Main and rod bearings

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Main and rod bearings

Post  blown473 on March 16th 2012, 11:47 pm

Questions to the experts here on rod & main bearings. First fully grooved mains, are they better or worse than 1/2 or 3/4 grooved? I just was reading a machinists rebuilder's magazine that stated that half grooves are better, it stated that the grooved goes to the upper part of the block, and the non grooved 1/2 to the bottom, the non grooved part retains oil & load better, that full grooves benefit the rods, but not the crank. True or false? And rod bearings Clevite or King, or coated? For the record I already have new fully grooved mains & King rod bearings new in the box for my 514 build, but have not assembled it yet, I thought I'd get opinions here first. This is a blown application, Paul Kane stage 1 pump and mods to block. 8 qt pan and 2 qt external oil cooler. This is not a drag strip engine, but a land speed engine and will have a slower rpm curve due to the very high rear gear set of 2.47 and 20% overdrive in high gear. Any input is appreciated as many of you have far greater knowledge & experience on this subject than myself. Thanks!

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Re: Main and rod bearings

Post  Paul Kane on March 17th 2012, 1:57 am

Relative to the main journal support and in complete layman's terms, the less groove in the mains the more hydro-dynamic wedge and resulting film strength. More specifically, as you add groove to the lower saddle the load bearing capability of the main bearing is decreased and of course pressure-fed oiling to the rods is increased simultaneously. It's a trade off.

But that does not automatically mean that the load bearing and lubrication of the mains is inadequate with a full-groove bearing. Adequate lubrication is a function of so many variables such as the oil used (many factors within the oil alone), the bearing clearances, amount of oil pressure, amount of oil volume, temperatures, etc, etc, etc.

One big "caught with the pants down" irony in all of this is that Clevite did a major study which concluded that in regular passenger car applications the half-gooved shell was not only adequate but also deemed best since it provided the most main journal protection...and yet Clevite still manufactures and sells full-groove bearings!

In the late 1960s during the days of only half-grooved bearings, Top Fuel reached a point where they were regularly grenading their engines due to lack of lubrication for the amount of horsepower being generated. The fix at the time was to place the crankshaft in a lathe and groove the main journals which--in effect--is exactly the same as going to a full-groove bearing. With this modification, Top Fuel engines lived again and continued to do so even as power was further increased.

So there is no "best" bearing...just as there is no "best" head, or "best' intake manifold, no "best" rocker arm, etc. And as is often the case there is plenty of application-crossover from build-to-build. But will the full-groove main bearing provide adequate support for the main journals in your blown 521 engine? I can tell you they've been known to handle three times the power of your build. That doesn't mean that a 3/4-groove won't do as well and it probably would do just fine. I'd say to some extent it's a matter of personal preference. If it were me pushing for my 200 mph red cap for 3-5 miles of wide open throttle I'd definitely want the lower end as oiled up as possible.

Paul

P.s.: We have talked about only one feature of engine bearings, and that's groove type. There's soooo much more, such as bearing construction and bearing material which is a whole other chracteristic to consider...and there are as many opinions on which is "best" in this area as there are bearing materials multiplied by engine combos and their appications.
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Re: Main and rod bearings

Post  rmcomprandy on March 17th 2012, 11:33 am

blown473 wrote:Questions to the experts here on rod & main bearings. First fully grooved mains, are they better or worse than 1/2 or 3/4 grooved? I just was reading a machinists rebuilder's magazine that stated that half grooves are better, it stated that the grooved goes to the upper part of the block, and the non grooved 1/2 to the bottom, the non grooved part retains oil & load better, that full grooves benefit the rods, but not the crank. True or false? And rod bearings Clevite or King, or coated? For the record I already have new fully grooved mains & King rod bearings new in the box for my 514 build, but have not assembled it yet, I thought I'd get opinions here first. This is a blown application, Paul Kane stage 1 pump and mods to block. 8 qt pan and 2 qt external oil cooler. This is not a drag strip engine, but a land speed engine and will have a slower rpm curve due to the very high rear gear set of 2.47 and 20% overdrive in high gear. Any input is appreciated as many of you have far greater knowledge & experience on this subject than myself. Thanks!

Well, if you wish to believe the SAE reports, even the half groove is to much, (about 145 degrees shows to be optimum), for best main bearing oil load carrying capability. However, the rod bearings suffer when the oil supply is cut off for longer than 160 degrees of crankshaft rotation; (that leaves 200 rotational degrees).
Depending upon where the particular weak spot is in the particular engine in question, the best compromise will vary among them.
It turns out that crossdrilling a crankshaft is not so bad after all, IF it doesn't measurably weaken the crankshaft and is done correctly without too large a cross-passage.
Now, of course, the SAE is concerned mostly with durability and "shafted" horsepower only comes 2nd.

Draw your own conclusions...!

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Post  blown473 on March 17th 2012, 2:34 pm

Interesting. In trying to build the best most reliable possible, you look through the many great posts & try to gather as much info as possible, then make you choices. After seeing similar 514 engines running cast crankshaft combos break at the #2 rod journal, you wonder if a full grooved bearing might have helped, but cranks break for many reasons, not just a lack of oil. You also see these new coated bearings & wonder if they can help as well, seems everything has a coating on it these days, piston skirts, etc. Thanks for the input, it's appreciated!

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Re: Main and rod bearings

Post  rmcomprandy on March 17th 2012, 3:49 pm

blown473 wrote:Interesting. After seeing similar 514 engines running cast crankshaft combos break at the #2 rod journal, you wonder if a full grooved bearing might have helped, but cranks break for many reasons, not just a lack of oil!

A crankshaft breaking at the first rod or second main journal usually has a balance or frequency problem.

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Re: Main and rod bearings

Post  cooter on March 18th 2012, 9:47 am

with a supercharger application where the belt has the possibility to pull up on the crank snout as the belt tightens, i would expect the first main to require the most lubrication capabilty on the block half amd the cap which would make me want as much bearing material in the top and bottom.

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