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Big blocks have a tendency to "fret" the caps, which means the caps "bounce". Has nothing to do with bending of the block. Its all about clamping force. When the caps bounce the webs take the brunt, because the crank IS hammering the caps and webs. Attempting to "lock down" the caps with a girdle, forces the webs to take ALL the pounding and they simply are not strong enough in a stock block.

The pic you showed, from a small block, could be a number of things. Maybe the main housing bores are not perfect. The caps could be moving and "egg" shaping the hole, which would result in what you are showing. Could be crank flex. A big block usually shows bearing wear at 6 and 12 oclock on the bearing, when you are hammering the caps and fretting them..............At least the SEVERAL I broke looked that way. And the fretting is not always caused by something being wrong, as in tune or rpm. Sometimes it is simply POWER and more of it than the general design can hold

Monte



Certainly bouncing the cap from detonation (or as you pointed out big power) is hard on the block. But there is a bending moment from the crank that contributes to it. To say that stopping this bending with a girdle does "Nothing", ignores

The girdle does nearly nothing to help keep the caps from bouncing. It does not lock them down to the block up/down, and certainly does not add stress to the bulkhead. If anything it takes part of the vertical force and transfers it to the pan rail which LOWERS the stress in the bulkhead.

That being said...the metal is flat as it goes across the cap, so it's not very strong in that vertical bending direction. It does a very poor job of helping clamp the main down. If any of you have ever put one of these together, you can see that it takes VERY little torque to close any gaps to the cap. The girdle is however strong in the fore/aft direction because of the section width.

I agree that the markings on my SB could be from a bunch of things. But I know the line bore was right on, so I don't think it's that. The crank COULD be flexing, but considering it's cast, I would believe that it would have broken by now if it was flexing enough to make those marks. Which leads me to the most likely thing being the cap moving. If the cap was "egging" why would it only be on the inside edge opposite of the cylinder on power stroke?





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I disagree on your thoughts on the crankshaft trying to push the main caps forwards and backwards by bending the crank on the firing stroke, if that was the case the thrust bearings wouldn't last would they Also when the motor is firing on the power stroke the rod is pushing the against the cranksahft at or around 30 to 45 degree sideways trying to push the crankshaft mains out towards the oil pan rail, not straight down or forwards or backwards in the main saddles, correct Metal transfer(fretting) from the block to the main caps is usually cause by detonation from a bad tune up, it can be RPM related also Most failures in race motors are caused by some part being overstressed or a flaw in the metal or machining in my opinion



Funny you mention the thrust. The center main cap/bearing on mine was the ONLY one that didn't look like this. The thrust face however didn't look very good, which leads me to believe that the thrust WAS taking all of the abuse from the cap moving around.

Force on the mains is most definitely at TDC. See the attached pic... this is force vs. crank rotation of a main bearing. Notice that there is a peak pressure curve on each firing point near TDC (Red circles). The crank has very little bending moment on it as it rotates to 90 degree's because most of its energy is rotating the crank. Not to mention the pressure is SIGNIFICANTLY lower at 90 degrees. Thrust force on the cylinder wall is a different story though.

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