Inliners International Forums General Discussion Engines 1955 261 engine Oil Pressure - What's normal?

Hi there,

I have a question about my rebuilt 261 engine and oil pressure. I'm questioning what's 'normal' on these engines.

I have a rebuilt 1955 261 with no filter. It now has 1700 miles on it. I currently am running straight 30W in it and when it's cold, I will get close to 60lbs pressure in it. When it starts getting warm, it will drop to about 30lbs, and when it's really hot (on our 100 degree days) it will go down to nearly nothing on my 0-60lb gauge when i'm running about 2000RPM. When I rev it up, it does come up off of 0, but barely much maybe 4-6lbs - after running it for 30 minutes or more.

My truck is a 49 suburban and I'm using a 0-60lb factory rebuilt and converted gauge from classicparts

The engine had a complete overhaul 10; new cam bearings, new pistons, rings, new melling oil pump (for better or worse) crank bearings, etc. I am getting oil to the top of the rockers, but I didn't replace the rocker arm shafts. I'm wondering if my shafts are worn so badly it's affecting my oil pressure - and if so - should I replace them? When I got the motor, it had the infamous external oil line run from the side of the block to the passenger side of the head with a 'T' on it.


If replacing the rocker arm shafts is the next step, which rocker arm shafts do I purchase? I have a 235 "848" head on the truck now. I also have the "pinched" return oil line on the rocker arm shaft (NOS)

The motor sounds great and runs just fine - I'm just concerned about the hot oil pressure, valve train is a little noisy, but it's a solid lifter setup and I have nothing to compare it to!

What else can I check (or am I going to have to start checking bearings next)

Thanks

Are you using a mechanical or electric guage (I'm not sure what a converted guage is) and is the real low pressure just on the real hot days?

I don't know about the oil on hot days. It's all we've had around here since the rebuild! 90+ degrees.

Yes, the gauge is mechanical and I have another one I'm going to screw into the passengers side of the head to check against my current gauge. From what i've read to date, 60lbs cold may never happen due to the pressure relief valve in the oil pump - maybe it is my gauge?

Sounds like a clearance issue for the pressure to drop off to near zero. What kind of bearing clearances did you end up with on the rods and mains. Were the connecting rods reconditioned, did you have the housing bores on the mains checked to see if it needed to be line bored or if they were in spec?

Oil pressure wise a 261 shouldn't be different from a 235. So warmed up 35-40 lbs is good. 15 lbs or so at idle. I use the old 10 lbs per 1,000 rpm. Yes try another gauge first. I doubt that you would lose that much pressure at the rocker shaft without totally flooding the top of the engine. There could be a leak where the pump feeds into the oil galley in the block, a weak or stuck bypass spring, a bad pump, a clogged line to the gauge, or Heaven forbid excessive clearance in one or more bearings. Take it a step at a time and eliminate the simple stuff first. Don't run it with no pressure. Good luck! It sounds like a fun ride.

Chevy did make attempts to limit oil to the head. I'm more familar with the 58 and up blocks that had either a .062 restriction somewhere in the oil galley leading to the head or the restriction was the design of the rocker arms. . Or the so called special center head bolt with a .062 restriction to limit oil when using a 59 or later block with an earlier head..But I don't know how much of an oil pressure loss occurs if done wrong.
For little money and an hours work,loosen the rocker shaft,remove the center oiling piece,figure a way to plug the oil feed hole.See if it makes any difference.
Low oil pressure after an overhaul might be a missing oil galley plug,it can happen.Or excessive bearing clearance.
Obviously use another gauge to be sure of the oil pressure.

A single fixed leak such s the rocker feed will have great effect at low RPM, and almost none at full speed.

To be loosing almost all oil pressure at hotter temps is something more serious than a worn rocker shaft. Its excessive rod and/or main bearing clearance. Did you even check it when the engine was assembled? Also, where are you tapped into the engine to read your oil pressure?

I made an oil pump test fixture to adjust the oil pressure and just play around.More or less submerge the pump pick up in a pail of oil,screwed a pipe into the outlet with a variable orifice bleed and a pressure gauge.A 1/2 inch heavy duty variable speed drill motor rated at 3000 rpm maximum drove the pump.A 235-261 pump lost 75 percent pressure at all speeds pressure with a 3/16 bleed inch opening.I set the pressure at 50-55 psi.
This same pump was used in my 261 build with .002 on the rods and .0025 on the mains,oil to the head was restricted by a .062 Holley carb jet pressed into the rocker oil feed tube.The engine has a full flow oil filter,the gauge hooked up to the outlet of the filter.
After a one hour ride at hiway speeds with 20/50 oil,this engine has 25 psi at 700 rpm and rises to 40 psi at 1200 rpm with 45 psi maximum at higher speeds
A real world test....

Tony P Thank you for your test. I have a 57 261 and that just what my oil pressure gauge reads at those rpm. I always wonder if that what it should be reading or not. Again, thank you.
Luke

Over the last 15 years I've built two 235's and three 261's with new bearings on a reasonably sized crankshaft,new cam bearings,good oil pump and the rockers and oil feed to the rockers as it should be.All of them had about the same oil pressure 25-35 at a hot idle ,depending on idle speed. And 40-45 running down the road.
I'm just a hobbyist not a pro engine builder,but if your oil pressure is a lot lower ,your engine has issues.I believe some machine shops grind cranks on the loose side because they are afraid of engine seizure come backs.

3-4 lbs at idle, 17lbs at 1500, 22 lbs at 3000. The oil pan is off and a few main bearings are out. Something doesn't look right after 1700 miles for sure. If you look at the crank, there is a ridge where the oil galley in the bearing is suppose to ride. The ridge is raised and it feels around .005 There is also some pitting in the bearings as well and there is about a tablespoon of really fine metal in the bottom of the oil pan. It's ground so fine it looks like metallic paint. I haven't had a chance to pull the crank, but at this point I may as well pull the engine. The crank was turned .020/.020 and I did read the bearings and they were stamped 20 over as well, so I'm not sure what happened. Is anybody good at reading bearings? I guess I really won't know until I pull it and start measuring. They certainly don't look oil starved at all. Also, look at the ridge on the edge of the one bearing. My first suspicion - Bad machining, but we'll know soon enough. I hope I can turn that crank again. What's the largest size bearings I can get for these motors?

Since the engine is out and it's tore down again, is there any other modifications I should look at while I'm in there? The head is completely rebuilt, but I'm thinking about decking the head again for a little more compression. How far should I go?

http://www.flickr.com/photos/45290812@N03/sets/72157630828532764/

Looks like the result of a "soft" crank, hence no wear in the oil groove areas of the crank journal. Very common to grind thru the cranks surface hardness when their ground undersize like this, leaving the crank journals soft and vulnerable to rapid wear. Some cranks are heat treated deeper than others and have no isssues when reground. A sharp crankgrinder operator can usually tell by how the journals "spark" against the grinding wheel whether the crank is soft or not if he is aware of this in these cranks. Regrinding this crank wont make it better unfortunately.

CNC Dude, I think you're onto something....

Measured the crank. All signs point toward a bad crankshaft or bad grind when I had it turned. I am going to gauge the bearings again, but the crank has almost .005 taper in the con rod journals, and there is about .004 taper in the mains. Also, I had the rods re-done with the engine, but again, they seem out of round with how the bearings are wearing. I haven't measured them yet. I guess I could have had some premature wear from something else, but I don't know what!

Where is the best source for a crankshaft?

Got a crank from jjinlines coming.

I currently have a "3/4" cam from Patrick's in the engine. Can somebody give me some specs for a decent camshaft for bottom end and torque for this engine (preferably single pattern)? I may want to change the camshaft that's in there now, but i'm not sure what my options are. I have a friend at Lunati and he said he'd grind me whatever I want - but I don't know what I should try!

Patrick's specs are intake 19/53, exhaust 65/19 lift .436. Should I just stick with that or is there something that would give me a little more?

Make sure you thoroughly flush the oil galleys in the block out and us a rifle bore brush and soapy water to remove the metal trash that has circulated thru out the engine before getting it running again. Also, do the same to the new crank before installing it as well. Wash the oil pan to remove all the metal junk from the old crank.

All forged cranks are hardened to some degree.Crank shaft shops say the depth of the harness varies and often the manufacturers were sloppy about it.If the original engine manufacturer listed .030 as the maximum oversize that's a message about crank durability.
Many guys turn cranks well beyond the manufacturer limits,.060 bearings are made for GMC and Chevy I believe.Sometimes it's not a problem and sometimes it is.
A friend lost a crank in a modified GMC 270.A reputable crank shaft shop told him the .030 crank was only hardened to about .015..Loss of hardening can lead to wear and cracks,especially in a modified engine.Sending out a crank for treatment is expensive insurance.

Your oil pressure looks a lot like my JNIX 235, while one its way to spinning a bearing, after trying TWO Melling oil pumps - with no change in pressure between the two. Dang good thing you pulled the mill for a look-see, as I think you would have also been headed towards a spun bearing!

I would double-check the clearance on that Melling pump, between the gears and cover. With the gasket, you should have between .002-.0025", as per Tom Langdon. Stock gasket I believe is .009" thick. I have seen gaskets in full gasket sets measure thicker - best to check thickness. I made my own for the 235 turd motor now running in my '57 car from a file folder measuring .008". I used gasket sealer on both sides, then installed the cover and tighten the screws. Removed the cover, then cleaned up the squeeze-out inside and installed the cover for good. I did this with a GM pump and came out with BETTER oil pressure than I had with the JINX (good clearances), despite the sloppy bottom (rods -.003", mains - .004"+)!

Can you look at the cam card that came with Patrick's cam and tell us what the DURATION @.050" is? Are you looking for a bigger or smaller cam? Might be worth a call to Tom Langdon to see what he might recommend. He also might have GM oil pump gaskets available.

Sorry to hear about your bad luck on a fresh rebuild - man, I know the feeling! The JINX I speak about began with me as a $300 used 3/4-race motor with a fully ported head, Crower cam and full kit, assembled in 1978. First, the PO's builder left out the tin baffle in the crankcase for the road draft tube - I could only slow down the mess under the '57. Then, it dropped a intake valve leaving a gas station near home - due to a broken CROWER valve spring. Took the head in, only to find out it was cracked above a water jacket. I checked compression before pulling the head, #6 was 20-lbs. lower than the others, so tore it down for molly rings and rod bearings, being as clean as a home builder can be during assembly. Two years later it spun a bearing on #5.

...I'm still looking for a solid referal to a shop in SoCal that can weld up the crack to save the head.

BEST of Luck in putting it back together!

- Tim