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?

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

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 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?

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

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?

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

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

Originally Posted by Itoldyouso

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

The engine is a fresh rebuild (first time torn down) from a local (reputable) machine shop. I installed the oil pump though.
I the distributor is taking some effort to seat on the driveshaft, I've never had so much problem with one. The distributor looks good, but the oil pump shaft looks a little boogered up. (technical term)
To preprime it, Im using an old distributor or with the cam gear removed.

Lay your dummy distributor and your real distributor side by side and compare them. Make sure the distance from where it sits down on the manifold to the tip where it goes into the driveshaft is exactly the same. Make sure they are identical in all respects, ie, gear in same spot etc. What I see as a clue to your problem is that the dummy will turn the shaft and pump oil but the real one does not. (apparently, by the lack of oil pressure)

When you say the shaft is boogered up, how so? It could be damaged enough that it just isn't long enough for those tangs to engage the distributor. You might also try a second distributor. The fact you have oil pressure when priming is good, and I imagine you are turning the shaft in the right direction? ( I know, it's a dumb question, but since I play with Chevies AND Fords I have to stop and think each time which one goes which way )

It could be that you are going to have to drop the pan and put in a new driveshaft. If it were me, I would spend the extra money and get a good one from someone like Summit, some high performance shaft. I would also be tempted to spend another $ 25 and buy a good Melling standard volume pump and put that in for giggles. Dropping pans or pulling engines to do that are not fun things, so while you are in there.....................just sayin'. I don't think your problem has anything to do with faulty work on the part of the machinist or you putting it together, something is just not turning your oil pump when the real distributor is in place IMO.

Don

Oh, sometimes when you remove that gear to make a real distributor a primer it lets the shaft drop down just a tad more. I have one modified from a Ford distributor to do my Ford engines, and the shaft moves up and down quite a bit. So maybe that slop is allowing your dummy one to click on the driveshaft, but the real distributor doesn't have that slop and won't index on. Something else for you to look at.