Rod bearing clearance how tight is to tight

[blykins]

The most accurate way to measure clearance is to use a bore gauge/ball anvil micrometer combo.  Measure your bearing shell thickness with the ball anvil mic.  Add that to journal diameter.  Set a flat anvil mic to that dimension, then the bore gauge to the mic as normal.  Measure the bore without the bearing shells in them.  The  bore indicator will leave indentation marks in the soft bearing material and that does affect accuracy.  Using the bore gauge on the hard iron bores does not indent.

That's another way of doing it as well. 

However, if you have a good, quality bore gage, the anvil will not hurt the bearings.  There will be no precision difference between those two methods, but a quality mic won't leave tracks that dig.

[BigBlueIron]

Just heard from the machine shop. They where able to bring clearance up to .002.

[frnkeore]

The reason I asked about using a bore gauge, is that if your using factory rods, the big end bore, can fall between, 2.5907 - 2.5915 a .0008 tolerance and they won't all  be exactly the same in any set I've measured.

They can be honed to the same upper limit on all the rods and pick up maybe .0005 there but, in any case, you will have the same size on all rods and if the journals are all the same, you should have the same clearance on all of them.

Regarding measure the rod journals, with a mic, as Brent says, you need to use a "standard" to set the mic (gauge blocks are best) but, you also need the test the end of the travel, in this case a 3" standard, to make sure nothing in the mic is warn. But, the actual best way, is to use a gauge block set and check the mic at 2.438.

For the inexperienced mic users, they make a "indicating" mic. They are expensive new but, can be found at a affordable price on Ebay. All users of that kind of mic, will get the same reading:
https://www.ebay.com/itm/134142947458?hash=item1f3b8af082:g:KTUAAOSwtSViolIu

If you do indeed have .002 clearance, for your application, I think you'll be fine with good lighter wt oil.

[Joe-JDC]

Take your micrometer to the machine shop and check yours against theirs.  You will be surprised.  We had the problem with telling the machine shop what we wanted on rod and main journal sizes, and they always came back a "little off".  Took the micrometer and checked their standard against our micrometer, and they didn't calibrate theirs to the standard.  It was their machinist mistake, and the boss had him start calibrating their micrometers for each crankshaft grind job.  My local machine shop has quit grinding crankshafts because of the cost to repair the machine and the cost of replacement machines.  It is getting nearly impossible to find a reputable shop in the greater San Antonio area that can even do a multi-angle valve job or turn a performance crankshaft.  We are the 9th largest city in the USA, but haven't got a really good professional machine shop.  EPA has killed most small shops.   Joe-JDC

[Barry_R]

Using a mic and adding up the numbers is not as accurate as a dial bore gauge.  The mic method does not take housing bore distortion and bearing crush distortion into account.  A good quality dial bore will have a very light spring and will not drag a meaningful line into the bearing - just cosmetic.  My local crankshaft guy can consistently remove a half thou or less from a crankshaft journal with a grinder.  As mentioned, a heavy hand on a polisher ain't the tool for that task - it will just indent around the oil holes and around the radii

[BigBlueIron]

Clearance confirmed at .002. Decided to live with it, good or bad. Rings gapped and pistons are in. Now to order a reduced center timing chain. Way to much slop, block was aligned honed and it was mentioned it was pretty bad.

Just an update for anyone that was wondering.

[chilly460]

Piling on if that’s ok…

Any issue with .003”?  Have one with a couple at that size, just a cheap hotrod combo, figuring on maybe running 40wt and calling it good?

[blykins]

I always aim for rod bearing clearances between .0025-.003 and like it when they hug .003”. 

You won’t need to run 40W oil.  JJ ran with .003”/.003” and 5W-20.

[Falcon67]

>Unfortunately, you won't be able to get +/- .001" bearings for a .010" under crank (.009"/.011").

Why I have a crank and a set of 351C rods+pistons in a box.  The shop that did the crank did it a bit "short" and bearing clearance mics out at .0011~.0008.  No .010 bearings with -.001 available unless I pay for custom.  Not worth it for stock parts.  If I decide to use them, I'd take it to a different (better) shop and turn that crank .020 as needed to get the .002~.003.  Last time I looked at the set, I'm pretty sure the guy put the pistons on the rods in the wrong orientation.  Needless to say, that shop isn't around anymore.