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Discussion Starter · #1 · (Edited)
Can we get in-depth regarding main bearing sizes? Specifically with the SBC----we have the "350" size, 2.45". And the "400" size of 2.65?

i'm ASSuming larger main bearing equals larger main journals on the crank----which means more strength for the crank? BUT more friction (how much though?) and thus more heat?

Another disadvantage to larger mains MIGHT be less block strength---because more of the block had to be radiused away to accomodiate the larger diameter bearings?


Smaller main bearings equal less friction, which should equal more power. But again, how much? My gut feeling is not much?

Is that all it boils down to?

Everyone says more friction with larger mains, but my gut feeling is not much; Note: i'm only going to be dragging. Not road-racing where you have to go flat out for 500 miles....
 

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Basically what you are saying is correct. The thing with smaller bearings is that it is usually more beneficial when done on rod journals to reduce both friction and rotating mass. The main bearing will reduce friction as well but is probably less beneficial in its result. Something else to think about is the "overlap" that is created where tha journals meet. Two larger diameter journals will overlap each other more and the juncture will be much stronger. At higher rpms, this strength is beneficial to help limit crankshaft flex and harmonics. If you are building a high dollar racing engine that will receive lots of maintenance it may be worth doing. If you are doing a street/strip engine, I wouldn't .
 

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Pro racers tend to use smaller diameter bearings especially on the rods because the forces are a lot different on the mains. But they also use exotic forged steel cranks which have significantly better physical properties which in the instance of crank pin overlap are much stronger therefore not nearly so dependent on pin journal overlapping the main for strength as is needed in nodular cast iron. So you as builder need to appreciate in an engineering and materials strength sense as to why these configurations are what they are.

As I’ve said many times pro racer engines are like everything else on the car they are sacrificial items in the need for speed. These are designed for a season, or a race not necessarily to last tens of thousands of miles. So they do things that just won’t work well on a street performance vehicle that doubles as a part time racer and full time daily driver.

As far as the SBC is concerned it just doesn’t have significant structural issues. For crankshaft failures and specifically rod journal and bearing failures the FE Ford was the poster child. That was a long time ago. The problem it had really was rooted in using the Lincoln Y block tooling for crank and block. This drove bore centers which drove engine length which drove journal and bearing widths on the crank. It got mixed into bearing area philosophy at the time which only considered load per unit area of the bearing surfaces and didn’t consider surface speed and the combined effect of oil wedge retention. So between tooling constrained bore centers affecting block length and the bearing design philosophy that area is the only consideration then to get the needed area inside the package constraints they stayed with the existing narrow but large diameter rod journals and bearings of the already 10 year old Lincoln design.

The narrow but large diameter solution worked fine till factory built performance engines running on the pro racing circuits like NASCAR reached the power and rpm levels of the late 390’s and certainly the 406 where suddenly the engine started having bottom end problems with the narrow rod bearings and high rpms. At first this was crutched with putting the oil pressure relief on the end of the main oil galley instead of the pump, then cross bolted mains. By the time the 427 came out these crutches no longer were sustaining so the blocks got priority oiling to the mains first that bought a tiny bit of breathing room and grooved mains for full time oiling of the rod bearings which then introduced main bearing failures to those of the rods. But finally they arrived at the the loads on the large diameter but narrow rod bearings was blasting the oil wedge out of the rods. The fix was more care in rod side clearance to make it harder for the oil to be blasted out, in the end the last patch was to widen the bearing surface to give more width to preserve the oil wedge against the imposed loads and reduce the journal diameter to bring the surface speeds down, this ended at the LeMans and the NASCAR cranks.

Now before you get to LeMans and NASCAR rotating assemblies from Ford you get those ‘good ‘ol southern boys figured out the root cause of too little rod bearing width and too much diameter and they started modifying the Ford crank to take a Chrysler rod. Obviously something Ford was not pleased about which led to the 1963 disaster year of the factory mandating that sponsored cars ran as Ford built the engine. Back to Chrysler rods between Ford pistons and crank is now there is an engineering design problem as the force alignment between piston pin and crank pin isn’t correct it’s a bit off center which puts some angularity in there so forces on the piston want to tip it in the pin length direction where the piston has no bore wall support and it wants to do the same thing between the big rod end and the crank pin so the already pretty busy rod bolts have to deal with an added side loading. You can see where this leads to short lived pistons and perhaps bore walls and bottom end venting with con rods hanging out. In the end the only solution was a fresh design from a clean sheet of paper, hence the 385 model engines (429, 460).

All of this FE adventure stemming from the decision to salvage as much Lincoln Y block (279, 302, 317, 332, 341, 368) tooling as possible. So you see the finance departments impact on design and manufacturing engineering essentially locking in already obsolete design features and philosophy going into the future. Now back in my senior high school year (57, 58) and my college years I drove a 53 Merc rag top yellow with black interior and top (babe magnet) with a 56 Lincoln 341 punched an eighth inch a 3/4 race reground on a truck solid lifter cam using solid lifters and truck adjustable rockers, mounting the then new Carter AFB. For the period it was a damn good motor. But it never turned 6000 plus rpm for a steady two and a half hours.

Bogie
 

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cranks usually break at the rear right in front of the rear main journal where the force from all 8 rods is transmitted, where torsion loads are greatest. Lightweight cranks that are machined down to less than about 40 pounds start to suffer longevity problems back there if hp is much beyond about 450
 
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