Main Bearing Clearance

[bbarrett]

Another question, Ive started assembly on the motor and have ran into all kind of problems. But im working them out one at the time. My question is, I need to know what kind of main bearing clearances everyone is running that uses a stock pump and has good oil pressure. I have gotten mine down to 001 vertical and 0015 at horizontal or at about the 2:30 position. As all know im just building a highway motor, But i want good oil pressure. I build Race motors for a secondary living but have never built a in line Jeep. Now i don't want to make anyone mad but i need actual bearing clearance measurements done with a dial bore gauge, Not plastic gauge. I'm probably going to try to get the rods down to this measurement also. Has anyone ran one this tight? Any and all answers will be appreciated. PS. I know the service Manuel says 001 is the minimum, But i would prefer to hear from real life experience.

[IH 392]

Sounds good to me.
A std oil pump should do just fine, I think a high volume pump in a street engine is just wasted HP and over heated oil.

[yuppiexj]

1988 FSM says
.0010-.0025 mains clearance
.0015-.0065 mains end play
.001-.003 rods clearance
.010-.019 rods side play

2000 FSM says
.0010-.0025 mains clearance
.0015-.0065 mains end play
.001-.003 rods clearance (.0015-.0020 preferred)
.010-.019 rods side play

per the FSMs you are on the tight side of spec but I'd prefer to right on the tight side.

[Cheromaniac]

I have gotten mine down to 001 vertical and 0015 at horizontal or at about the 2:30 position.

But i want good oil pressure.

Those clearances are on the tight side (0.0020" is preferable) but should be OK. Just run a stock oil pump and a lowish viscosity oil to keep the cold start oil pressure not higher than 60psi.

[gonridnu]

Your vertical oil clearance number is the only number you should be measuring. Bearings are manufactured with "ovality" (is that even a word?) that is in direct relationship with the direction of load to counteract bore distortion. Because we do not know, or care, what the rate of taper is in the bearing we should only measure bearing clearance 90 degrees to the parting line. For instance the ovality of an L6 engine may be slightly different than that of a V8 engine due to the difference in load angle.

Housing bores are a completely different subject and can be measured for size and roundness at any point except the parting line.

Clearance specifications are the result of arithemetic ranges possible with parts that meet factory specifications not recomended clearances. Most engine builders prefer .001" of oil clearance per inch of journal diameter (+.0005 for heavy duty applications).

.001" is very tight for 2.5" mains but I have run engines there with no adverse affects other than kinda high oil pressure. One question I would have is if your rod clearances are equally tight? If they are not bear in mind the oil from the mains will supply the rods. I personally do not like tight mains with loose rods.

Are you measuring the journal and then using that very same mic to set your dial bore gage?

I have gotten mine down to 001 vertical and 0015 at horizontal or at about the 2:30 position. As all know im just building a highway motor, But i want good oil pressure.

You'll have plenty of that oil pressure thing....getting rid of of some of it might be a problem though

[bbarrett]

Your vertical oil clearance number is the only number you should be measuring. Bearings are manufactured with "ovality" (is that even a word?) that is in direct relationship with the direction of load to counteract bore distortion. Because we do not know, or care, what the rate of taper is in the bearing we should only measure bearing clearance 90 degrees to the parting line. For instance the ovality of an L6 engine may be slightly different than that of a V8 engine due to the difference in load angle.

Housing bores are a completely different subject and can be measured for size and roundness at any point except the parting line.

Clearance specifications are the result of arithemetic ranges possible with parts that meet factory specifications not recomended clearances. Most engine builders prefer .001" of oil clearance per inch of journal diameter (+.0005 for heavy duty applications).

.001" is very tight for 2.5" mains but I have run engines there with no adverse affects other than kinda high oil pressure. One question I would have is if your rod clearances are equally tight? If they are not bear in mind the oil from the mains will supply the rods. I personally do not like tight mains with loose rods.

Are you measuring the journal and then using that very same mic to set your dial bore gage?

I have gotten mine down to 001 vertical and 0015 at horizontal or at about the 2:30 position. As all know im just building a highway motor, But i want good oil pressure.

You'll have plenty of that oil pressure thing....getting rid of of some of it might be a problem though

I ended up with my tigest main being .001, The rest of them after mixing and matching bearings are around .0015. My rods are mostaly in the .002 range. I will be running a stock pump, And 10 w 30 oil. After break in it will be put on synthetic. I do use the same mike to set the dial bore gauge. Should have it running in the next few weeks.

[lafrad]

Those are REALLY tight clearances. keep an eye on your oil pressure, and make SURE you get a VERY GOOD coating of assembly lube on everything involved....