Main/Rod Bearings

Hey, I'm a noob here. I searched around a good bit trying to find a wright up or thread about changing the Main and Rod bearings in a 4.0 but I had no success.

So I have a 92 MJ with the 4.0 HO. It has 8 quintillion miles, burns little to NO oil but it has very little oil pressure, like around 7psi at highway speeds. Once I figured this out, I parked it and was in denial for a while that its probably the gauge or the sender. But I hooked it up to a mechanical gauge...and it confirmed my fears. So I'm off to change the bearings.

I've changed the RMS a while back, so I won't be doing that. But I will be changing the Rod and Main bearings, oil pump, timing chain, and water pump (including all required gaskets).

Now, I'm planning on changing this stuff with the engine in the truck. Will I have to drop the crank, and if so, do I have to drop the trans to do that????

thanks in advance for any help!

Have you dropped the pan and checked the bearings? Maybe it's just the pump that is bad.

It's tricky but can be done. The rod bearings are cake. The mains however- there are very sharp edges on the corner of the block, and you'll have to roll the bearings in and out. If you cut it too close and shave any of the bearing material from the backside of the mains they won't seat properly and can chatter and fret.

You can gain some clearance by dropping the trans and letting the crank down, but this will drop the crank in the front seal too. A new, pliable seal won't mind- but if it's old and brittle it will probably crack from this treatment. Reviewing your post, though, I see you'll be into the front cover anyway so no big deal.

Changing the mains will be just like the rear main seal, hold it tight to the crank and rotate it around. Notice the index notch, it only goes in and out one way.

Welcome to .

Not to dissuade you from what you have decided to do, but you do realize that all that work may not fix your oil pressure problems if the cam bearing clearance is too big. Don't the mains and rods usually knock if they are worn out too bad?

winterbeater said:

Not to dissuade you from what you have decided to do, but you do realize that all that work may not fix your oil pressure problems if the cam bearing clearance is too big. Don't the mains and rods usually knock if they are worn out too bad?

Click to expand...

The mains are less likely to make noise. If the rod bearings went out they'd be much more noisy and violent, but it's definitely possible to have bad main bearings and hear absolutely nothing.

In my personal experience though, my 302 sbf race motor had a slight "increase" in oil pressure before one my main bearings welded itself to the other half of the bearing and my crank. Never main a sound. Just stopped entirely. I've never built up the 4.0 personally, but the low oil pressure issue just doesn't sound like a rod or main bearing problem to me.

Get yourself a good magnet, or a magnetic drain plug and check for metal shavings in the oil.

Well, when I replaced my rear main seal, I went ahead and replaced the mains since I was there. With 250k on em, the new mains brought my oil pressure up quite noticeably.

thanks for the input guys. I know the cam bearings are pretty rough to change, thats why I'm skipping them....for now. Eventually I'll rebuild the whole thing with new stuff, I'm just trying to keep it running decent right now. A whole rebuild isn't in the budget yet.

So the general consensus is, the upper half of the main bearings will slide out and I won't have to drop the crank.

I'll definitely use a magnet to see what kind of glitter I have in my oil. Hopefully I haven't done too much damage.