The bearing clearance limits taken from various sources of engine specs give a range of .0008 to .0028 clearance.  When I take a crankshaft to be ground the standard has been .002.  Asked the shop why a more middle of the road clearance wasnt the objective and he gave me an answer, but I dont remember it. 

So why .002, and not .0017 or .0015.  Is it not important?  Would there be benefits or problems with the tighter figure? 

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

The problem with too little clearance is that when they get hot, the clearances get smaller. It can be just a nuisance like not turning over to start after they have been run to seizing and spinning a bearing shell.

Kenneth

Fredricksburg, Texas

Yeah I considered the spun bearing mess.  Wouldnt that be a disaster .  A person sure wouldnt want to run tighter with 20/50 oil.  Another poss. bad problem could be increased oil pressure, more oil bypassed at oil pump, and less getting to top end.  I say possible because am unsure.  But referring back to the specs, .0015 or .0017 isnt too little clearance.

Are there more negatives, or positives?  Longer crank life, better for stroked cranks, less harmonics, etc.? 

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

The reason there is such a wide variance in the spec manual for the ‘as installed’ bearing to journal clearance is because of the normal tolerances associated with the journal diameters, bearing holes, and the bearing thicknesses.  When you have several variances that work against you when putting them together, you have what are called ‘stackups’.  A worst case scenario for the clearances being on the tight side while all the associated dimensions are within specification are a top of the spec journal diameter, bottom of the spec bearing hole, and top of the spec bearing thickness.  Likewise, the opposites of all these puts the bearing clearances on the loose side.

 

Tighter clearances just require some extra thought in regards to the oil viscosity being used.  Standard weight oils being used with tight clearances can cause an overheated bearing due to inadequate oil flow through the bearing.  As Kenneth mentions, spun bearings now come into play and pretty well ruin your day.  From a performance standpoint, tight clearances will not be as free reving as with looser clearances.  There's some free horsepower and fuel economy here with the proper bearing clearances.  I’ve actually torn a stock street engine down in the past that was not running up to expectations and simply added an additional 0.001” clearance to the mains and rod bearing clearances which made a significant difference in how the engine ran afterwards.  As a general rule, better a little loose on the clearances rather than tight.

 

If I’m not grinding a crankshaft to a specific journal size as a result of performing a size check with the bearing halves installed in their respective holes, then I’ll grind the crankshaft journals on the low (small) side of specification.  This is worth an easy 0.0004”-0.0005” in additional clearance with the crankshaft still being ground within specifications.  There are those instances where I’ll grind the journals either a ½ or 0.001” under the low side depending upon the intended use or application of the engine.  This usually occurs after performing a size check of the bearings in their bores and finding the clearances to be on the tight side if the crankshaft is being ground within specifications.  The connecting rod bearing clearances can also be manipulated by resizing the big end rod holes either towards the large or small side of the spec rather than honing them to size using the middle of the spec value.  There are a multitude of ways of altering the clearances but measuring with anything other than a good set of mics and/or a dial bore gauge will not get you there.  Plastigage is not accurate enough for any kind of serious blueprinting.  Just had to throw that out there.

Lorena, Texas (South of Waco)

Ted, thanks for the explanation.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Common oil clearance spec for performance engines is .001" per inch of journal diameter. That works out to .0022" for rods and .0025" for mains on a Y-Block. I think a street engine could use a little less than that but as has been pointed out, as bearing clearance is reduced oil should get thinner. Since we are mostly using heavy oils in Y-Blocks I look for a minimum .0015" on rods and .0019" on mains even for street engines.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

John Mummert! I just got my crank and block back from the machine shop.The crank did not need to turned down and was simply polished and is still standard size.I will be plastigauging to check it.What is considered to be acceptable clearance in this case?I believe the shop manual lists .002-.0025.The motor is being left stock.I am also taking your advice and having the oil groove in the new cam deepened to .035.Is that sufficient?Thanks