replacing main bearings

Getting ready to do a main bearing replacement in hopes of getting some oil pressure back. My question is...according to the service manual you have to measure the gap and then mix and match the bearings in order to acheive the correct clearence. Am I reading this right or is thier something else I should be looking for?

01 4.0

You're referring to the FSM?

If so, there should be an explanation of "stampings" you may find on the passenger side of the block - I'd look there, to see if the crank had to be ground undersize at the factory. Failing that, I'd take a 2-3" micrometer and measure the journals, if it were mine (but I'm a little more thorough than most...)

It's theoretically possible to "mix and match" bearing shells to clean up oil clearance issues, but it's not recommended, and you'll usually only see it done with .000/.001" shells anyhow. Better to have the crankshaft reground if it's that far off.

What's your oil pressure, anyhow?

5-90

I've had 20w-50 in it and its showing 35 on the highway and 13 at idle

pikey77 said:

I've had 20w-50 in it and its showing 35 on the highway and 13 at idle

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What is wrong with that. How many miles? FSM says 13 at idle is fine.

I have to use 20w-50 in it to get that.

With 10w-30 ill lose oil pressure at idle after coming to a stop.

The motor doesn't knock or anything so I'd figure I'd try and recoup some lost oil pressure and get back to a regular weight oil.

120K


As for the bearings Ill re-read it tonight but I believe it said something about using plastiguage to measure the gap then using over/under sized bearings. However the write-ups I've read on replacing the main/rod bearings haven't mentioned this at all.

Does your pressure get better immeadiatly after an oil change and go downhill from there and, any mysterious loss of coolant?

natep1 said:

Does your pressure get better immeadiatly after an oil change and go downhill from there and, any mysterious loss of coolant?

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Not really. Its always around 50 on startup once it warms up it drops to around 35 on the highway and 13 or so at idle. I do have a slight coolent leak but its from a bad radiator. I've looked inside the oil cap and haven't seen any signs of a cracked head. Someone told me I should be able to see a greenish tint. I flushed the radiator about a month or so ago and it still filled to the brim with shiny green fluid. No oil residue in coolent and the oil is/has been black and doesn't appear to have water in it.

Couldnt the oil pressure sending unit also be reading a little off?

I had good luck replacing all my rod bearings with 0.001" undersize shells and the mains with standard shells. My '90 4.0 had about 230k on it at the time, and the oil pressure was a hair over 0 psi at hot idle with 20W-50. Your engine isn't in quite as bad shape as that, but I'd guess that you'll still be fine with 0.001" under on the rods, considering that your oil pressure indicates some significant bearing wear. Just be sure to check clearances with plastigage as you change the shells. Although getting a micrometer on the journals would be ideal, plastigage is good enough to tell you if you're way off or close enough, especially if you want to keep things quick and cheap (meaning keep the crank in the engine). Also, as long as you've got the pan off, you should change the oil pump and might as well do the rear main seal. This is how I did my bottom-end bandaid, and 8k miles later I'm running strong with about 25 psi of oil pressure at hot idle with 10W-40.

Urban...where did you get your bearings from and whatbrandif you remember. Also if I'm reading it right you should plastiguage after putting on the top bearing and bottom bearing. If this is the case do I just guess at .001 and then measure after while installing.

I do plan on doing the rear main and putting in a high volume pump.