low oil presure in my fresh re-build

[lw181]

hello; I have a 41 Chevy business coupe with a just built 56-57 235 hydro lifters. I had a machine shop clean and bore the block 30 over re ground crank 10 under new cam all new bearings new oil pump also had them drill and tap the holes for the full oil filter system. and tap and plug the 1/8'' hole in the block where the oil pump line goes in. the head was checked for cracks and valves re ground. I put the motor together. all the main bearings and rod bearings plastic gaged between 1 and 2 thou. after getting the motor in I started turning the pump with my drill to get the oil to the lifters, took a little while. I fired up the motor and saw I was only getting 18 pounds oil pressure. do not know why. I really do not want to drop the pan. just to check again I pulled the distributer at the number 4 firing spot so I could put it back in the same place turned the pump with the drill and got 30 pounds oil pressure. what the heck is going on. well I thought I will try doing the same at the number one firing spot. I got 18 pounds. wow. I decided to check every firing point. I ended up with cylinders 1, 5 , and 3 came up with 18 pounds. 6, 2, and 4 I got 30 pounds. also when the engine was running at 2500 rpm the oil pressure would drop to 12 lbs. I would like some thoughts before I pull the pan. thank you

[lw181]

hello; does not look like any response. I picked up another oil pump and I'm going in. I will let you know what I find and yes the plug is in the oil pump line hole.

[Pre68Dave]

I have worked on a lot of 235’s and thought about it a lot, and I can’t come up with a good explanation for your problem.
I would double check bearing clearance while you have the pan off.
Since crankshafts are cross drilled from the mains to the rod journals I suspect that the position of the holes in the crank at any particular place it stops could have some affect on the pressure you are checking.
keep us informed on the outcome.

[Deve]

That is the feeling I had too Dave. Its possible that is totally normal behavior because of where the holes are drilled. If you run the oil pump (drill motor in the distributor hole using a large flat tipped screwdriver end) and you get the correct pressure, and each rocker weep hole is outputting a little oil, you are good to go. This question has never come up before as far as I know.

[lw181]

hello again; got the pan off, pulled some bearing caps. looked good put in another new oil pump. bolted everything back together got the drill out and started priming the pump. had a real hard time getting the pump to grab the oil. what is going on now. I pulled the outlet line from the oil filter, blew a little air in the filter fitting pushing oil to the pump. put the fitting back together ran the drill, nothing. took the inlet fitting loose from the filter and blew air in while turning the drill and the I felt the pump grab. I put the fitting back together then started the drill again. I got pressure, thank god. ran up to 30 pounds ran till oil got to all the lifters. put that distributer in and fired it up. 30 pounds whoopee. as it warmed up it dropped to 25 pounds. as it got to temp at slow idle it was 15 pounds when I raced the motor up the oil pressure would go up. wow I think I got it. thank you to the guys that responded. what are your thoughts now ?

[Pre68Dave]

The oil pressure you have now sounds great.
Now put some miles on it 😁

[Deve]

Most people do not realize the power of Dave. If Dave says it, you can bank on it. He is a 2nd generation super mechanic where these particular vehicles and engines are concerned. I am very thankful for my friendship with Dave (and everyone here of course) and appreciate it anytime he weighs in. Thanks Dave and LW, enjoy your new rebuild. Even though it was a pain in the ass, you learned new things about your engine. That is a gift. Thanks for posting!