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401 crank is reground, does it need rebalance |
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Trader 
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Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881 |
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 Posted: Sep/22/2020 at 11:52am |
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6PakBee, in your calculations did you put the bearings back in the rods? I get 0.272g total, all 8, with a 0.010" undercut crankshaft and putting tri-metal bearings back in. That's 0.034g per rod so one of us likely has the 0's wrong through the conversions. Regardless it is nothing to be concerned about for rebalancing.
Most of the bearing is steel backing. ). 0.030" is what I used for bearing coating as per Clevite. Aluminum bearing material would be slightly more but they are not available. Main bearings don't count at all as they are not rotating. |
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rang-a-stang
AMC Nut
Joined: Sep/25/2018 Location: Camarillo, CA Status: Offline Points: 272 |
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Posted: Sep/22/2020 at 4:29pm |
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I think it was balanced to the gram. My motor does see 5500 RPM but I have my EFI set to cut out at 5600 so it never goes over that.
Copy all. My thought was if I am removing about the same metal from all 4 throws (all 8 rod/pistons/pins/rings), they should still be, generally, about balanced, right? I do not see 8K rpm, and only rarely see anything over 4K.
Copy all. Great news! I wish I was as good with my math wizardry as you guys. You put this into perspective real quick.
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79 Cherokee, 401
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6PakBee
Supporter of TheAMCForum 
Charter Member Joined: Jul/01/2007 Location: North Dakota Status: Offline Points: 5457 |
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Posted: Sep/22/2020 at 5:42pm |
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Trader, you are absolutely correct. I neglected to consider that whatever is removed from the crank throws is made up by the increased bearing thickness. So the actual difference would depend to a large part on the different densities of the crank and the bearing materials. In the end, as you so correctly pointed out, the difference would be insignificant.
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Roger Gazur
1969 'B' Scheme SC/Rambler 1970 RWB 4-spd Machine 1970 Sonic Silver auto AMX All project cars. Forum Cockroach |
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BassBoat
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Posted: Sep/22/2020 at 7:59pm |
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before you worry about your oil pressure, try changing your oil. I have run 20W50 and found 10 psi difference between Kendall GT1 and Valvoline. I have run Rotella 15W40 with success. My street car ran on Mobil 1 15W50 for the last 20 years and has really loose bearings and oil pressure is 40 hot idle and 60 plus with any rpm. I recently changed to Mobil 1 0W40 European spec based on the oil wear testing data posted elsewhere on this forum. I am still about 60 psi so maybe lost 5 psi with the nominally lighter oil. My main bearing clearance is between 3 and 3 and a half, with the rear main almost 5. So I don't think bearing clearance on the crank is your problem unless you are running 10W30. Cam bearings can cause problems, as others have mentioned lifters can allegedly be a problem although I personally have not had issues. Oil Filters can be a problem. Back in the day the dealer told me AMC, AC or Fram. Fram has had reported problems, but I still have good luck with AC and K&N filters. I wish you the best.
BB
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Trader
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Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881 |
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Posted: Sep/22/2020 at 10:45pm |
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As for engine oil change, personal opinion, is to go to a 5W40. If you read the specifications on 5W40's of Mobil, Quaker State and Valvoline, they are better cold and hot then their 5W30's.
Cannot explain it, just reading the specifications, but over a 15Wx or 20Wx, they would be my first "go to", to try. Better cold start, higher shear points and better hot viscosity temperatures then their 5W30's! All well rated oils. Add - SN Plus only.
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rang-a-stang
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Joined: Sep/25/2018 Location: Camarillo, CA Status: Offline Points: 272 |
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Posted: Sep/23/2020 at 1:10pm |
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That all makes sense.
I think the bottom line is, my oil pressure is too low and I need to open the engine up to find out why. Maybe it's bearing clearances, maybe it's lifters, maybe it's oil pump, maybe it's oil, maybe it's a little of all those; but the engine needs to come out to verify. I am just to paranoid about damaging my crank. I am hopeful that I will not have to regrind my crank, though (after the discussion we all had in this thread) and even if I do, I won't have to re-balance. Oil filter: I have been running Baldwin B9s since a REALLY bad experience with a FRAM on cam break in. This time I ordered a WIX, though (it's sitting on the shelf waiting to go on after the engine comes out/goes back in). Oil: I think I have Castrol GTX Conventional 20W50 in there right now. I know it's conventional 20W50 but I am not 100% positive it is Castrol GTX...
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79 Cherokee, 401
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rang-a-stang
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Joined: Sep/25/2018 Location: Camarillo, CA Status: Offline Points: 272 |
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Posted: Oct/11/2020 at 8:27pm |
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Started a new thread with my inspection. Follow along here:
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79 Cherokee, 401
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6PakBee wrote: