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Main Bearings .001 over or not?

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elks

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So working on a motor right now and need to determine if I should order standard main bearing or get a set that is .001 over. The Tolerances for the crank mains are 2.7484 to 2.7492. Each main has been polished and measures out at the following:

2.7485, 2.7484, 2.7484, 2.7483, 2.7483.... So All of them are on the bottom end of the spec. the machine shop said that if I was on the bottom end to order a set of .001 bearings, but in my research I have found conflicting info. I know that no matter what we will need to plastic gauge and confirm oil clearances, but would rather not order 2 different sets of main bearings if I can order it right the first time.
 
It is a street motor going into a Torino. Slight build above stock with an RV cam intake upgrade and header. Nothing major just a decent running street motor.
 
any work been done on the main caps/block?

are measurements taken @ 90* to the parting line?

are the measurements consistent 45* from the parting line (in both directions)
 
If .001 unders were too tight some builders claim you could use just one "half" of the .001 under inserts with a standard sized half to get a .0005 undersized bearing..GM used to "select fit" bearings that way,and if you were not careful to see what sizes they used it could lead to troubles if the crank was not turned or polished and miked for the correct size bearings..once in a while they had a crank with a journal that they ground undersized rather than scrap it,and used appropriate sized bearings to fit that one journal...later on if someone rebuilt the engine and assumed all bearings were standard size--oh-oh!..one ended up being too loose..

I've seen a lot of high mileage engines with cranks that measured within specs but on the "loose" side have .001 under mains installed and they didn't have any troubles with them being too tight,but maybe those guys were just lucky..
 
76zimmer said:
any work been done on the main caps/block?

are measurements taken @ 90* to the parting line?

are the measurements consistent 45* from the parting line (in both directions)
Click to expand...

The numbers were taken at close to 90 from the parting line but not all are exactly at 90. The numbers were really close and consistent at both 45angles. Variances maxed on one main at .ooo4 and the rest were at +/- .0003 and under. I am by no means an experienced machinist/builder but those numbers seemed pretty good considering the main was just polished and not turned? I know perfect would be.0000 but not sure that is ever achieved?

I am just curious as to what would be the point at which a std bearing should be replaced with a .001?
 
those look pretty dam good for a "used" crank...I would try for the tighter side of the tolerance being a moderate street engine, but I think the .001 might be too tight? You are within only one ten thousandth of being in std by the book.
 
elks said:
It is a street motor going into a Torino. Slight build above stock with an RV cam intake upgrade and header. Nothing major just a decent running street motor.
Click to expand...

I knew that diameter sounded familiar.

I shoot for .002 on the mains and .0025 on the rods.
Have you read up on the cleveland oiling system?
 
63chevyll said:
Based on those measurements I woul use a STD bearing set
Click to expand...

i seem to agree with this, however, the only thing that will tell you for sure is when you plastigauge it, then you can see in what part of the range you are in for oil clearance and can make a judgement call from there.
 
I have done so many engines lately with measurements all over place... from loose ground cranks for race engines to tight stock applications.
Everyone has lived and done well.

You.all.would sheet if you knew what ive done just to "see" what would happen. All have lived and had good oil pressure
 
diesel4me said:
If .001 unders were too tight some builders claim you could use just one "half" of the .001 under inserts with a standard sized half to get a .0005 undersized bearing..GM used to "select fit" bearings that way,and if you were not careful to see what sizes they used it could lead to troubles if the crank was not turned or polished and miked for the correct size bearings..once in a while they had a crank with a journal that they ground undersized rather than scrap it,and used appropriate sized bearings to fit that one journal...later on if someone rebuilt the engine and assumed all bearings were standard size--oh-oh!..one ended up being too loose..

I've seen a lot of high mileage engines with cranks that measured within specs but on the "loose" side have .001 under mains installed and they didn't have any troubles with them being too tight,but maybe those guys were just lucky..
Click to expand...

Standard bearings or if what diesel4me is stating is feasible I would go .ooo5 under. Its one thing to run an engine on the loose end, even on the street you can compensate via an oil pump upgrade. However you couldnt convince me to build something over tight...
 
Ok I am going to go with std and see what the clearance specs at. I have a feeling it will be just fine, but if it is on the big gap end I will try switching in and going for .0005 to see what it feels like.

BTW we are building this motor as a school project so as a result I am learning a ton as our my students. Thanks for the help.
 
63chevyll said:
I have done so many engines lately with measurements all over place... from loose ground cranks for race engines to tight stock applications.
Everyone has lived and done well.

You.all.would sheet if you knew what ive done just to "see" what would happen. All have lived and had good oil pressure
Click to expand...

I recall a customer who was rebuilding a Ford 302 buying .010 under rod bearings from me at the parts store,he had the crank ground .010 undersized--after the engine was back in the car and running,it would lose most of its oil pressure after being run about 15 minutes,and you could hear a definite woodpecker type slight knock,which got worse under acceleration...the owner had the shop take the engine back out and sent it back to the machine shop that did the rebuild...they discovered the bearings in the box I sold the owner were actually "standard" sized ones!..no one bothered to look at the markings on them before installing them..the car had been driven that way a good 3000 miles before the noise got disturbing enough for the owner to complain about it too!..

Another customer I knew well got hammered one night at a bar,and on his way home (about 3 miles from the bar),his Ford pickup decided to have the oil filter back off and drop to the pavement,and the rear tire ran it over...he said the oil light came on,the engine started clacking BAD,but he kept driving,knowing if he stopped he'd likely be stuck there--and get caught DWI!--so he headed straight home,and parked it in the garage..

Next day he came and bought a set of rod and main bearings from me,and an oil pump,pan gasket,and oil filter..I told him I doubted he'd be lucky enough to have the engine last more than a few hours,the crank HAD to be scored up pretty badly I assumed..

But he went ahead and installed the bearings with the engine still in the chassis (a "hellatious job I would never do again,he stated!)--and he even used strips of crocus cloth to "polish" the journals where some scratches and melted babbit had stuck to the crank,till they were smooth --despite this crude treatment,the darn engine once again ran just as quiet as before,had 55 lbs oil pressure,and he decided to keep it after driving it a few weeks ,saying "if it aint blown up yet it probably never will"...he drove that thing for years without it ever knocking too...:eek1:
 
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