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drilling out main bearing feads

 
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solid
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PostPosted: Sun Aug 06, 2006 4:38 am    Post subject: drilling out main bearing feads Reply with quote

hi from australia
when my new engine was being machined the guys drilled out the main bearing feeds and the bearings supposedly for more suppy???
but when the guy that assembles my engine see this he was shocked and had never heard or seen it before.....he sugested that if you enlarge the feed holes it will lose pressure to other places in the engine!!!!
so he asked around as did i nobody down here seem to no about it ether...
as we didn't have a problem before we fitted undrilled bearings instead....the engine is about to be freshened up and i was wondering it there an advantage in doing this????
thanks
danny smith


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Hanz
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PostPosted: Sun Aug 06, 2006 5:55 am    Post subject: Reply with quote

It is common to drill out any small or poorly factory drilled passages. The oil pressure is determined by the point of final leakage, primarily rod/main bearing clearance. (btw, connecting rod side clearance is NOT a factor in oil pressure contrary to some beliefs)

With that said, in your pic the main bearing feeds do seem quite large, I only clean up my small blocks to 1/4", and I drill the holes in the upper bearing half to match.

Hanz

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artinla
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PostPosted: Sun Aug 06, 2006 7:29 am    Post subject: Reply with quote

Solid-

Those holes look just fine. I would run unmodified bearings if I were you, I have never been a strong proponent of modifying crank feeds or the bearings. Some people make ridiculous grooves and chamfers around the oil feed holes in crankshafts. Lightly chamfer, debur and remove any flash or obstructions. Leave the rest alone. Oil timing mods can get you into serious trouble unless you know exactly what you are doing.

Hanz is correct about the side clearance not determining oil pressure, although there are reasons other than oil pressure to keep side clearance low. Don't just ignore it. If a rod gets too narrow you can run into crank radius to bearing interference and also the rods could start banging into each other or the rod thrust area if the clearance does not support enough oil pressure between the rods to hold them apart.

Art
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Big Dave
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PostPosted: Sun Aug 06, 2006 8:34 am    Post subject: Reply with quote

It is hydrostatic pressure that is the actual bearing surface, not the metal of the crank or the babbit. If, as has been mentioned, you start getting into artistic acts of sculpture with grooves and chamfers you are decreasing the oil pressure in those areas (pressure is inversely related to volume) and thus decreasing your bearing area. All that is required is that the hole in the bearing not cover the supply feed hole in the block (restriction to the oil feed).

Any irregularities in oil feed (the bearing hole not matching the feed hole) is an example of a poorly machined block (bit wandered from were it was supposed to be due to a worn spindle or inattentive machinist). If it is off at the point of entry it probably drifted through out the bore and may not have made proper contact with the intersecting feed within the block.

In such a case you would be justified in remachining a larger hole to verify proper oil volume. Pressure in any closed hydraulic system is in theory instantaneously the same at every point.


Big Dave
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af2
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PostPosted: Sun Aug 06, 2006 11:12 am    Post subject: Reply with quote

Hanz wrote:
The oil pressure is determined by the point of final leakage, primarily rod/main bearing clearance. (btw, connecting rod side clearance is NOT a factor in oil pressure contrary to some beliefs)

Hanz


Oops. Shocked I knew that. retard
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solid
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PostPosted: Sun Aug 06, 2006 9:47 pm    Post subject: Reply with quote

thanks for the help
im glad i trusted my engine builder hes old friend thats been building engines and heads for as long as i can remember......
were pulling the caps of this week so i will let you no how bad the bearings are after 12 burnout and some cruising.....
was hammering bearings before using 25/70 went to 40/70 heaps better under the heat and pressure of doing burnouts at 7000rpm for 2-3 min poor little small block....broke a few till we got it right....

not many good engine builders i the state i live in as our drag strip has been closed for 7 years with no 1/4 mile comp racing ....speedway is not big over here now to....very sad

so were doing burnouts

giving smoke to the car gods Laughing

till later
danny

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