bearing tolerances

[maui]

I keep forgetting about this but what is the wear limit on bearing before it has to be machined. 000 is to tight and .003 under to loose or. So maybe 2.000 to 2.003 is good and maybe 2.165 to 2.168 is good.

[martin]

Do you mean thrust cutting the case for oversize bearings?

[DonPauter]

for a street application I would just use the basic .0010/.0015" per inch of journal diameter, though you can run slightly less I would not exceed the larger number. if racing only you can increase it slightly to .002" per inch.
 before starting inspect both journals and rod housing bores they should be round within .0004" and parallel within .0003", check with bearing or rod supplier for min / max housing bore diameter values. We use 2.1245- 2.1252" for Chev/buick and 2.2755/2.2762" for VW rods and most are held within .0002" of bottom tolerance
 Clearances need to be mic'd and bore gaged for true accuracy with bearings torqued to spec in their rod bores and measured at 10-4, 6-12 and 2-8 o'clock positions , do not measure bearings straight across 3-9 as most all bearings are thinner for about 10/15 degrees above and below parting line of the rod and will show up as having more clearance than the positions previously stated.
keep in mind there are thicker and thinner bearing shells available for some of the GM 2.000" & 2.100" rod journals commonly adapted for use in ACVW combos which may save you time and expense of reworking parts to achieve target clearances.
  Given the wide range of new, used or reworked cases available  main bearing clearance is often a take what you can get situation and any long term stabilization of their size borders on mythology, I will say that they are suprisingly reliable for as unstable as they can become thru both thermal and dynamic assault.  When line boring either mag or aluminum we've always targeted the bottom side of clearance here @ .0015/.0020" actual, because they're going to loosen up when warmed anyway so why start out with a target operating clearance set at room temp ...  actually this is the same logic we've applied to our aluminum BigBlocks and it's proved to be sucessful so far as well. having said that I would not like to see more than .003 "actual on a fresh rebuild (room temp)
   One more thing... in racing venues where excessive clearance is used the lubrication demands often require specialized high vicosity,high volume, high pressure systems.

[maui]

Mahalo Don, my crank mains are right at 002 under so that must be good but the rods are from 0023 to 0035 under so it's off to the shop. Also, I didn't know that some bearings can be ordered looser or tighter for fine tuning so it no need go to the shop and I will remember that.

[DonPauter]

unfortunately the various +/- size bearings are only available for use with standard diameter cranks not regrinds, however as long as reground rod journals are within usable specs (round/taper) as noted in the previous post you can adjust clearance by tightening up big end rod bores. for example the crank that is in my Ghia was originally used in a record holding sand dragster and had been reground and nitrided at least twice to it's current .020" under with an added -.0015" to reach the previous owners target of .004" clearance. In order to achieve my target clearance of .0015" I was forced to resize the rods  more than .002" under the 2.2245" minimum recommended diameter and actually increase bearing crush to get there. There are limits to how far you can go with this process before you reach a diminished return and it's not something for inexperienced builder but it is a viable option for reaching a workable solution to a sticky problem.

[maui]

Mahalo, I know what you mean because I had a set of rods cut before and it got about .004 shorter but was right on round and all good.

[Ohio Tom (DdK)]

Thanks Don for all your input.
Great stuff for us "novice" engine builders to know.

I recently built a 2332 with Chevy journals.
The crank is a used Scat volksracer (very hard steel).
The crank journals were just a fuzz small and gave me .003" clearance.
The crank grinder didn't want to grind undersize as the journals were perfect in all respects except O.D.
I purchased 1/2 set of standard bearings and 1/2 set of .001" under bearings. I installed 1/2 shell of each kind on each rod.
This brought my clearance to .0025"
The motor is alive and working very well.

[maui]

Wow Tom, you guys think of all kine stuff and it got me thinking so the next time around I can try your idea.