Bearing clearance
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Bearing clearance
I was checking bearing clearances on my new Scat 383 crank and rods last night, and am coming up with .002 on the mains, .003 on the rods, using Plastigage. According to my chiltons manual, these are about the loosest clearances I should run. I will sign out a 2"-3" mic today(in the .0001 range if it is available), and try it again, but before I go and buy a set of .001 under bearings, I'd like some input on what clearances you forum members use, and if my readings are ok to use without damaging something
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Originally Posted by 682XLR8
I was checking bearing clearances on my new Scat 383 crank and rods last night, and am coming up with .002 on the mains, .003 on the rods, using Plastigage. According to my chiltons manual, these are about the loosest clearances I should run. I will sign out a 2"-3" mic today(in the .0001 range if it is available), and try it again, but before I go and buy a set of .001 under bearings, I'd like some input on what clearances you forum members use, and if my readings are ok to use without damaging something
If you increase the clearance .001 the oil consumption per bearing goes to 2 1/2 gallons, the bearing runs only slightly cooler, the crank gets washed by all that oil and the bearing carrying capacity goes way down.
I try to keep my mains at .002 to .0025 and my rods at .0015-.002
I do break the motors in slowly to let the clearance work itself in.
Don't run .003 to .0035
Build it tighter, break it in for a few thousand miles before beating on it and you will have a better motor. I even set my forged bigblock pistons at .0045
Racer
Joined: Apr 2004
Posts: 401
Likes: 0
From: IL
Who's bearings are you using? Sign out a 0-1" mic w/ball attachment, measure each Rod bearing half. Depending on bearings you'll see up to .001" difference between halves (typically .0006"). This is because of the variance in the final "coating" of each half.
Lay them out on a sheet of paper and mark the results (measured at center), when all 16 halves are done you'll see what I mean. If their .030" normally, and two are .0306", you would be .0012" over and end up with tight tolerence. Same if their both .0294", would be .0008" too loose. These values are for discussion only, not actual thicknesses.
This is why FM Race Bearings don't have the final tinning, they are within .0001" of each other. Use to sit and measure before these bearings came out, would grab extras just to get the correct clearances. But that was when we were looking for all the hp we could get.
Double check everything, once it's installed your done. No warranty or exchange after it's in. Your main bearing clearances are fine, check the rods as discribed and see where your at.
Later.
Lay them out on a sheet of paper and mark the results (measured at center), when all 16 halves are done you'll see what I mean. If their .030" normally, and two are .0306", you would be .0012" over and end up with tight tolerence. Same if their both .0294", would be .0008" too loose. These values are for discussion only, not actual thicknesses.
This is why FM Race Bearings don't have the final tinning, they are within .0001" of each other. Use to sit and measure before these bearings came out, would grab extras just to get the correct clearances. But that was when we were looking for all the hp we could get.
Double check everything, once it's installed your done. No warranty or exchange after it's in. Your main bearing clearances are fine, check the rods as discribed and see where your at.
Later.
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
I am using Clevite 77 CB-663 P rod bearings from Summit. They are still mounted in the rods, so I'm going to re-check with the 2"-3" mic and snap gauge, and back it up by measuring the bearing thickness with the ball-end mic. In fact, I'm going to go grab a Bud Light 
and head out to the garage right NOW! Thanks for the tip
and head out to the garage right NOW! Thanks for the tip
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Done measuring, and found the rod bearings to be .062 in the center and .061 at the ends. This doesn't sound right at all, and it's compounded by the fact that the rod journals on the crank were ground to the smaller end of the tolerance scale(2.099). So to sum things up, it seems to be a matter of tolerance stacking adding up not in my favor, so I will have to spring for a set of .001 over rod bearings.
Thanks to those that responded, you guys were a big help, if anyone has any suggestions for a different make of bearing, I'm open
Thanks to those that responded, you guys were a big help, if anyone has any suggestions for a different make of bearing, I'm open
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
I've found plasti gage usually is about 0.0005 too big when checked against a mic. I had to buy .001 U/S rod bearings (663P1) and just use one half with the regular 663P on my Scatt rods.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
