Thread: SBC Oil pressure loss.. What the Heck??

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I have a new (rebuilt) 355 from a reputable machine shop in my area. Bored, new pistons, bearings, heads re-done....the typical non performance rebuild.
Anyway, when I priimed the oil pump prior to start-up, I got a lot of resistance. the primer (modified distributer, turning it with an air ratchet) was very hard to turn. It was kinda like stirring cold molasses with a stick. Is that normal? It's been years since I've done that so I don't remember. I was able to get about 70 lbs of oil pressure priming it.
Question #2, this is the big one... When I started the car, it started very quickly and I brought it up to around 2000 rpms to break in the cam. After a minute or so I looked at the gauges and saw that the oil pressure was on zero... I immediately turned it off. How is this possible if I had good pressure when I primed it? Has anyone here experienced the same thing?

I had exactly the same thing on breakin, luckily, my Son was watching the oil pressure gauge and we shut it down the second it hit zero. It turned out to be an oil galley plug in the timing chain area that blew out. The machine shop blamed it on "oil pressure spike" but I think they didn't strake it correctly. I tapped threads and put screw in plugs in their place, no more problems.

Not saying that is what you have (could be a broken oil pump drive shaft or something else), but it is one place to start looking. I rounded off the hex on a drive shaft one time and lost oil pressure immediately too.

Don

That would have to be a huge oil pressure spike.... Get this though..I removed the distributor and put the primer back in...and the pressure started building up again. I'm stumped.

Check the bottom of your distributor?????

Oil drain holes in block-heads open/clear?

Is it pumping the oil pan dry and all of the oil up in the heads/lifter valley?

how soon after oil pressure loss should one hear lifter clatter .. i rarely pay attention to guages till i hear a problem ..

Originally Posted by cffisher

Check the bottom of your distributor?????

Bingo. It would appear that your primer is driving the pump but not the distributor when installed. As Charlie suggests, try another drive shaft in the bottom of your distributor while the distributor is out to see if it is tight in there. Inside of the bottom of distributor might be rounded out, or too short to reach the driveshaft.

Don

If I can get pressure by priming, I have to assume I don't have a problem with passages and drains..(but I don't know for sure) The bottom of my distributor looks good, but I'm going to take some measurments and see if the distributor is seating in the oil pump shaft.
Here's a kooky theory...could I have driven the oil pump shaft down while I was priming it? Is that even possible?

Originally Posted by HOSS429

how soon after oil pressure loss should one hear lifter clatter .. i rarely pay attention to guages till i hear a problem ..

I wish I had your confidence. I don't want to wait until I hear a problem.

Is this an aftermarket distributor with an adjustable collar?

nope. I didn't even know such a thing existed. It's a factory HEI

Originally Posted by 65ny

I wish I had your confidence. I don't want to wait until I hear a problem.

i think i worded that wrong .. i`ll have to think on what i said and clear it up a bit

No, the driveshaft should bottom out in the oil pump so it will stay at the right length. Is this engine and all the components one that has been together and running at one time, or has it been put togethr from various parts? I'm leaning toward the distributor not indexing on the driveshaft correctly for some reason. Is the distributor taking some effort to get it down over that driveshaft? Usually, you need to play with turning the shaft just a tad at a time until it will index correctly and slip down. If yours just fell into place on the first try, that might mean the insides of the distributor where it goes on the shaft is worn or damaged.

What kind of tool are you using to preprime the engine?

Don

Hmmm.....that has me thinking... I installed an aftermarket intake too. There is a chance (small one) that the distributor mounting surface is higher (further away from the oil pump) than the original. That doesn't sound likely though...just thinking out loud.

Shouldn't be a difference, but anything is possible. Compare it to your old one. Is the distributor seated all the way down?

Don