Rocker Arm bearings theory
01-18-2012, 08:34 AM
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Rocker Arm bearings theory I have a theory why GM's stock rocker arm bearings are so loose and rattle at idle.
If you have removed the stock rocker arms, you will notice there is a lot of movement of the bearings. Some say sloppy. Like me, I thought it was terrible quality or cheap design.
Here is my theory. GM designed them to be like this intentionally. As the cam lifts the pushrod which in turn pushes on the rocker arms, the rocker tip pushes down on the valve tip. The least about of wear on the valve guides is when the rocker arm tip stays centered on the valve tip through it whole cycle. Below is a picture of the rocker arm to valve tip geometry that assumes the bearings are tight:
From this illustration, the rocker arm tip does not go straight up and down centered on the valve tip. It goes through an arc so you would assume it would scrubs across the tip of the valve. Now, since the bearings are sloppy, my theory is the tip of the rocker arm actually remains centered on the valve tip. Pretty ingenious design if you ask me.
But the OEM rocker arm bearings still have design issues. First, because of the sloppy bearings, they rattle quite a bit at idle. At higher RPM, the exhaust noise drowns them out. Second, the roller bearings are not caged which is fine for stock lift cams. However, if a high lift cam is installed with higher pressure springs, it creates more stress on the bearings and the tiny rollers may escape and find there way down to the oil pan. Also, the bearing races and caps are stamped sheet metal, which are not as precise as components that are machined.
Comp Cam came out a few years ago with a very high quality caged bearing replacement kit. Great, they eliminated the slop so the valve train rattle at idle is gone. The caps and races are machined and snap rings are used to keep the bearings positioned correctly in the rocker arm. Its a great bearing. Unfortunately, this eliminates the design feature I outlined above so the rocker arm scrubs across the valve tip which results in increased side load on the valve which results in accelerated valve guide wear. Its not too extreme with stock cams lift, but as cam lift increases, the problem is much worse.
One aftermarket head manufacturer (Trick Flow) apparently recognized this problem and require full roller rocker arms such as the Yella Terra. Because Yella Terras have a roller tip, the valve to rocker geometry is much improved. See the illustration below:
Conclusion: In my opinion, with a stock cam, stick with the OEM bearings if they are in good condition and just accept the rattle at idle. With an aftermarket cam with lift below .550" the stock rocker arm and OEM bearings are still OK. Over .550" lift, consider upgrading to a full roller rocker arm. For really big lift cams, look at shaft mounted full roller rockers such as Jesel or T&D. These are expensive, so they are not for everyone.
If you have removed the stock rocker arms, you will notice there is a lot of movement of the bearings. Some say sloppy. Like me, I thought it was terrible quality or cheap design.
Here is my theory. GM designed them to be like this intentionally. As the cam lifts the pushrod which in turn pushes on the rocker arms, the rocker tip pushes down on the valve tip. The least about of wear on the valve guides is when the rocker arm tip stays centered on the valve tip through it whole cycle. Below is a picture of the rocker arm to valve tip geometry that assumes the bearings are tight:
From this illustration, the rocker arm tip does not go straight up and down centered on the valve tip. It goes through an arc so you would assume it would scrubs across the tip of the valve. Now, since the bearings are sloppy, my theory is the tip of the rocker arm actually remains centered on the valve tip. Pretty ingenious design if you ask me.
But the OEM rocker arm bearings still have design issues. First, because of the sloppy bearings, they rattle quite a bit at idle. At higher RPM, the exhaust noise drowns them out. Second, the roller bearings are not caged which is fine for stock lift cams. However, if a high lift cam is installed with higher pressure springs, it creates more stress on the bearings and the tiny rollers may escape and find there way down to the oil pan. Also, the bearing races and caps are stamped sheet metal, which are not as precise as components that are machined.
Comp Cam came out a few years ago with a very high quality caged bearing replacement kit. Great, they eliminated the slop so the valve train rattle at idle is gone. The caps and races are machined and snap rings are used to keep the bearings positioned correctly in the rocker arm. Its a great bearing. Unfortunately, this eliminates the design feature I outlined above so the rocker arm scrubs across the valve tip which results in increased side load on the valve which results in accelerated valve guide wear. Its not too extreme with stock cams lift, but as cam lift increases, the problem is much worse.
One aftermarket head manufacturer (Trick Flow) apparently recognized this problem and require full roller rocker arms such as the Yella Terra. Because Yella Terras have a roller tip, the valve to rocker geometry is much improved. See the illustration below:
Conclusion: In my opinion, with a stock cam, stick with the OEM bearings if they are in good condition and just accept the rattle at idle. With an aftermarket cam with lift below .550" the stock rocker arm and OEM bearings are still OK. Over .550" lift, consider upgrading to a full roller rocker arm. For really big lift cams, look at shaft mounted full roller rockers such as Jesel or T&D. These are expensive, so they are not for everyone.
Last edited by Mez; 01-18-2012 at 11:28 AM.
01-18-2012, 03:50 PM
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I have a theory why GM's stock rocker arm bearings are so loose and rattle at idle.
If you have removed the stock rocker arms, you will notice there is a lot of movement of the bearings. Some say sloppy. Like me, I thought it was terrible quality or cheap design.
Here is my theory. GM designed them to be like this intentionally. As the cam lifts the pushrod which in turn pushes on the rocker arms, the rocker tip pushes down on the valve tip. The least about of wear on the valve guides is when the rocker arm tip stays centered on the valve tip through it whole cycle. Below is a picture of the rocker arm to valve tip geometry that assumes the bearings are tight:
From this illustration, the rocker arm tip does not go straight up and down centered on the valve tip. It goes through an arc so you would assume it would scrubs across the tip of the valve. Now, since the bearings are sloppy, my theory is the tip of the rocker arm actually remains centered on the valve tip. Pretty ingenious design if you ask me.
But the OEM rocker arm bearings still have design issues. First, because of the sloppy bearings, they rattle quite a bit at idle. At higher RPM, the exhaust noise drowns them out. Second, the roller bearings are not caged which is fine for stock lift cams. However, if a high lift cam is installed with higher pressure springs, it creates more stress on the bearings and the tiny rollers may escape and find there way down to the oil pan. Also, the bearing races and caps are stamped sheet metal, which are not as precise as components that are machined.
Comp Cam came out a few years ago with a very high quality caged bearing replacement kit. Great, they eliminated the slop so the valve train rattle at idle is gone. The caps and races are machined and snap rings are used to keep the bearings positioned correctly in the rocker arm. Its a great bearing. Unfortunately, this eliminates the design feature I outlined above so the rocker arm scrubs across the valve tip which results in increased side load on the valve which results in accelerated valve guide wear. Its not too extreme with stock cams lift, but as cam lift increases, the problem is much worse.
One aftermarket head manufacturer (Trick Flow) apparently recognized this problem and require full roller rocker arms such as the Yella Terra. Because Yella Terras have a roller tip, the valve to rocker geometry is much improved. See the illustration below:
Conclusion: In my opinion, with a stock cam, stick with the OEM bearings if they are in good condition and just accept the rattle at idle. With an aftermarket cam with lift below .550" the stock rocker arm and OEM bearings are still OK. Over .550" lift, consider upgrading to a full roller rocker arm. For really big lift cams, look at shaft mounted full roller rockers such as Jesel or T&D. These are expensive, so they are not for everyone.
If you have removed the stock rocker arms, you will notice there is a lot of movement of the bearings. Some say sloppy. Like me, I thought it was terrible quality or cheap design.
Here is my theory. GM designed them to be like this intentionally. As the cam lifts the pushrod which in turn pushes on the rocker arms, the rocker tip pushes down on the valve tip. The least about of wear on the valve guides is when the rocker arm tip stays centered on the valve tip through it whole cycle. Below is a picture of the rocker arm to valve tip geometry that assumes the bearings are tight:
From this illustration, the rocker arm tip does not go straight up and down centered on the valve tip. It goes through an arc so you would assume it would scrubs across the tip of the valve. Now, since the bearings are sloppy, my theory is the tip of the rocker arm actually remains centered on the valve tip. Pretty ingenious design if you ask me.
But the OEM rocker arm bearings still have design issues. First, because of the sloppy bearings, they rattle quite a bit at idle. At higher RPM, the exhaust noise drowns them out. Second, the roller bearings are not caged which is fine for stock lift cams. However, if a high lift cam is installed with higher pressure springs, it creates more stress on the bearings and the tiny rollers may escape and find there way down to the oil pan. Also, the bearing races and caps are stamped sheet metal, which are not as precise as components that are machined.
Comp Cam came out a few years ago with a very high quality caged bearing replacement kit. Great, they eliminated the slop so the valve train rattle at idle is gone. The caps and races are machined and snap rings are used to keep the bearings positioned correctly in the rocker arm. Its a great bearing. Unfortunately, this eliminates the design feature I outlined above so the rocker arm scrubs across the valve tip which results in increased side load on the valve which results in accelerated valve guide wear. Its not too extreme with stock cams lift, but as cam lift increases, the problem is much worse.
One aftermarket head manufacturer (Trick Flow) apparently recognized this problem and require full roller rocker arms such as the Yella Terra. Because Yella Terras have a roller tip, the valve to rocker geometry is much improved. See the illustration below:
Conclusion: In my opinion, with a stock cam, stick with the OEM bearings if they are in good condition and just accept the rattle at idle. With an aftermarket cam with lift below .550" the stock rocker arm and OEM bearings are still OK. Over .550" lift, consider upgrading to a full roller rocker arm. For really big lift cams, look at shaft mounted full roller rockers such as Jesel or T&D. These are expensive, so they are not for everyone.
Have you tried shimming up your factory rocker stands and looking at the scrub pattern?
01-18-2012, 04:30 PM
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The shims thickness must be based on what is needed to achieve the best wipe pattern not what comes in the kit. After shimming, then use an adjustable pushrod to determine the correct length because that determines the lifter preload. Then order custom made pushrods. I ended up with 7.425 instead of the stock 7.40. My engine is very quiet considering the high lift cam. There are a number of threads that talk about shimming Yella Terra rockers.
01-18-2012, 05:41 PM
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Yes. That is standard procedure for installation of all after-market rockers. When I installed the Yella Terra, I used a much thicker shim than what came in the kit. I expected that since my heads had been ported and new valves installed.
The shims thickness must be based on what is needed to achieve the best wipe pattern not what comes in the kit. After shimming, then use an adjustable pushrod to determine the correct length because that determines the lifter preload. Then order custom made pushrods. I ended up with 7.425 instead of the stock 7.40. My engine is very quiet considering the high lift cam. There are a number of threads that talk about shimming Yella Terra rockers.
The shims thickness must be based on what is needed to achieve the best wipe pattern not what comes in the kit. After shimming, then use an adjustable pushrod to determine the correct length because that determines the lifter preload. Then order custom made pushrods. I ended up with 7.425 instead of the stock 7.40. My engine is very quiet considering the high lift cam. There are a number of threads that talk about shimming Yella Terra rockers.
01-21-2012, 04:18 PM
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Hmm. I always thought the roller tip in and of itself did nothing to correct rocker ARM geometry. I thought the roller tip reduces friction and to an extent might reduce the valve guide loading but geometry was all up to the dimensions of the pushrod, rocker arm, and adjustments.
01-21-2012, 08:14 PM
#6
^^^ Pushrod has nothing to do with geometry. You are right though the geometry has to do with the rocker arm and adjustments (shimming). Look at the illustration above and think about it. Geometry is how the rocker rides on the valve tip. The pushrod simply pushes the rocker up thereby opening the valve and letting the valve set down... The geometry is on the other side of the rocker from the pushrod cup....You can picture how shimming the rocker up changes where the the rocker rides on the valve tip.....
01-21-2012, 09:09 PM
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On the LS engines, the only thing the pushrod length does is set the lifter preload. The geometry is determined by the relationship of the rocker fulcrum to the valve tip. Google "rocker arm geometry" and you will find many explanations.
In the second illustration above, although its difficult to visualize, the roller tip stays close to the center of the valve during its up/down path because roller tip radius is much smaller than the stock non-roller rocker tip. So as it goes up and down, you can visualize how the roller stays closer to the valve tip center.
However, I would like to bring this thread back to the reason I made it. That is because of the stock bearing design intentionally being very loose, the whole rocker moves and the rocker tip stays centered on the valve tip more than the first illustration demonstrates.
In the second illustration above, although its difficult to visualize, the roller tip stays close to the center of the valve during its up/down path because roller tip radius is much smaller than the stock non-roller rocker tip. So as it goes up and down, you can visualize how the roller stays closer to the valve tip center.
However, I would like to bring this thread back to the reason I made it. That is because of the stock bearing design intentionally being very loose, the whole rocker moves and the rocker tip stays centered on the valve tip more than the first illustration demonstrates.
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01-22-2012, 06:20 AM
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Comp Cam came out a few years ago with a very high quality caged bearing replacement kit. Great, they eliminated the slop so the valve train rattle at idle is gone. The caps and races are machined and snap rings are used to keep the bearings positioned correctly in the rocker arm. Its a great bearing. Unfortunately, this eliminates the design feature I outlined above so the rocker arm scrubs across the valve tip which results in increased side load on the valve which results in accelerated valve guide wear. Its not too extreme with stock cams lift, but as cam lift increases, the problem is much worse.
Have you ever seen any real world examples of excessive wear after the comp cams bearings were installed and driven for a period of time?
01-22-2012, 09:32 AM
#9
On the LS engines, the only thing the pushrod length does is set the lifter preload. The geometry is determined by the relationship of the rocker fulcrum to the valve tip. Google "rocker arm geometry" and you will find many explanations.
In the second illustration above, although its difficult to visualize, the roller tip stays close to the center of the valve during its up/down path because roller tip radius is much smaller than the stock non-roller rocker tip. So as it goes up and down, you can visualize how the roller stays closer to the valve tip center.
However, I would like to bring this thread back to the reason I made it. That is because of the stock bearing design intentionally being very loose, the whole rocker moves and the rocker tip stays centered on the valve tip more than the first illustration demonstrates.
In the second illustration above, although its difficult to visualize, the roller tip stays close to the center of the valve during its up/down path because roller tip radius is much smaller than the stock non-roller rocker tip. So as it goes up and down, you can visualize how the roller stays closer to the valve tip center.
However, I would like to bring this thread back to the reason I made it. That is because of the stock bearing design intentionally being very loose, the whole rocker moves and the rocker tip stays centered on the valve tip more than the first illustration demonstrates.
Stated much better than I
I too would like to know this....After breaking a couple YT's I now have the aforementioned upgraded stockers with a cam in the low .600's lift..... The YT's were great for 27k then broke 2 (#5 #7) in 1k miles. I will say your theory seems plausible....
01-22-2012, 08:52 PM
#11
trick flow uses bronze lined valve guides, they do not tolerate any scrub at the rocker tip. THAT is why they require roler tip rockers. the guides will be dead very quickly.
on another note, I have a good wipe pattern on my L92 head's valve tips with a .65X lift cam, stock rockers
on another note, I have a good wipe pattern on my L92 head's valve tips with a .65X lift cam, stock rockers
01-23-2012, 10:24 AM
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One of the issues I have with the L92 (and other LS heads) is the tiny 8 MM bolt holding the rocker in place. With increase cam lifts and spring pressures this is not a optimal size bolt to carry the extra load of these performance parts. I plan to go with T&D which I know ae pricey and not for everyone, but I will feel good when I pound on it at the track. I know that you can machine the 8mm to a larger stud but my heads are on and have no plans to remove them. Shaft mounts are worth the investment IMO
01-23-2012, 12:58 PM
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trick flow uses bronze lined valve guides, they do not tolerate any scrub at the rocker tip. THAT is why they require roler tip rockers. the guides will be dead very quickly.
on another note, I have a good wipe pattern on my L92 head's valve tips with a .65X lift cam, stock rockers
on another note, I have a good wipe pattern on my L92 head's valve tips with a .65X lift cam, stock rockers
01-24-2012, 04:32 PM
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Trick Flow does offer powdered metal guides.
01-25-2012, 05:13 AM
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01-25-2012, 08:04 AM
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One thing that I feel is being overlooked the the rocker isn't loose as the valve opens. Once the pushrod starts moving the rocker to open the valve, there is upward force on the rocker bearings from the pushrod's movement. The friction between the rocker tip and valve would have to be so great that it would have enough sideways force to overcome the upward force applied by the pushrod. One would think that the rocker tip would scrub the valve before it moves that whole rocker body sideways...