I hope you are able to sort this out. Good suggestions here.

Main bearings remain the original factory bearings with 60k miles on both Darts.

New rear main seal was the only change as far as the mains were concerned for both Darts.

Rods bearings were changed since the originals were at max clearance. for both Darts.....maybe SL-6 may wear the rod bearings faster then the mains?

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

As already noted, all aftermarket oil pumps do not come with an anti-dranback valve.
It is an easy task to install one, then see if that solves the problem.
DD

You have a lot of inexpensive, easy things to do before you pay any heed to a spurious assertion that your bearings are shot.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Main bearing knock? that goes away?I highly doubt its a main.I would check pressure relief valve first and put 1 washer under the spring.Mark

Yes, you're right.

Easy guys.
I think you should Always be prepared to replace necessary componants when you work on an engine. No matter how small the original task.
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

I like Dan's idea on the thinner oil. 15-40 Valvoline in my van clatters in the cold weather.

I do have a piston slap on #2 (360) It has been there since I built it 170K ago. It lessens and disappears with warming of the engine.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

With nice fresh bearings such as you describe, 10w30—even synthetic—is on the too-thick side of things for fast oil pressure on cold start. Try a switch to 0W30 (I use Castrol Syntec) and see how that does. I found a noticeable improvement in oil pressure speed on startup even switching from 5w30 synthetic to 0w30 synthetic. Don't be afraid of oils that would have been considered unreasonably thin in the past![/quote]

We switched my daughter's Dart to 0W30 as recommended using Mobil 1 to maintain brand. Temps were low 30's. There was no discernable knock so Dan gets a thumbs up for this recommendation!

At what temp do we switch back to 5W or 10W?

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

We switched my daughter's Dart to 0W30 as recommended using Mobil 1 to maintain brand. Temps were low 30's. There was no discernable knock so Dan gets a thumbs up for this recommendation!

At what temp do we switch back to 5W or 10W?]

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Hoorah for cheap and easy fixes!
Oh, don't worry too much about it. The right-hand half of the viscosity (the "30" part) means you're covered. This is not 1970 or 1980 when you had to pick between thin oil that was always thin or thick oil that was always thick; today's engine oil is really quite amazing stuff. With nice fresh bearings in your engine, the oil pump evidently has no trouble maintaining good film strength with the 0W30. Me, I will likely carry on running 0W30 year round in my low-miles '73. If you want to go to a 5W30 or 10W30 for whatever reason, do it in late Springtime.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I am glad the switch to lower viscosity oil worked however you now know the engines are not identical.

When trying to force a high viscosity cold oil through the oil filter during startup, it may invoke the by-pass-valve (BPV). On some brands with valves made of inferior material, the BPV may stay open even when the pressure sinks as the oil gets hot! The WIX/Napa filters have an excellent silicone rubber valve so they should be in the clear, but with unknown brands or if in doubt, change the filter.
Olaf

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Aspenized

Identifying what is different will start with the easy...how the new oil pumps compare. It's likely that pressure & volume are performance indicators.

I'm going to tee in oil pressure gauges at the oil pressure switch locations when the weather breaks. Measuring volume may take some research on how that's done.

My daughter has been monitoring the oil pressure light for how quickly it goes out and it's continues to be much quicker after the oil change to 0W x 30 with no brief knock sounds.

It's possible that when we changed the oil & filter that we actually replaced a defective NAPA 1806 filter without picking that up as the culprit.
We used NAPA 1806 again however as that seems to be getting SL-6 Forum thumbs up.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

When we did an oil change on a friends '80 van I told him to get a Pure PL30001 for extra capacity. It works on all my stuff.

The stand pipe was so long that the filter hir on the head and would cross thread.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick