Re: JPOR: Main bearings

[Obi-Wan <bvh@xxxxxxxxxxxx>]

> I'd suspect the plastiguage first, recheck it.

I checked the rod bearings today & got numbers just a hair outside
spec, so I think the plastigauge is fine.

> If that's not the case
> then it the last person to go through it may have accidentally gotten
> one undersize bearing and not checked it or they may not have kept track
> of which main cap was which.  The bearing could be in backwards (the
> little tab not in the slot), who knows.

Ug.

>> Have any of you out there replaced rod & main bearings before?
>> The bearings obviously all need to be replaced, but with what size?
>> Since the #3 & #4 bearings are within spec, should I just get a
>> set of stock-size bearings, or do I probably need undersize bearings?
>> Can you get a mixed set?
> 
> Mic the crankshaft itself, there are specs for how worn or out of round
> it can be before it has to be ground and the undersize bearings need to
> be run.  Generally 401 cranks are very durable though (they are all
> steel).

I don't have a micrometer, and like you said, 401 cranks are
reputed to be very durable.  The journals all look fine, despite
the less-than-ideal condition of the bearings.

The TSM talks about .001 & .002 undersize bearings, yet the PAW
kits apparently only come with .01 & .02 undersize bearings.  Are
the smaller increments harder to come by?  If so, who sells them?

>> Also, when the #1 bearing cap was off, I stuck a screwdriver into the
>> timing chain cover & roughly measured the play.  It seemed like it
>> flexed about 1/2", maybe a little more.  Hard to tell using that
>> method.  The TSM says 1/2" is the max before replacment is necessary.
>> Is that a hard & fast rule, or is 1/2" still functional?  What about
>> 5/8" of play?  How much slop will a brand new chain have?  If enough
>> stuff needs replacement, I may just get it all in one rebuild kit from
>> PAW rather than piecing things together myself from local shops.
> 
> You've gone this far, might as well replace the chain too.

Doesn't the timing chain require a different angle of attack?  I
thought you had to yank the all the accessories & stuff off the
front of the engine.  I wasn't aware you had to remove the oil
pan or any of the main bearings.  Am I mistaken?

I'm not saying I don't want to replace the timing chain; I'm just
wondering if it makes sense to do it now or if I can do it in a
month or two without any extra work.

> If you've
> got that kind of wear on the lower end I'd do the whole engine but if
> you want to get by cheap and easy I'd at least do the timing chain,
> cam/lifters (and their bearings), and oil pump.

I rebuild the oil pump a couple weeks ago.  Its failure to improve
my oil pressure is what prompted me to inspect the crank bearings.

> That way you're not
> paying for any machining but you're replacing the stuff that would be a
> real pain to get to once this thing is back in the truck.

The engine is still in the truck.  I just put the front wheels on
ramps & yanked the pan.  Clearance really hasn't been an issue so
far, except for torquing down the main bearing cap bolts.

				-- Obi-Wan
				   '95.5 YJ, '77 Wag (under the knife)

-- 
Ben "Obi-Wan" Hollingsworth                                   obiwan@jedi.com
Many people don't understand this type of devotion; in fact some think of our
mindset as a sickness or as an aberration of the human mind. -- Rick Pewe
              Anybody want to buy a Jeep XJ Dana 44 rear axle?