Optimal Clearances for Different Setups
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From: Wichita, Ks
It seems like we are getting some good information here and I appreciate everyone's input. I wonder what the difference should be for long term durability versus an engine that's going to be refreshed about once a season. It seems to me that the tighter the clearances (within reason) the bearing life will be longer but the friction will be greater. On the other hand, looser clearances (again within reason) will result in less frictional loss (especially at higher RPM's) but the extra clearance could cause the bearing to become eccentric or oval shaped due to the metal to metal contact that occurs at TDC and BDC.
Steve
Steve
Originally Posted by Steve Bryant
It seems like we are getting some good information here and I appreciate everyone's input. I wonder what the difference should be for long term durability versus an engine that's going to be refreshed about once a season. It seems to me that the tighter the clearances (within reason) the bearing life will be longer but the friction will be greater. On the other hand, looser clearances (again within reason) will result in less frictional loss (especially at higher RPM's) but the extra clearance could cause the bearing to become eccentric or oval shaped due to the metal to metal contact that occurs at TDC and BDC.
Steve
Steve
I am not sure if we'll ever know which way is "better". Thats why the main and rod numbers I use are right in the middle of GM's spec. This kind of blends all the factors in one clearance!
Seems to be working great so I stopped think about it! LOL!!!
Last edited by 9D9LS; Jan 15, 2006 at 09:52 PM.
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From: Wichita, Ks
9D9LS,
I'm kind of **** about best practices myself. I used to be a Detroit Diesel mechanic and my dad was very much a master mechanic back in his time. He had his own independent garage and in time became a new car dealer. He even taught depot overhaul of engines and drivetrain components in the Army. He was a stickler for detail and he was my first teacher.
Now, at age 56 I'm an aircraft electronics engineer (been in the industry for 30 years) but I never put up my wrenches. I used to do it out of economic necessity. Now I just do it for fun and am proud to consider myself a gear-head.
Steve
I'm kind of **** about best practices myself. I used to be a Detroit Diesel mechanic and my dad was very much a master mechanic back in his time. He had his own independent garage and in time became a new car dealer. He even taught depot overhaul of engines and drivetrain components in the Army. He was a stickler for detail and he was my first teacher.
Now, at age 56 I'm an aircraft electronics engineer (been in the industry for 30 years) but I never put up my wrenches. I used to do it out of economic necessity. Now I just do it for fun and am proud to consider myself a gear-head.
Steve
You guys keep saying that the expansion of the aluminum block means different main clearances. What about the fact that the main caps aren't aluminum? Doesn't that reduce the effect?
FWIW my new aluminum shortblock has .0025" main, .0022" rod clearance. Oil pressure is 60 psi at startup when cold and idling around 1000 rpm. By the time the motor warms up the combination of the expansion, the thinning of the oil, and the rpms dropping to 750 leaves me with only about 25 psi oil pressure.
FWIW my new aluminum shortblock has .0025" main, .0022" rod clearance. Oil pressure is 60 psi at startup when cold and idling around 1000 rpm. By the time the motor warms up the combination of the expansion, the thinning of the oil, and the rpms dropping to 750 leaves me with only about 25 psi oil pressure.
I realize that, i'm saying won't the steel caps resist the expansion of the aluminum, at least from side to side? Or would you end up having a different clearance on the top half of the bearing than the bottom half with an aluminum block?
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From: Wichita, Ks
P Mack,
What happens (in my opinion) is that since the two halves of the bearing are installed in the cap (lower) and block (upper), the aluminum block grows away from the center of the crank journal with increasing operating temperature. Since the block is backing-up the upper bearing half, the bearing clearances for all of the main bearings increase more than in comparison to a cast iron block. This increased clearance at operating temperature with the aluminum block can cause relatively lower oil pressure when compared to the iron block. This is true even if both blocks have the same main bearing clearances at a given room temperature.
You are right that the rate of expansion of the caps should be the same. It's the expansion of the block that makes the difference.
Steve
What happens (in my opinion) is that since the two halves of the bearing are installed in the cap (lower) and block (upper), the aluminum block grows away from the center of the crank journal with increasing operating temperature. Since the block is backing-up the upper bearing half, the bearing clearances for all of the main bearings increase more than in comparison to a cast iron block. This increased clearance at operating temperature with the aluminum block can cause relatively lower oil pressure when compared to the iron block. This is true even if both blocks have the same main bearing clearances at a given room temperature.
You are right that the rate of expansion of the caps should be the same. It's the expansion of the block that makes the difference.
Steve
Last edited by Steve Bryant; Jan 16, 2006 at 07:38 PM.
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From: Spring Hill, TN
Originally Posted by Steve Bryant
P Mack,
What happens (in my opinion) is that since the two halves of the bearing are installed in the cap (lower) and block (upper), the aluminum block grows away from the center of the crank journal with increasing operating temperature. Since the block is backing-up the upper bearing half, the bearing clearances for all of the main bearings increase more than in comparison to a cast iron block. This increased clearance at operating temperature with the aluminum block can cause relatively lower oil pressure when compared to the iron block. This is true even if both blocks have the same main bearing clearances at a given room temperature.
You are right that the rate of expansion of the caps should be the same. It's the expansion of the block that makes the difference.
Steve
What happens (in my opinion) is that since the two halves of the bearing are installed in the cap (lower) and block (upper), the aluminum block grows away from the center of the crank journal with increasing operating temperature. Since the block is backing-up the upper bearing half, the bearing clearances for all of the main bearings increase more than in comparison to a cast iron block. This increased clearance at operating temperature with the aluminum block can cause relatively lower oil pressure when compared to the iron block. This is true even if both blocks have the same main bearing clearances at a given room temperature.
You are right that the rate of expansion of the caps should be the same. It's the expansion of the block that makes the difference.
Steve
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From: Wichita, Ks
jyeager,
You summarized the intent of my thoughts exactly! Thanks. I'ts good to see another Tennessean chiming in on this thread. I live in Kansas now, but I am from Cleveland, TN.
Steve
You summarized the intent of my thoughts exactly! Thanks. I'ts good to see another Tennessean chiming in on this thread. I live in Kansas now, but I am from Cleveland, TN.
Steve
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From: Spring Hill, TN
Originally Posted by Steve Bryant
jyeager,
You summarized the intent of my thoughts exactly! Thanks. I'ts good to see another Tennessean chiming in on this thread. I live in Kansas now, but I am from Cleveland, TN.
Steve
You summarized the intent of my thoughts exactly! Thanks. I'ts good to see another Tennessean chiming in on this thread. I live in Kansas now, but I am from Cleveland, TN.
Steve
In theory doing an align hone could be improved if done with the block heated to operating temp.
