under/oversize bearings

Belgian1979vette

Belgian1979vette

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I have a question about under and/or oversize bearings.

A couple of bearings i took out of my original block are indicated .0006 and .0012. There is no plus or minis.

Does this mean they are .0006 and .0012 oversize ?

They are GM originals
 
Yes GM used a lot of .0005 oversize bearings on the X rods in these motors too.
 
IMG_2589.jpg


These came out of 1 main. Rest is likewise. Looks awfully strange to me. Will have to measure, maybe tomorrow.
 
I found out these are select fit bearings as used at the factory to customize clearances. Not for sale to the general public.

I need a bearing that is .0012 thicker. Closest i can get is .0010. Guess it'll have to do.
 
gkz said:
What final clearance are you shooting for?
Click to expand...

I was shooting for a .0024" on the mains and on the rods. Even with the rods it's difficult to get close like this, but some guys solved my problem. I leaving the mains as is, since the difference is only .0002. As for the rods i will be using one half of .001 oversizes and one half standards. Should be perfect.
 
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I should be a bit worried of mixing different bearing sizes!
The risk is that the sharp edge in the thicker bearing will scrape out the oil from the crank.... I really don't like it.

I would select the thicker bearing set and then to polish the crank with a sand band to reach the correct clearance!

Just my opinion, of course!
 
Stroker-427 said:
I should be a bit worried of mixing different bearing sizes!
The risk is that the sharp edge in the thicker bearing will scrape out the oil from the crank.... I really don't like it.

I would select the thicker bearing set and then to polish the crank with a sand band to reach the correct clearance!

Just my opinion, of course!
Click to expand...

Actually the groove in the upper bearing has to move around a step as well to go to the lower part. Besides, were talking about a .0005" (0,012 mm) difference. Low pressures due to excessive bearing clearances are by far more destructive than a marginally lower flow.
 
Sorry but I don't agree..... :-)

Actually is the flow lubricating the engine, not the pressure!
The pressure in the circuit only tell you that there is enough flow to warrant that the oil is anywhere in the engine!

In fact the pressure in the bearing is near to zero, due to the fact that this is the area where the oil come out from the circuit!

Neither the rod load can alter the pressure in the circuit because the hole in the crank is in a position without loads.

With the today's syntetic oils you can even neglect the old rule "10 psi each 1000 rpm".
Is the oil that need to have the ability to withstand the loads before to break he's molecules.

In my engine (4" of stroke with 7000 rpm) I chosed a bit higher bearing clearance with very good high volume oil pump.

.....still my two cents...... like you wrote in your signature.... ;-)
 
The difference in bearing thickness will manifest itself where the 2 halves meet and this will not be a problem or a simple reason. Big ends are not round, they are oval/out of round with more bearing clearance on the sides where the 2 halves meet. This is done so that there's not a clearance problem after the bearing halves crush (new bearings stick out past the mating surface). I have mixed bearings before, the old ones were GM originals w .0005 oversize so I used half a .001 set to get the proper clearance.
 
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