Main bearing clearance
 

Here are the clearances I got ,arp studs torqued with arp ultra lube 130lbs rear 110lb 1-4

Oh 400 stroker

Those clearances are so next year! Sorry, I had to.

If you just switched to studs then the clearance is secondary to the line bore being straight.
This needs to be checked first with a checking bar.

If it were mine i'd buy some .00 1 under bearings and put half on the uppers.

For a 3" main engine it's loose, assuming measurements are correct. Were these measurements taken with plastiguess or a dial bore gauge?

When i built my 4" stroker, I shot for .0025-.00275" on all the mains. I think the rear main came out about .003. I was using a Scat crank with the journals on the small side and the main bores were on the larger side of spec. So I had to use FM 4040M1 bearings, they are .001" tighter than STD.

That seems loose to me based on my minimal experience. My mains are all around .0025 with the largest around .0027. I'd try some different bearings.

You could also have your bearings coated to decrease the clearances. I have done that before and Calico's thick coating decreased the clearance by 0.0007-0.0008". But as Steve said, I would first check your main bores without bearings to check for concentricity and size. I have put main studs in before and they changed the size of the main bore compared to factory bolts and I ended up getting the main line done.

IMO, too loose. I'd shoot for .0025 - .003

All measurements were done with a Fowler bore gage crank was measured with starlet mic ,I'll be getting it linehoned regardless .The vertical clearance was excessive due to crank being worn by .0005.Should the clearance get bigger or smaller as measuring closer it gets to parting line ?Also when checking saddle bores should they be perfectly round ?

I used to mock it up with bearings and crank, light film of oil, torque to value, rotate crank a few times, remove the crank, and take measurements w bearings in place.

That will give you the most accurate measurements.

Is the workshop very cold? Temp-wise?



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I had to use King 0.001 larger halves to get mine just right plain STD were too tight. Mixing the 2 sets got me right on.

It's been my experience that the bores themselves should be fairly round until you get right at the parting line. The parting line taper is in the bearing inserts themselves. The clearance should be tightest 90* to the parting line then get looser by about .0005 as you approach the parting line. This is to prevent the bearing crush causing a high spot at the parting line. The taper can change depending on the bearing crush, the bearing type, torque value, etc etc. Rod bearings can have more parting line taper because of how the rod bearing bore can distort during operation.

Yes. And that's why I mock it up first, remove crank, torque caps to value, and measure with the bearings in place. (After crush)

That should provide readings as it would be in use. Or as close as possible.

The lock/locating tabs on the bearings is an area that will not 'crush' much, or not as much as the rest of the shell.

Old gearhead machinist suggested I do it this way a long time ago, and continue to do so. Try it both ways and take readings and see what you get. Will make a believer out of most.


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Skip that's how I got those clearances is with bearings installed.Just noted today that my bore gauge has spec sheet showing is calibrated to + - .0005.Anyways got out the plastic gauge and front bearing showed 2.75

Id run those clearances ...

Take it from a stroker guy that got .00495 on his mains and 8psi hot old pressure and yet the builder is going to be fighting me in court about it, those initial clearances of .0035 are great to him. LOL

New engine is .0028, great oil pressure.

even without a spec sheet. Basic rule of da thumb-----between .0007 and.001/per 1" dia holds true for the TOP end of pressurized oil clearance fit. On a 3.500" main....your good. On a 3" main your OK...not the shoot for number but OK

Did you mic your bearings before setting them? Rule of thumb: clean and mic the bearings, mic the main journals then set the bearings and measure the bore diameter. Fowler is a pretty good basic bore gauge, but invest in a quality bore gauge graduated in .0001 instead of .0005.

I almost had the same clearance numbers . #1) 3.0 #2) 3.5 #3) 3.4 #4) 3.6 #5) 3.4

Been driving it all summer . Hot in gear oil pressure is 38 psi Cursing at 2000 rpm is 80psi.. I use 20/50 VR1 for oil..