gonridnu wrote:Cam bearings are considered a "low load bearing".
I wouldn't consider cam bearings as "low load bearing", when there are 12 valves at different points of opening or closing, with max pressure per valve at over 200psi, on the lower surface of just 4 bearings. Now take all that force and move that cam back and forth, it is probably going to cut through the .002-.003 oil film like a saw.
Now I agree that the oil galley plug being out probably killed the cam bearings, but while you are in there I would have the shop install a solid pin. I can't hurt, it's not expensive, and doesn't take long to install.
Relative to the pressure a single rod bearing sees on ignition they are low load bearings. On a standard duty engine that pressure is in the neighborhood of 750 PSI and on a top fuel engine can be 3000 PSI and higher. This is distributed over 1 bearing that, while slightly larger, is made of the same basic materials we use and is an instantaneous load.
V8 engines only have 5 cam bearings and have 16 valves at various states of opening and routinely run roller cam springs that are 600+ open in just about every form of racing there is. My point is that unless you have mechanical interference or are running pro stock 1100 PSI open springs you will not shear the hydraulic wedge on a cam bearing.
If every cam lobe (12) was in the fully open position and you had 300 lb open springs that would be a total of 3600 lbs of pressure distributed over 4 bearing journals which equates to 900 PSI each. This is within the range of a single rod bearing, but every lobe isn't open at the same time so it is really significantly less than the hypothetical. I'll let someone else do the math, but it is a lot less.
Seeing a cam bearing installed at a machine shop and then carved on (with what is essentially a deburr knife on a stick) to create needed clearance would clear this issue up. That is the way cam bearings were installed forever. Now we have better install tools that keep the bearings straighter, and many shops have cams that have cutting edges installed so that once the bearing is installed it can be "scraped" to size by rotating the "cutter cam" followed by a treatment with a fine emery. They are still far less concentric and smooth than a rod or main bearing yet manage to last 100's of thousands of miles.
This fitting is necessary because cam bores migrate ever so slightly on seasoned blocks and there is no way to align hone them unless you install roller bearings which is only done on high end race applications.
gonridnu wrote:Cam bearings are considered a "low load bearing". When compared to the pounding the hydraulic wedge takes on a rod bearing, the cam bearings are seeing almost nothing. The oil system on the 4.0's sends oil to the cam bearings first, mains 2nd, and rods 3rd so they are last in line.
I have heard that the cam bearings are low load as well. You also have to take into consideration that they don't see more than the full load of the spring due to the rocker arm ratio. However I doubt this to be an issue on our engines. In my case the rod and main bearings looked fine. Also, the cam bearings we have access to are much softer than the rod and mains. Some engines have aluminum cam bearings available in the aftermarket, not ours though.
If the rod bearings look ok and there is cam bearing failure it is related to either inadequate valvetrain clearance (as in something binding at full lift)
Ive checked that on mine numerous times as have two machine shops.
it is not uncommon to need to massage cam bearings in order to get a good feel when the cam rotates, which should be fairly easily by hand.
While all of mine were easy to turn by hand blueing the bearing showed some rubbing by the oil hole and other places, which was correct in one build.
I'll also go with the surface finish of the cam itself as described in a thread here.
Don't forget the direction in which the cam is turned while being polished. Going the wrong direction lays the metal fingers in the wrong direction which can act as an abrasive.
gonridnu wrote:If every cam lobe (12) was in the fully open position and you had 300 lb open springs that would be a total of 3600 lbs of pressure distributed over 4 bearing journals which equates to 900 PSI each.
It would be greater than 3600 lbs in that example, probably more like 5760 lbs. However, as you point out not all the valves are open at the same time.
This thread is becoming deja vu. However, I seriously doubt this is his case as he did have an oil problem.
FINALLY got the motor back last week- two months later. The first cam was sent out to be re-nitrided, and the box the cam was in was bent in half when it came off the UPS truck... that´s not a good start- had to order a new cam again, and Ultradyne was having trouble finding a new core to grind- apparently all of the aftermarket cams are actually leftover 258 core stock, that´s why they have the fuel pump lobe, and why the cam lobes are thinner than the 4.0 lobe- but anyway it took like three more weeks to get a new cam, then reassemble everything again...
I spent the weekend getting everything hooked up, will probably head home from work around lunchtime to do the cam breakin (didn´t have the heart to do that to my neighbors yesterday when everyone was outside enjoying the nice weather-- everyone will be at work today).
Fingers crossed that this will be the last time I´ve had to pull the motor- (I´ve had it in/out FOUR times in the 15 months).
So, I went to break in the cam yesterday afternoon- and I cannot for the life of me get the motor running right- It won´t start on its own, and if I ¨help¨ it start, it runs super rich, backfires out the exhaust, knocks loudly, and I shut it down.
I have re-checked the distributor indexing 1000 times, and I know it is 100% correct according to the FSM, and the last two times I put the motor in, it started right up like this. It is definitely not 180 degrees off, either, I am at Cyl 1 TDC end of compression stroke, and have done this countless times.
Anyway- if I try to start it with the distributor where it is supposed to be, it sputters for a bit, then won´t fire up, and then floods the cylinders, and when I pull the plugs they´re soaking wet with gas. If I then disconnect the injectors, it will run for a couple of seconds on the residual gas in the cylinders/manifold.
If I loosen the dizzy hold-down bolt, and advance the distributor slightly whlie having my girlfriend crank it (turn the dizzy counter-clockwise), THEN it will fire up, but it will only idle, and any attempt to give it gas results in backfiring, stumbling, black smoke, and knocking. I don´t understand this, because the distributor should not be advanced at all- it should run where it is supposed to be, and has been for the last two times I´ve broken in the cam.
Of course, fuel pressure is spot on.
Any ideas? I´m wondering if the injectors are stuck open? I never touched them- they remained installed in the intake manifold as they were when i pulled the motor in march- how would I test the injectors? It ¨seems¨ like the timing is retarded, but that can´t be because the ignition timing is controlled by the PCM, not by the distributor- all the distributor does is distribute the spark from the coil, and house the CPS, which determines the injector timing.
What about the CKP? I left it installed in the bell housing of the transmission when I pulled it in march- would it even fire with a bad CKP?
I am getting so frustrated, and I´m worried by now of course that I´ve probably worn all the moly off the cam lobes, and that this constant cranking trying to start it is destroying my brand new cam. I just want this thing running, already, it´s been down over 5 months except for the disaster with the oil galley plug in march when I had it running for two weeks.
Please, I am not one to normally ask for help, but I am all out of ideas and completely frustrated- if I can´t get it started I am going to just push the whole thing off of a cliff and never look back.
Bad CKP- that just adds insult to injury in this whole debacle.
ANYway, the good news is it fired right up, and I was able to get it broken in, changed the oil, charged my a/c, topped off my coolant, and we look ready to roll.
So, she made it to her 600 mile oil change without issue; oil pressure is still awesome, lifters are a little noisier than I remember but it could be the hardened exhaust seats. Got my tune dialed in, she´s still a pig as far as fuel consumption, but power is smooth and present, and nothing is leaking from anywhere. Still have my fingers crossed, but at least I can call this a success for the short term.
Thanks to everyone who helped me when I was pulling out my hair.
