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Select (selective?) fit main bearings

Norwegiandiesel

Norwegiandiesel

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Hi guys.
When first assembled Service manual says the main bearings are selective fit. What do you do when you replace main bearings? When explained in the service manual it also seems they mixed different sizes upper and lower halves of bearings in the same bearng , sounds insane to me? Do you do it like this or do you put in standard size bearings if no machining has been done to the crank or web?
Thanks
 
The factory has a wider variety of bearing halves to get the clearances they want.
During a production run tooling wears and allowances need to be made.

Might want to get your crank polished/sized so that journals end up the very same,then plastiguage or mic/bore gauge the bores/crank to be sure that clearances are in spec.

Selective fit bearing installation is common practice on racing engines where a wider variety of over/undersize bearings are available.

1 note is that if you use a standard shell and an over/under size shell on the mains/rods-always use the standard shell in the same position on all the mains-top or bottom,don't mix the positions of like 1/2 shells-I prefer that all the standard shells go in the upper bearing location.

Having your crank ground undersize should eliminate the need for this and can actually make the crank stronger if the grinder adds a generous radius to the fillets.
 
Hi.
Thanks. I understand and it sounds ok to selective fit the bearings except for mixing halfs of different thiknesses. As long as the outer diameter of the halves are equal, there has to be an edge on the inner diameter of the halves, where the halves meets in the journals? I see that I've to machine the crank journals to line up, and then selective fit the bearings at each journal. Mixing fillets of different thickness still seems wrong to me?
 
It;s common practice and is not wrong in any way,but it is always better when things are machined for 1 thickness bearing and everything fits that way the first time-thats why I say have the crank ground undersize and you will be fine with that undersize bearing.
 
Possible to grind the crank anyway? Seems like its a common opinion that its not possible or at least not something to recommend, to weak as is?
Thanks again
 
Sure the crank can be ground-and if the machinist puts what they call a race grind on it-then they leave a generous radius where the journal meets the throw that actually makes the crank stronger.
Really has more to do with the shape of the stone used than it does the machinist-just pay extra to have the standard stone swapped out and the radius stone put in the grinder most times.
Metal tends to be weakest where a sharp corner exists-make that corner rounded and it adds strength.
 
You would be better off getting a new crank shaft. The stock shafts have a coating and grinding them down removes the coating. Make sure the bearing use are the same. Diesel Depot built my engine and used miss matched bearing and just short of 24K lost a rod bearing.
Be safe and buy a Scat crank and use Clevite bearings.
 
635 said:
You would be better off getting a new crank shaft. The stock shafts have a coating and grinding them down removes the coating. Make sure the bearing use are the same. Diesel Depot built my engine and used miss matched bearing and just short of 24K lost a rod bearing.
Be safe and buy a Scat crank and use Clevite bearings.
Click to expand...

There isn't a coating that gets removed when ground.

The treatment done to the crankshafts is hardening-that hardening depending on the process,meloniting,tufftried ect. extends on average 0.20" to .030"-a crank ground undersize should remain in the hardened range.
http://www.inductoheat.com/pdf/Taking-The-Crank-Out-of-Crankshaft-Hardening.pdf
http://www.enginebuildermag.com/2005/04/crankshaftsstock-and-performance/
 
Hi. Thanks again. Seems like the crank is beyond saving. The #1 rod seized, bearing spun and ate the bearing fillets, crank s "ground" at least 0,5mm and the rod is way beyond to be re-used.
Does anybody know if the reason for performing a selective fit of bearings is because of un-even crank journals or misaligned machining of the block and bearing caps? Or a combination of both? I've a SCAT crank, suposedly the journals are even, still need to measure and fit each bearing individual? Or use standard size bearings and possible be good to go?
Thanks
 
Not really if the crank or bores are out of line niether 1 should be used without it first being corrected.

Take the time to check clearances and have the big end of the rods resized to ensure trhe bores are round and on size.
 
The coating is a nitride (did I spelled that right?). I think its a gas process. Its been my understanding that any grinding would take the coating off.
Racer55 do you have knowlege of the process and how deep in go's? Time for a google search me thinks.
 
quote from the Diesel Progress magazine by Dick Hamparian, GM's chief engineer, deep rolled fillets are an important design element contributing to the strength of these 6.2/6.5 crankshafts. GM service manuals indicate that these crankshafts are not suitable for re-surfacing. Re-surfacing journals incorporating deep fillets will produce a new journal surface with somewhat shallower fillets. This weakens the crankshaft. Engine machine shops usually encourage their customers to authorize resurfacing the crankshaft as part of a complete engine rebuild, allowing them to use a matched set of main and rod bearings. This makes their work easier (and helps pay for their expensive crank resurfacing equipment), but other than a polishing, you should insist that your crankshaft NOT be ground. Replace any crankshaft not suitable for re-use. Typical 6.2/6.5 crankshafts, as originally manufactured by GM, may require a "select fit" set of replacement rod & main bearings due to minor variations in diameter between journals.
Click to expand...

There the info, do what you want with it.

:)

Optimizer nodular iron cranks are induction hardened. Cant recall if the gm ones were....
 
Depends on how the grinder profiles the stone.
Most grinders have more than 1 stone with different profiles.
The common grind stone is set with very shallow fillets,but a race profile on the stone has larger fillet relief and is why I said repeatedly to have a race grind done-costs more because they have to change stones.
 
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