Unsure what to do with 0.0020" Main Clearance

[Dave Koehler]

I almost did not want to get in on this but I am a glutton for punishment.
I have a blown alky BBC that I sometimes get to play with. it is not a big show car or something on the edge.
I run .002 on the mains....on purpose.
Now it was a lot of measuring and triple checking to arrive at that point successfully.
I suppose it helped that I personally ground the crank and prepped the main saddles, etc.
I did this because I run a solid aluminum block.
I went this tight because in earlier incarnations I kept a watch on oil pressure vs clearance vs bearing wear.
Things are better for me with "tight" mains.
I warm it up and don't whack it until it's got heat in the oil.
All seems to work.

[n2omike]

Typically there is quite a bit of raised, displaced steel around the letter and numbers stamped on the back of the shells. Many times the shells will also have displaced steel all along the edges lengthwise. A skilled machinist armed with a fine precision file or diamond lapp can detail the back of the shells to gain a few tenths of a thou clearance and IMPROVE accuracy and heat transfer. Now try to get paid for your time!

Looking at these 'Race' bearings with their ugly surface finish... on the front AND back side, I decided to try and smooth the backs out a bit. I figured it was just some of the same coating that was on the front, but not really needed on the back. So, I took a sanding sponge, and lightly went over the backs, focusing a little more time around the letters, edges and tabs. They still have the copper color, and I smoothed out the entire underside of the bearing to avoid any issues with eccentricity.

After doing this, I re-checked clearances by using the mic on the crank, looking at the reading, locking it... then zeroing the dial bore gauge to that size. I then placed it on the crank, and had gained three tenths of clearance! So, it's right where I want it to be, and I don't think there's been any harm done.

I don't LIKE putting the stylus of the dial bore gauge directly on the bearing, as it does scratch... so I believe it also provides an artificially large reading. I partly blame this for getting close to a half thou larger reading when using this method vs. Plasti-gage and Subtraction of all the thicknesses. Before, I was getting 0.0025" with the dial bore on the bearings, and 0.0020" to 0.0022" using the other methods. After smoothing out the backs, I'm now getting a solid 0.0028" with the dial bore on the bearings. Even subtracting for the scratches, it puts the clearance right where I want it!

Below are a couple pictures of the bearings. One taken in the daylight, where you can see more detail... but the sun washed out the remaining copper color from the coating... and one inside to show the color still on the bearing.

I'm more comfortable doing this than having a half thou micro polished off the crank, or getting it turned.

Thanks for all the information! LOTS of good knowledge on this board!!!

Of course, feedback is always welcome.

Thanks again!

[vortecpro]

Your a bearing sander...........you can't do that......LOL

[racetowin]

Typically I find myself in the bearing sanding mode when I get a crank from lets say Scat and its big and I don't want to grind the crank or open up the housing bore outside of spec, and I need a few tenths extra clearance to sleep good. I have a friend that machines and builds engines and there no end of the -hit I get from him over sanding bearings in rare instances, but again he really knows very little of anything............
[/quote]


Oh, come on, really Mark??? Not this again with the sanding of the bearings..... I thought we had agreed to disagree about this before...... And, I know a little about lots of things! (Just enough to get me in trouble)

[gmrocket]

Your a bearing sander...........you can't do that......LOL

If you count the strokes, it works out perfect.

[BigBlocksOnTop2]

About the stylus marring the bearing...that would not bother me at all, it is so slight it doesn't matter. Glad to hear you whipped that thing.

[jed]

An option that has not been brought up is installing and torquing the bearings in the block and line honeing
the bearings with 2/3 strokes with light pressure. This will remove .0002/.0003 and solve your problem.
However I don't see any problem with .002 main clearance in this application.