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Old 01-06-2006, 05:46 AM
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SBC low oil press but no rocker arm noise?

Hey guys,

Been lurking here for two or three years and usually find what I need by searching....however, after slogging through 7+ pages of "low oil pressure" posts I havent found what Im looking for so I figured Id ask.

Question: Does a stock block SBC have a small welch plug that is fitted into the main oil galley under the rear main cap like the bow tie blocks?

Reason(s) I ask is that I recently rebuilt the 350 in my father in-laws 93 K1500 truck and now have low oil pressure when the temps come up to operating levels. The motor is bone stock other than being bored .030 and all bearing clearances in the rotating assembly were mic'ed (not plastigauged...mic'ed) and found to be perfect before assembly. New cam bearings were installed by the machine shop as well as hot tanking the block and installing the freeze plugs. Oil galley welch plugs behind the cam gear and the behind the flex plate were "staked" in place so I dont think any of those have flipped (least the ones behind the flex plate havent since its not puking oil out of the rear of the motor).

Cold idle with 10-30 oil was 60psi but as temps came up it dropps to 13psi at idle in gear which is lower than it should be in my experience on a fresh rebuild (less than 1 mile on this engine/cam break in time only). I pulled the oil pan and yanked the Melling 55 pump off and disassembled it and found a broken pressure relief spring (first time Ive ever seen that one) and debri from the spring had scored the pump rotors and body slightly. Since I had the pan off I swapped in a M55A pump since I had a few on the shelf which brought the cold idle pressure to 65psi and hot idle pressure to 18psi in gear which is still low from my experience. High RPM pressure (4000 in this case) oil pressure while hot is ~40psi which is about 20psi lower than I normally see with the M55A and there has never been any lifter bleed down/clatter with this engine other than a few ticks when its first fired which is kind of odd but it goes away immediately.

No debri in the oil other than the typical cam break in stuff that I normall see after dropping the oil out after breaking in a flat tappet cam. Oil has been changed twice since then with no bearing material/metal noted.

The one thing that worries me that I didnt think of until I had everything buttoned back up is that when I had the pan off I didnt pull the rear main to verify if the welch plug was installed by the machine shop in the main galley or if there is even one in there on stock block SBCs. I know there is a plug there on the bow tie blocks since thats what I usually mess with but didnt even think of it on a stock block.

Can anyone verify whether or not there is a welch plug under the rear main cap on a stock block SBC? Id normally look at one of the two or three orphaned SBCs that normally take up space in my garage but for once I dont have any orphans that arent installed in anything to look at LOL.

TIA

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Last edited by myclone614; 01-06-2006 at 05:59 AM.
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Old 01-06-2006, 06:05 AM
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The long answer is yes. But, I would think that if it was missing you would have an across-the-board drop.
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Old 01-06-2006, 06:20 AM
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Quote:
Originally Posted by MI2600
The long answer is yes. But, I would think that if it was missing you would have an across-the-board drop.
Thanks for the "long" answer . Seriously though I appreciate the info.

I may pull the pan back off and yank the rear main to verify the plug is in there since my machinist has gotten kind of sloppy in his services with the last couple of blocks Ive sent to him. Stuff like "forgetting" to install freeze plugs, sending the wrong parts back, and other minor annoyances. So far his machine work is dead on (I check all tolerances before assembly) but I guess I need to start checking every little detail now.....or find another machinist that knows what he's doing which is almost impossible out here in BFE.
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Old 01-06-2006, 07:45 AM
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You'll have to lower the crank too to see the plug.


Where are you tapping the oil from? I assume it's at the back of the intake mounting rail. You might look at a pressure loss near or at the cam bearings. Otherwise I might just leave it alone. It's a ton of work messing around with the oil pan with the engine installed. 20 and 40 hot are pretty normal. Remember that pressure is only a restriction to the system. All you really need to do is get oil to the bearing.
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Old 01-06-2006, 12:50 PM
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Quote:
Originally Posted by johnsongrass1
Where are you tapping the oil from? I assume it's at the back of the intake mounting rail.
Yeah the pressure readings are from the rail at the back of the intake. I thought the original guage sending unit may be bad/weak so I removed it and temporarily installed an accurate mechanical gauge. Both the factory gauge and the mechanical read the same.
Quote:
You might look at a pressure loss near or at the cam bearings Otherwise I might just leave it alone.
About the only thing I can think of would be one of the galley plugs behind the timing cover has flipped/pushed out but like I stated previously they are "staked" in place. Only way to check that is to drop the pan and pull the timing cover off which, as Im sure youre aware, is a pain.

Quote:
It's a ton of work messing around with the oil pan with the engine installed.
Actually its not that bad really since its a full size truck and there is a decent amount of room to work. Drop the exh Y pip, pull the torque converter cover, take out the two motor mount bolts, loosen the oil filter/cooler adaptor, and lift the motor up ~3". The pan will slide right off once you get that stuff out of the way or loose.

Quote:
20 and 40 hot are pretty normal.
I would agree if the motor had ~100k on it or was set up loose but its not high milage nor is it set up loose which in my experience yeilds 70+psi cold idle pressure and ~45psi hot idle (~700rpm).

I may let him have the truck back so he can drive it and see what happens. It may run for 200k miles like that but I know something isnt quite right and if it scatters then I have no choice but to yank it back out. He wont be happy but considering he's getting free labor I doubt he'll complain too awfully much....well wait...yeah he will.
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Old 01-06-2006, 02:12 PM
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Concerning the front galley plugs, if the center one is pushed in to far it blocks the oiling hole behind it.

I suppose the machine shop could have installed the bearings in the wrong order. Putting the bigger one in back or not lining up the oil holes correctly.
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Old 01-08-2006, 10:31 PM
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oil pressure problem

I would think that leaving the plug out would really mess up the oil press but you are right in you thinking though there is a plug under the cap and it is such an odd shape that they don't seat well I would check that one out.I know guys that like to thread a plug in in it as it will come loose and leak we had an engine in a dirt car that was fine cold but when raced in the turn would lose oil press we changed out the oil pump and switched pans still it happened finally we checked the plug by pushing a piece of steel brake line from the top where the oil pressure sender is down to the plug it was in there but loose as hell replaced it and that cured the problem.If I'm not mistaken that plug keeps the clean oil from mixing with the dirty oil I've only seen that happen one other time though! We always used to use melling pumps in the street engines we built the hv pumps came with a couple of different springs we didn't use the hv spring we used the optional spring and had pretty much 60psi cold abd 40 psi hot even at idle. You could have another plug leaking but as you stated oil would be running out of the bell housing or you would have very low press all the time hope this helps
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Old 01-17-2006, 03:23 AM
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Update:

Short version...\/
Changed the oil/filter again looking for debri in the oil and got the dreaded "shiney stuff" (metal flakes) floating around in the oil. I yanked the engine and tore it down only to find chewed up main bearings and scored main journals on the crank. Rear cam oil galley plug (the big one at the back of the cam) was driven in too far causing it to rub the back of the cam and circulate metal through out the oiling system. Bearings do not like digesting chunks of welch plugs I guess .

Long version...\/
I messed with the truck for a few days after I first posted this thread but I knew something wasnt right so I yanked the motor, put it on the engine stand, and pulled the oil pan. I cut off the screen of an old oil pump pick up, attached the tube to the oil pump, and ran a 5/8" hose from it to the drain pan under the engine. I filled the drain pan with 10/30 and had the wife (she's a trooper) run the oil pump with an air drill/priming tool and call out oil pressure readings while I watched the bottom end of the motor looking for internal oil leaks. At first I got a steady 70psi of oil pressure due to the cold oil so I thinned it with some gas to try and simulate the viscosity of hot 10/30 oil. Once the oil was thinned out the oil pressure dropped to 25psi and I could see that the mains were hemoraging(sp?) a LOT of oil so I pulled the rear main cap. Once I found a beaten up main bearing I aborted any further tests and tore the motor down and found the rest of the mains chewed up and a couple of the rod bearings with some minor gouges.

IMO since the cam blanks arent machined on the rear face to remove the saw marks when the blanks are cut as well as the fact there are a couple of indexing holes drilled in the back of the cam (for machining I suppose) the back of the cam acted like an end mill and chewed into the non hardened metal of the rear welch plug. This put metal shavings into the oiling system which migrated to the bearings and chewed them up.

I take most of the blame for not catching that the cam bottomed out against the rear welch plug at the point it was flush with the front cam journal (it should be recessed slightly in relation to the front cam journal face with no timing gear bolted to it to verify clearance between the back of the cam and the rear cam tunnel welch plug). The machinist shouldnt have driven it in so far as well but its up to the builder (me) to double check everything and obviously I dropped the ball there. That'll learn me to get in the mind set of "its just another SBC so just put all the parts in a box and shake it and it'll run for 200k miles".

So the lesson here is if you get to a point that youre doing a fair number of motors dont get complacent and start over looking anything. I over looked one small thing and it cost me a few hundred bucks by the time you add up the new crank that was needed, new rings/bearings, another gasket kit, several qts of oil, and half a dozen oil filters. This isnt counting the aggrevation of yanking a freshly installed motor back out and tearing it down. Lesson learned.........the hard way as it usualy is with me .

Last edited by myclone614; 01-17-2006 at 03:29 AM.
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Old 01-18-2006, 04:32 PM
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sbc engines

heres one for the record books,a buddy rebuilt a sbc for a blower motor,all excellent parts(not stock) anyhoo,he put in teflon buttons in the piston pin reliefs.(full floating pins)he must have dropped on of the buttons ,cause after a few hours running time,we heard a kind of low grinding sound. we pulled the motor and found that the piston pin had worked its way out (remember...a full floating pin) and machined the neatest gouge? in the cylinder wall.curved at top and bottom but nice and smooth. of course bearings were ate up. did NOT replace block used dremel tool to roughen the gouge and filled the guuge with JB WELD...NO B.S. ran a stone down the bore to get a nice cross hatch on the bore. put that dog gone sbc back together and it was still running with a street blower 25,000 miles later. no problems. is thar JB weld good stuff or what> NO i dont own any part of the jb weld company.
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Old 01-20-2006, 05:53 AM
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Had a press fit pin come loose in an IH 392, said to be quite common. All Inters now get spiralox.
I hope that sbc cam welsch plug was a gennie one with the short lip and not a cooling system replacement - yeah, been there.
A bloke I new did a customer engine, forgot to tighten the cam bolts (sbc), delivered, fitted, 3 weeks later the big crane solid slid back and cut the flex plate in 2 - at high road speed. OOPS.
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Old 01-22-2006, 03:19 AM
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Sigh...... I tell ya fellas I might have blamed the wrong thing for the failure. Even though the plug was driven too far in and indeed was scored by the rear of the cam the same motor after being rebuilt again with a new crank, bearings, rings, oil pump, cam bearings, hot tanking, etc etc hadthe same low oil pressure problem on start up. I shut it down and dropped the oil out only to find another nice batch of shiney particles (bearing material) floating in the oil. UGHHH!!

To make a long story short after a few days of cussing/head scratching I found an issue with the oil cooler thats mounted inside the end tank of the chain store replacement radiator that was installed (I didnt install it) not long before the motor spun a bearing which is why it ended up in my garage to begin with. Come to find out that the oil cooler is of such poor design that when the factory preformed aluminum oil lines were inserted and the fittings tightened that the opening of the oil lines were only .020" from bottoming out on the back of the oil cooler when measurements were taken with a dial caliper. IMO the outlet and inlet of the oil lines being so close to the back of the cooler was severely restricting the oil flow to the motor enough to give me the low pressure and starve the bearings of oil. You'd think I would have caught this when I had to thin the oil down with a LOT of gas due to higher oil pressure on the engine stand like I stated in my last reply since the only thing that was different from when the motor was in the truck was that the oil was going through the radiator cooler instead of a loop of hose attached to the oil cooler lines while on the engine stand. Then again IMO thats an extreme case of an odd ball problem and who'd have thunk the oil cooler was manufactured wrong? Obviously not me nor any of my local gear head friends that discussed the problem .

One brand new SCAT 350 crank, main/rod bearings, oil pump, gaskets, etc down the toilet for the second time and the truck moved but maybe one mile. Nothing I like better than flushing money down the ol crapper due to some third world manufacturers POS since Im footing the bill for parts .
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Old 01-22-2006, 10:46 AM
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the oil cooler should have been changed the first time.. as my machine shop says.. theres no way to clean an oil cooler.
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Old 01-22-2006, 03:52 PM
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Quote:
Originally Posted by StevenC
the oil cooler should have been changed the first time.. as my machine shop says.. theres no way to clean an oil cooler.
The cooler wasnt stopped up it but it was manufactured incorrectly. The radiator was replaced with a cheap chain store import that had such sloppy manufacturing that the actual cooler wall was too close to the opening of the oil line which restricted the oil flow.
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Old 01-23-2006, 07:44 AM
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if the bearings got chewed up from a spun bearing in the first place, usually you got metal floating around which can make it into the oil cooler and be resident. it coudl then flake off later when you got the new motor in there and cause havoc with the bearings.. this is what my machine shop explained to me and have a huge sign on the wall about it.
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Old 01-23-2006, 04:16 PM
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Quote:
Originally Posted by StevenC
if the bearings got chewed up from a spun bearing in the first place, usually you got metal floating around which can make it into the oil cooler and be resident. it coudl then flake off later when you got the new motor in there and cause havoc with the bearings.. this is what my machine shop explained to me and have a huge sign on the wall about it.
They are 100% correct but Im an odd ball case where as I have access to large ultra sonic cleaning systems at work that will litterally clean anything down to a molecular level (long story that would bore you to tears).

Youre average Joe Shmoe usually doesnt have access to that type of equipment and it would be extremely hard to completely clean internal passages in something like an oil cooler. So your machine shops sign is actually a pretty good thing and offering good advice.
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