Ever add a thousandth to top rod bearing clearance?
12-16-2014, 11:44 AM
#1
Ever add a thousandth to top rod bearing clearance? You guys ever add a thou to the top rod bearing clearance in order help to keep it off of the crank on higher pressure combustion chamber builds?
Almost every time someone posts pics of a used top rod bearing it shows more wear than any other bearing surface. Am sure that this is due to deflection of the rod arch that supports the bearing. The deflection is greater once more chamber pressure is applied (higher HP builds). So, in addition to stronger rod materials (less deflection), doesn't it make sense to add a little more bearing clearance? Just the top half of the bearing, not the cap/bottom side. Maybe .003" top, .002" bottom. How much could we add, .004" or .005" w/ heavier weight oil?
Almost every time someone posts pics of a used top rod bearing it shows more wear than any other bearing surface. Am sure that this is due to deflection of the rod arch that supports the bearing. The deflection is greater once more chamber pressure is applied (higher HP builds). So, in addition to stronger rod materials (less deflection), doesn't it make sense to add a little more bearing clearance? Just the top half of the bearing, not the cap/bottom side. Maybe .003" top, .002" bottom. How much could we add, .004" or .005" w/ heavier weight oil?
12-17-2014, 08:51 AM
#2
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I noted Tony's name in your sig, have you considered asking him? Reading through his engine builds, he pays extra attention to bearing clearances, however never heard him specify what numbers he aims for. Would be worth a question. I may ask him when he gets back to me sometime soon.
12-17-2014, 09:50 AM
#3
I noted Tony's name in your sig, have you considered asking him? Reading through his engine builds, he pays extra attention to bearing clearances, however never heard him specify what numbers he aims for. Would be worth a question. I may ask him when he gets back to me sometime soon.
Yah, ask him when he gets back to you. It makes sense to me to add more clearance as combustion chamber pressure increases. However, I wouldn't want to create other unforeseen issues in the process. Would be good to know how much is too much, as well.
12-17-2014, 04:03 PM
#4
You don't want to run anymore than .0005" difference between an upper and lower bearing. So for example, if you have .003" bearing clearance with std. bearings, and you half shell with an X bearing, it should give you .0035". Now I'm assuming you're talking about a 2.100" rod journal. With that, I like around .0028"-.0032". My emc engine last year had .0028" rod and .004" main bearing clearance. With 5w20 oil, it had 60psi hot at 6500 rpm. You have to make sure that you have a pump that will keep up with the demand. Thicker oil isn't the answer for that.
12-17-2014, 07:33 PM
#5
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The top shell as alot of load on it from compression stroke and ignition advance/combustion its enevitable that is wears out and the other looks better
You need to run a H bearing that has a higer eccentricity built into it to address the cap distortion and enough clerance but that is dependant on rod design what clerance it wants
You need to run a H bearing that has a higer eccentricity built into it to address the cap distortion and enough clerance but that is dependant on rod design what clerance it wants
12-25-2014, 09:57 AM
#6
Uncontrollable factors like bad gas, mid grade only stations, warm/hot IAT, bunch of passengers/weight, can cause the spark before TDC which tries to back the crank throw even though its moving north. No amount of bearing clearance, material, or oil blend can eliminate shell/throw contact when these conditions are present.
01-05-2015, 10:07 AM
#7
You make the problem worse, not better.
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01-05-2015, 12:03 PM
#8
You don't increase the leak rate if you have enough pump to maintain the same pressure as it had prior to adding clearance. Also, all top bearings kiss the crank shaft @ some point. Every pic we've seen show wear on top & no wear on bottom half of rod bearings. Since bearing clearance spec is .002-.003, I'm inclined to go for .003".
01-05-2015, 01:02 PM
#9
The system is the sum of the pressure drops. Unless you can measure the pressure drop at the rod bearing, you can not be sure you are maintaining the film "strength" at that location.
Remember, the rod is at the end of the line.
If you exceed the design load, or rub during cranking before the pressure rises, the bearing will contact the crank pin.
If you are collapsing the film during operation, you will collapse it regardless of clearance.
Remember, the rod is at the end of the line.
If you exceed the design load, or rub during cranking before the pressure rises, the bearing will contact the crank pin.
If you are collapsing the film during operation, you will collapse it regardless of clearance.
01-14-2015, 03:01 PM
#10
I have always thought of it like a bicycle sprocket. With just a smooth pedal(small cam) the the wear on your sprocket is going to be pretty even, with slightly more on the teeth used at each down stroke. Now with a heavy pedal (big cam or more compression), where you are standing up with each stroke, the wear on those same teeth is of course going to be greater. This is just how I always had it in my little pea brain and may not make sense to anyone else. There may be ways of mitigating that wear, but it can never be eliminated.
01-14-2015, 07:03 PM
#11
I'm not convinced. Big blocks which have more area above the rod bearing are less susceptible to spun bearings due to rod bearing deflection. This has nothing to do with oil film, it's simply less deflection @ the same CC pressure. So, just because oil film has collapsed doesn't mean that there has been contact between the rod bearing & crank. The BB is deflecting into the oil film, but not enough to touch the crank.
Bearing wear @ start up is not the same condition as spinning a bearing due to high RPM high CC pressure related deflection.
Because there is no room within specs to add much clearance, stronger materials that will resist deflection must be the best solution...or maybe there are LS cranks & rods that allow the use of wider than stock rod bearings (more surface area to resist deflection)?
Bearing wear @ start up is not the same condition as spinning a bearing due to high RPM high CC pressure related deflection.
Because there is no room within specs to add much clearance, stronger materials that will resist deflection must be the best solution...or maybe there are LS cranks & rods that allow the use of wider than stock rod bearings (more surface area to resist deflection)?
01-15-2015, 11:28 AM
#12
I'm not convinced. Big blocks which have more area above the rod bearing are less susceptible to spun bearings due to rod bearing deflection. This has nothing to do with oil film, it's simply less deflection @ the same CC pressure. So, just because oil film has collapsed doesn't mean that there has been contact between the rod bearing & crank. The BB is deflecting into the oil film, but not enough to touch the crank.
Bearing wear @ start up is not the same condition as spinning a bearing due to high RPM high CC pressure related deflection.
Because there is no room within specs to add much clearance, stronger materials that will resist deflection must be the best solution...or maybe there are LS cranks & rods that allow the use of wider than stock rod bearings (more surface area to resist deflection)?
Bearing wear @ start up is not the same condition as spinning a bearing due to high RPM high CC pressure related deflection.
Because there is no room within specs to add much clearance, stronger materials that will resist deflection must be the best solution...or maybe there are LS cranks & rods that allow the use of wider than stock rod bearings (more surface area to resist deflection)?
If the oil film has collapsed I don't see how any additional clearance is going to help. There is going to be metal to metal contact at some point.
Truer bearings of the proper material (coated aluminum) and oil, truer journals, pins and bearings is the only fix.
01-16-2015, 01:37 PM
#13
No offense intended, but this section used to be a place where very experienced guys came to share their information. Where have they gone?
The deflection I am describing has nothing to do w/ misalignment.
-Think of the CC pressure as the force that a hammer applies to the head of a nail as it strikes the nail.
-The vertical section of the rod would be the nail.
-The top half moon part of the rod would be the piece of wood that the nail is trying to penetrate. In the case of deflection; instead of a nail penetrating a board, it is the vertical part of the rod trying to penetrate the half moon arch that houses the top rod bearing. Instead of penetrating the half moon, it pushes (deflects) the center of the half moon toward the crank. The greater the force, the greater the deflection.
-In all cases, the oil film is forced out the sides of the bearing when the half moon housing deflects under CC force. Once the force is great enough, the deflection created by the force will push the half moon housing & bearing into the crank independent of the amount of oil film because the oil is forced out.
-In order to make sure this doesn't happen, the deflection needs to be calculated & then stronger materials used to reduce the deflection or increase the gap between the crank & bearing. Since .003" is max allowable clearance, the only other solution is to use better material or create a larger cross section of material in order to resist deflection.
The deflection I am describing has nothing to do w/ misalignment.
-Think of the CC pressure as the force that a hammer applies to the head of a nail as it strikes the nail.
-The vertical section of the rod would be the nail.
-The top half moon part of the rod would be the piece of wood that the nail is trying to penetrate. In the case of deflection; instead of a nail penetrating a board, it is the vertical part of the rod trying to penetrate the half moon arch that houses the top rod bearing. Instead of penetrating the half moon, it pushes (deflects) the center of the half moon toward the crank. The greater the force, the greater the deflection.
-In all cases, the oil film is forced out the sides of the bearing when the half moon housing deflects under CC force. Once the force is great enough, the deflection created by the force will push the half moon housing & bearing into the crank independent of the amount of oil film because the oil is forced out.
-In order to make sure this doesn't happen, the deflection needs to be calculated & then stronger materials used to reduce the deflection or increase the gap between the crank & bearing. Since .003" is max allowable clearance, the only other solution is to use better material or create a larger cross section of material in order to resist deflection.
01-16-2015, 02:51 PM
#14
No offense intended, but this section used to be a place where very experienced guys came to share their information. Where have they gone?
The deflection I am describing has nothing to do w/ misalignment.
-Think of the CC pressure as the force that a hammer applies to the head of a nail as it strikes the nail.
-The vertical section of the rod would be the nail.
-The top half moon part of the rod would be the piece of wood that the nail is trying to penetrate. In the case of deflection; instead of a nail penetrating a board, it is the vertical part of the rod trying to penetrate the half moon arch that houses the top rod bearing. Instead of penetrating the half moon, it pushes (deflects) the center of the half moon toward the crank. The greater the force, the greater the deflection.
-In all cases, the oil film is forced out the sides of the bearing when the half moon housing deflects under CC force. Once the force is great enough, the deflection created by the force will push the half moon housing & bearing into the crank independent of the amount of oil film because the oil is forced out.
-In order to make sure this doesn't happen, the deflection needs to be calculated & then stronger materials used to reduce the deflection or increase the gap between the crank & bearing. Since .003" is max allowable clearance, the only other solution is to use better material or create a larger cross section of material in order to resist deflection.
The deflection I am describing has nothing to do w/ misalignment.
-Think of the CC pressure as the force that a hammer applies to the head of a nail as it strikes the nail.
-The vertical section of the rod would be the nail.
-The top half moon part of the rod would be the piece of wood that the nail is trying to penetrate. In the case of deflection; instead of a nail penetrating a board, it is the vertical part of the rod trying to penetrate the half moon arch that houses the top rod bearing. Instead of penetrating the half moon, it pushes (deflects) the center of the half moon toward the crank. The greater the force, the greater the deflection.
-In all cases, the oil film is forced out the sides of the bearing when the half moon housing deflects under CC force. Once the force is great enough, the deflection created by the force will push the half moon housing & bearing into the crank independent of the amount of oil film because the oil is forced out.
-In order to make sure this doesn't happen, the deflection needs to be calculated & then stronger materials used to reduce the deflection or increase the gap between the crank & bearing. Since .003" is max allowable clearance, the only other solution is to use better material or create a larger cross section of material in order to resist deflection.
01-16-2015, 02:53 PM
#15
No offence, I sir am too stupid to be offended. I guess I just have my definitions mixed up a little....what you just described is what I would call Rod Torsion. I thought deflection was always result of misalignment......I'll go back to the Racers Lounge now.
01-16-2015, 03:16 PM
#16
[QUOTE= I've even narrowed the existing narrow bearing over .100" with no detrimental results at all.[/QUOTE]
Damn that's alot! Last question and I'm gone for another 3 years. Was this just experimental? Like trying to reduce surface area for bearing temps or reduced drag....did you have to go with a heaver weight oil. Have you ever done something like this and then measured for any difference in rotational drag using a torque meter?
Damn that's alot! Last question and I'm gone for another 3 years. Was this just experimental? Like trying to reduce surface area for bearing temps or reduced drag....did you have to go with a heaver weight oil. Have you ever done something like this and then measured for any difference in rotational drag using a torque meter?
01-16-2015, 04:03 PM
#17
Damn that's alot! Last question and I'm gone for another 3 years. Was this just experimental? Like trying to reduce surface area for bearing temps or reduced drag....did you have to go with a heaver weight oil. Have you ever done something like this and then measured for any difference in rotational drag using a torque meter?
