Oil pressure gauge reading.. is it too low??

[mckpaul]

I'm sure this has been answered in the past, but can't search to find it.
Got a stock 305 with a 60 psi oil pressure gauge. What's normal for a reading on this gauge on a running engine at idle speed?... at driving rpms? Just finished a stock rebuild on the motor including a new oil pump, ran with the old sending unit for a while, replaced it yesterday. The gauge is responsive to changes in engine speed, but reads lower than what I would expect. At driving speed rpms it reads somewhere between 15 and 30, and at idle drops down below 15. I know I have enough pressure to close the oil pressure switch by the oil filter that sends voltage to the choke heater and turns off the choke light, so if I didn't have a gauge and only had an idiot light, I assume in reality I have enough pressure that the idiot light would be off, but it seems strange to me that the 'normal' range on the gauge would be down in the first 25% of the scale and not around the middle. Do they all read low like this or do I have an oil pressure problem????

[GMC AMI]

If the gauge is correct than your oil pressure is a bit too low. It should be around 30 lbs at idle is more normal, a little higher a highway speeds. You may have a bit too much clearance in the main berrings. If the oil pressure cold is higher than when warmed up, my guess is the mains and not an oil pump problem. By the time the idiot light would indicate and light up the damage is done so the saying goes and there is a lot of truth in the statement.

['85Silverado]

Before you get too carried away, I would verify the pressure using an aftermarket mechanical guage. What oil and filter are you using? On the rebuild, what did you do to the crank? Plastigauge the bearings?

[mckpaul]

On the rebuild, I took the crank, cam and block to my local machine shop. He was able to get a 'kit' from a supplier with everything for the rebuild (including the oil pump)tweaking bearing and ring sizes as needed. He was pretty impressed with the lack of wear on the cam and we reused it, he said the crank wasn't worn enough to merit going with over sized bearings, so the mains (and connecting rod inserts) are new standard sized on a slightly worn crank that he polished. (Keep in mind this isn't a performance vehicle, or even daily driver, just a second vehicle to have a truck around the house, wasn't even going to rebuild the motor before dropping it in, but one thing led to another while I had it on the stand and I did it anyway)

Thought about finding a way to measure the pressure with a gauge other than the one in the dash, not sure what to use or where to get it. If I find an aftermarket mechanical gauge, would I connect it in place of the sending unit (at the back of the intake manifold) or could I connect it where the other switch is by the oil filter?

As for oil, it's Pennzoil, I think 10W-30, and the filter is Fram.

Oh, and by the way, I've had the instrument cluster out recently to replace all the bulbs, and made sure all electrical connections and grounds were good. The ground strap from the head to the firewall is in place as well. After doing all that, the fuel, battery, and temp gauges are solid and accurate, unlike before when they seemed to wonder around.

[Captkaos]

That is normal range for a stock bottom end and pump. Typically it is about 10 at idle and 5lbs for every 1000 rpms.

[mckpaul]

^ That sounds encouraging. I used to have a 77 Silverado many years ago with a 350. Seems like I remember it reading low too, but it had a lot of miles on it. Guess I'm just being a worry wart since I just rebuilt this one, makes me go back through everything in my head wondering if was a little off on something somewhere in the rebuild process. It's not my first overhaul, done three or four over the years.

[cliffsta]

my 305 stays around 20-30 at idle in park, and in the gas it acts normal. Since its a new engine im assuming the oil has been changed a few times. if its hot outside and you're running 10w30 then that still is a bit low, but remember that if its hot oil pressure will be low.

[Destructo]

I get about 15 psi at idle on my 350 and its been like that daily driving it for over a year now with no issues. Goes up to around 30-40 while cruising.

[mckpaul]

No it's till the original oil, only put about 60 miles on it so far going out for short runs to get the bugs out, no inspection yet, no registration yet due to no passed emission test yet, working on getting all that put back in place now. Anxious to see if it changes after I get inspection and tags and can put some miles on it, get things seated and broke in and change the oil a couple of times. Gonna have to look and see what weight oil I used.

I apperciate the input!

[76HighSierra]

IMHO, that reading is too low. I have rebuilt a ton of SBC's and have never had one that read lower than 25psi idling hot. Personally, I rebuild my own engines (including checking all clearances) because I don't trust anybody else's opinion on what's "within spec". I trust my machinist, but I ALWAYS double-check his work. After all, it's not his money that was spent and it's not his engine. I've seen freshly machined cranks that were a mile out of spec, due to sloppy machining. Also, never just slap in a new oil pump. Many come with improper gear end clearance, right out of the box. Depending on just how far off it is, this can drastically affect oil pressure. The proper spec for BBC and SBC oil pump gear end clearance, is .0025". I recently bought a Melling HV oil pump that was at .0075"!! Took some serious time and elbow grease to blueprint that sucker. Proper clearances will ensure good oil pressure.

[mckpaul]

Hmmmm.... well, I did my own rebuild, machine shop polished the crank, did the heads and got the block ready for me, did the measurements and got the parts. It is a Melling pump, not a high velocity though.

Guess my next move is to get a pressure reading with something other than the gauge in the dash and see what the actual pressure is. I'm thinking a mechanical pressure gauge that goes to about 50 or 60 psi with a pipe thread? Just a basic screw in gauge? And would the pressure be the same at the location where the switch is by the oil filter as it would at the location that feeds the sending unit (the one by the oil filter would be easier to access to screw a gauge into)?

[screwballl]

with my stock 76 and 350, mine reads 35psi or so cold (just after startup) and when running down the road at more than 20mph. After its warm and idling liek at a stoplight in gear and at a complete stop, it usually reads around 10psi, if i put it in neutral or park then it goes up to around 15-18psi unless the oil is low then it stays 10 or so.

[1FaastC10]

Originally Posted by mckpaul

he said the crank wasn't worn enough to merit going with over sized bearings, so the mains (and connecting rod inserts) are new standard sized on a slightly worn crank that he polished.

there you go right there... if in fact your guage is accurate, there is your problem. excessive bearing clearances at the crank.

[MylilBowTie]

Dont forget that the idle rpm will have influence on the oil pressure reading. I get a different reading for the oil pressure for different idle speeds 500-900 rpm. At 500 rpm the oil pressure was a lot lower than when it was idled higher.

[aliencheyenne]

Switch to 20-50 and see if it makes a difference

[Destructo]

Probably should have used some .001" undersize bearings on a polished crank. I think they make bearings specifically for use on cranks that have been polished.

[mckpaul]

OK - Started it up last night and watched the gauge closely. Cold idle it came up from 0 and read slightly above 15 (assuming that mark in between 0 and 30 is 15) maybe about 17. When I'd rev the engine, it would go up about 2 more psi and drop back when I let off. What seemed weird to me it that the gauge doesn't just move up, it kind of clicks up, or immediately jumps up a little on a rev, and then jumps back down when I let off, a much faster response than I'd expect to see if it really was just a couple of psi increase.... OK, wishfull thinking on my part...

Assuming the pressure is a little low due to excessive clearances, would it help to change the oil pump to a high volume pump? Autozone shows to carry a high volume Melling pump with a comment "MUST USE INTERMEDIATE SHAFT WITH STEEL GUIDE". By that, are they referring to a steel equivilent of the teflon coupling piece that came with the regular Melling pump that keeps it engaged with the distributor shaft?

In hind site, guess I should have posted this in engines and drivetrains, but thanks for all the input, much appreciated!