When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
If you want to get feel for how much the oil pump pulls the cam, next time you are priming the oil system with priming tool and drill motor, note how hard the drill pulls as it is bogging down at a few hundred RPM. Then imagine the pull at idle speed and higher.
Since my point of contact will be out until at least Wed I'm having to work on this with an uneducated mind. So if we go with a stock 396 spring pressure of 325 open and multiply that by the rocker ratio, 1.7, I come up with 555 pounds on the contact area. AZ maintains that there is no difference standing on a scale at 200 lbs or standing on a 1" plug on the scale it's still 200 pounds. That put me to thinking yeah that sounds right. But if you are standing on the floor with both feet but you then raise one you just doubled the pressure on the floor without changing the weight. So with this in mind I'm sticking by my math. 555lbs concentrated on .0028 sq in becomes 275,000 pounds per sq in. Now the cam obviously doesn't see 275,000 pounds because it's not one sq in its .0028 sq in. So where am I wrong in saying that? Just remember what Mark Twain said 'never argue with a fool they'll bring you down to their level and beat you with experience'.
Doug, I built this engine back in 1970 using a Crane roller cam, with the rev-kit along with a few other performance pieces. Unfortunately back then, I did not know, what I did not know and assembled it without the benefit of a cam button. The engine/car ran like a scalded ape and would pull until you ran out of nerve. For me that was just short of 7k. Within a month and a few hundred miles on the engine including about a dozen or more passes down the quarter mile, I put the car in storage. Early on while in storage I tried starting the engine with no luck. At that point I was in no position to deal with a full race motor, so I backed off all the rocker nuts and decided I would get back to it.
Fast forward 36 years and this is what we discovered upon disassembly. I had also learned in the interim about the standard practice (?) back then of using a cam button, so I was not too surprised either.
It's all good and I'm currently using the shortblock, albeit after a thorough rebuild, in the car.
GUSTO
While it is clear that your timing gear hit the timing cover, it is not clear that the cause was simple cam walk. Your photo shows a fractured timing gear with a section that moved back toward the block. The resulting chaos could have pushed the rest of the timing gear and the timing chain forward into the timing cover. It's hard to tell what happened first (cam walk or fracture of the timing gear).
I thought a lot about the purpose of the cam button when I put a roller cam in a 383 that I was building from a 1967 small block that GM originally designed for use with a flat tappet cam. My conclusion was that the resistance of the oil pump would pull the cam rearward in the block, and I confirmed this with an actual test. I could not think of any force acting on the cam that might counteract the rearward thrust caused by the oil pump enough to push the cam forward.
A further concern was that even if I installed a cam button and the cam was somehow pushed forward, the stock timing cover would likely deflect quite a bit. So, the notion that timing variation caused by cam walk could be prevented by simply installing a cam button seemed unrealistic.
In the end, I decided to go ahead and install a cam button and reinforce the timing cover, based on the view that while these steps were probably not necessary, they were not harmful.
Attached is a short article I wrote for my NCRS chapter newsletter. While the article addresses three separate considerations for in installing a roller cam, most of the discussion is about the cam button debate. The article includes the results of some basic measurements on how well a stock timing cover can limit forward movement of a cam that has a cam button, and also contains some calculations of the relationship between cam walk and ignition advance.
I remain unconvinced that cam buttons are actually necessary for vintage Chevy V-8 engines that use a cam-driven oil pump.
I'm not surprised to see the fracture on the stock aluminum/nylon silent type cam gear. I have torn down several completely stock small blocks to find these gears missing a lot of the nylon teeth and fractured. And that is with stock springs that have maybe 90 pounds of pressure on the seat. I can only imagine the pressure Gusto's roller cam with rev kit had, and in my opinion, the cam gear fractured first, causing the resulting damage. Anytime we ever did a cam change that is the first thing we threw in the trash. Double roller bicycle type chain is only way to go in my opinion. YMMV of course.
When you don't control cam walk the cam can and will walk far enough that lobes contact other lifters then things go from bad to really bad. I just looked at Gustos pics. Classic cam walk. And as I mentioned earlier just installing a button does almost nothing the cam walk with a button alone will just push out the tin timing cover. Seen it time and again when people didn't know or wouldn't listen. This again is why performance water pumps and I don't mean a few brands every performance water pump has an added tab for a bolt to be adjusted to control timing cover flex. When every cam company, the people that actually design and make the cams, and every water pump company alters their casting for a particular purpose people should be smart enough to listen.
One more thing Gusto you should have never used a nylon gear with roller cam spring pressures. The extra pressure would have destroyed the timing chain had you run it very long. But that was a long time ago.
555lb / .0028 sq in is 198214 lb/sq in. That assumes you have 555 lbs 357 times to fill up a full square inch to actually get 198214 lb force
Pressure is not the same as force.
198214 lb/sq In x .0028 sq in= 555 lbs force.
Notice the sq in above the divisor cancels out the sq in below the divisor and you are left with pounds force on the cam lobe. I was using nominal SB spring for my 200 lbs, and yes is should be 300 lbs with the rocker ratio. but 300lb x .23*/90*= .76 lb per lobe x 16 lobes x.30 ( estimated integral) x 120*/360* = 1.2 lb average rearward force from lifter pressure on the cam. Notice the units of measure all cancel out except lb. Using your BB numbers , the rearward force on the cam from lifter force is 3.05 lb rearward force. Maybe the integral number is actually .5, well then the ave rear ward force would be whopping 5.08 lb rearward force on the BB cam. 5 lbs of course, is better than zero.
Doug
The only scenario I can come up with when the pump does not pull the cam to the rear is during brief cavitation episodes, or a broken oil pump shaft.
The cavitation at high speed is what would worry me about unloading the cam, esp during sudden RPM change downward that would cause some severe timing chain jerking, and that may be the reason for thrust buttons. Looking at the holes drilled in the rear cam journal, I can't imagine enough oil building up behind the journal to cause any forward thrust from that.
Doug
I'll admit I was surprised at the statements that the oil pump actually imparts forward axial force. This is from other sources not myself. It doesn't interest me enough to fret about. But the axial force from the lifters does. If the guys I ask will devote enough time to actually come up with an answer. I don't know how I arrived at 275000 but I did everytime until now so we'll go with the correct number 198,214. Hopefully I'll have an answer soon. This much I do KNOW oil pump pressure does not control cam walk.
pay particular attention to type b page b103. In you equation I believe you are calculating a column pushing straight down. When in fact it is a 114" diameter ball, the radius on the bottom of a flat tappet.
Any radius, or slope on the "pusher", the lifter, does not affect axial force on the pushed surface, assuming friction-less surfaces, as I am for a simplified equation. Adding friction into teh equation would reduce any axial imparted force. The axial force imparted to the cam by teh lifter would be the same whether the lift had the same/matching .23* slope, was perfectly flat, or has a slight radius, the only thing that changes with those three options is the point where along the front/back length of the cam lobe, the force is exerted.
Google this for some reading: force vector diagrams conceptual physics
I drew it out in cad. I was expecting the radius to contact the lifter a little diffetently. My cad is only 2d its not like MasterCam or something of that nature. It's a simple program to draw and create cnc programming. It's just my speed. I'm a analog guy living in a digital world. Did you look at the graph where the load increases with rpm to 3000n?
The cam load at 2000 RPM has that large lump in the graph during the lift phase, probably due to inertia on vertical acceleration of the valve, pushrod and rocker, while there is negligible force on the down side as the valve train has is trying to keep up with the cam on the closing side. That is all part of the " average integral", or area under the curve, I estimated, and it is only an estimate, but it is probably not far off.
Look at the area under the curve for the 500 RPM vs 2000 RPM, the areas are probably very close, which would give you about the same average.
My calc was for static, or near static system, as once you get to higher RPM, the equations become more complex and I am not qualified to attempt those.
I'll admit I was surprised at the statements that the oil pump actually imparts forward axial force. This is from other sources not myself. It doesn't interest me enough to fret about. This much I do KNOW oil pump pressure does not control cam walk.
I think that both analysis and actual testing will show that the helical gear arrangement of the cam-driven oil pump (and the cam-driven distributor too) will impart a significant REARWARD axial force on the cam. I know that some people think the resulting axial force is in the forward direction, but I think they are mistaken.
It's pretty easy to mock this up with a bare block, a cam with a timing gear on it, and a distributor (no need for an actual oil pump or oil pan). Simply use one hand to turn the camshaft timing gear while using your other hand to resist the turning of the distributor rotor. Then see which way the cam walks.
Absent any constraints on the axial movement of the cam, the cam will try to "walk" in whatever direction has more net force applied. I think everyone agrees that with a flat tappet cam, the lifters apply a rearward force. With some analysis or testing, these same people should agree that spinning the oil pump and spinning the distributor also apply a rearward force.
I have not been able to identify any other force acting on the cam that would push it forward. So, it seems to me that the net force will always be rearward, even if the lifter force is removed by using a roller cam.
I did some tests last year to try and demonstrate to some members here the effect of changing the bypass spring. I and I'm sure many have read articles about how much to it takes to run an oil pump. I am reasonably certain that a 1 hp motor will run an oil pump at full capacity. I know driving it with my mill it took no effort at all. While I didn't turn it to 4000 rpm the pump was dead headed against a pressure gauge. I hope to get a good answer on the axial thrust soon. I hope this doesn't peak my interest enough to set up an SBC with a flat tappet valve train just to measure the lbs of forward thrust. I have read your thread where you don't believe in roller cam thrust but you heeded everyones correct advice to prep your engine accordingly. I once was involved with strain gauge testing on pushrods in a Spin Tron. You cannot imagine the rotational forces on a pushrod in a roller cam.engine where the lifter doesn't spin. It broke everything we tried to keep the pushrods from turning. Now I could rig it up pretty quick to stop it. A little further off topic but food for thought on axial forces. What acts on a wrist pin so hard that it will extruder the metal away from spiral type locks. There's nothing to impart forces on a wrist pin axially.
Some of you guys refuse to listen to what I say or refer to. I've been doing this 44 year's and I'm just a newcomer compared to Pamotorman. Since you don't believe me maybe you'll listen to him. I've never met him but I've read his posts for several years on another forum, not to mention here. I've never seen him post anything that wasn't spot on.
if the oil pump is what holds the cam back in the block why does pre LS factory roller cam setup have a cam retainer plate ???
Back when I was trying to decide whether to install a cam button (which I did decide to do), the only way that I could envision sufficient net force to push the cam forward was if the engine momentarily spun backward due to a misfire during startup. Reversing the rotation of the engine reverses the rearward cam thrust caused by the load of spinning the distributor and the load of spinning the oil pump.
The main reason I was trying to understand the potential load on the cam button was that the industry offers cam buttons in the form of solid Teflon or a form of "roller tip." So, I had to consider when, and how hard, the cam button would be loaded. Another part of this consideration was the fact that the factory timing cover will flex considerably under load.
My conclusion was that under normal operating conditions, the cam button is never in contact with the timing cover. However, there might be some momentary conditions that would cause the cam to walk forward and bump the timing cover. I could not think of any continuous operating condition that would push the cam forward.
So, I decided to use the solid Teflon cam button instead of the roller type that is (theoretically) better suited to high and/or continuous loads. I also decided to reinforce the stock timing cover.
I set up my cam button to allow .010" of end play for the cam. If I ever have to take the timing cover off, I will be interested to see whether there is any evidence of the cam button actually contacting the timing cover,
I suppose dieseling on shut off could spin the motor backwards a rev or two also.
I have had motors start up and run backwards before, they don't very well, or very long but they will run backwards for few seconds.
Doug
12-25-2018, 12:32 PM
