'. Timken bearings are very good quality & it would be exceedingly rare to have one defective straight out of the box. What do you mean exactly that it's off-center to the low side? Visually? How much (measurement)?
Are you still experiencing any bad driveline vibrations with the new bearing in? Did you also replace the Cardan Joint during the process or no? What condition are your U-joints in? Any slack or play in them?

If the only issue you have is that the new carrier bearing is a bit off-center to the low side visually but don't have any driveline vibrations, I'd say you're OK. The rubber in the carrier bearing is meant to flex a little bit, & even when newly installed can look slightly low off-center, due to the weight of the driveshaft. It's not meant to statically hold the driveshaft in the center at all times, but to allow some movement under driving conditions.

Only other thing I can think of is this. You say you have a 4x4 truck. Is it lifted higher than stock? If yes, this changes your driveshaft(s) angles & puts more stress on the carrier bearing, causing premature wear/breakage/failure of the bearing & it's components. For lifted trucks the usual solution to this is to install a spacer plate (or plates) between the bearing & the frame, to try to restore the stock driveshaft angle(s).

 

Thank you for the feedback.
I edited the TIMKEN spelling.
Stock Truck. No Lift.
Have not replaced U-Joints, which, when inspected, were found to be free and without concern. However, I've decided to change them, just to freshen up the components and remove that possibility.

I still have a vibration, especially above 60mph through 70. It's feels like driveshaft. (I marked everything before beginning.) After I change out the u-Joints, I'll followup.
The On-Center issue of the Support Bearing really bothers me. I think it naturally rides too low, too close to the housing. It looks like it is stretching the rubber too much, on top. I can understand some off-center, but ...I don't know. (When I removed the original 370,000mi Toyota OEM bearing, it was permanently stretched Off-Center to the lowside.)

Again, thanks for your input!!