Bearing Clearance

[rfmaster]

This an age old question - how much bearing clearance should you run in street driven 350 SBC. One common recommendation is:
Rod bearings 0.0020 - 0.0025" , side clearance 0.010 - 0.020"
Main bearings 0.0020 - 0.0030"
Crankshaft end play 0.005 - 0.007 crankshaft end play

I am in a process of putting a budget 350 (880 block, 1peice RMS)

Micrometer: Mitutoyo 103-217 (2 to 3", 0.0001 resolution)
Bearings: King Bearing 557SI (STD and -001)

Mains Measured Plasti-Gauge Plasti-Gauge
103-217 | STD | -001
M1: 2.4500 | 0.002 | n/a
M2: 2.4495 | 0.003 | 0.002
M3: 2.4495 | 0.003 | 0.002
M4: 2.4495 | 0.003 | 0.002
M5: 2.4490 | 0.003+ | 0.002 to 0.0015 (Probably 0.0018)

Rods
R1/2 2.0995 TBD (to be determined)
R3/4 2.0995
R5/6 2.0995
R7/8 2.0995

Crank thrust movement: 0.005 to 0.007 depending how hard I pry my bar

So I am going with mix set of main bearings - on the mains I am keeping #1 STD and 2 through 5 -0.001" based on platigauge indication. My only concern that #5 Main is a bit on tight side:0.0018"

Comments regarding #5 being a bit tight....

//RF

[prostreetC-10]

Was there any machine work done? Was the crank turned or polished? Block line honed? ....or are you just replacing the bearings?

[rfmaster]

Crank was polished (it was not turned) - it had 140k on it when I yanked it in favor of 3.75" stroker.
Block probably was line honed at one time or another. Crank turns very nicely with bearings configured in original post. It is a budget build - just tossing new bearings and rings and put it back into service.

//RF

[manimal]

#5 is the thrust bearing right? .0018 is too tight. I always set that one at .0025-.003

[crazy longhorn]

Originally Posted by manimal

#5 is the thrust bearing right? .0018 is too tight. I always set that one at .0025-.003

Yup, I would run a tad looser on the thrust main also. Longhorn

[GRX]

^^ not only that, #5 gets the full flow of oil then sends it on down the line/s. Otherwise clearances look good. You are installing a new pump right? Std volume with high pressure spring.

[LONGHAIR]

If you are that concerned about it being too loose with the standard bearing and too tight with the -.001 bearing, have the machine shop polish it a little more.

[bobs86silv]

Are you using any lubrication on the bearings or journal when you are measuring? Even a splash of any goo can give you a false reading. Rods are a bit trickier because you can easily distort the strip when you are taking the rod caps off. Is the crank nice and round? Is taper in check, or are you figuring from hi/average? A bit to the looser side will keep you out of danger, a bit on the tight side can get ugly fairly quick. Lots of science to bearing load and oil film, but SBC is somewhat forgiving on clearances.


Good luck! Robert