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After a nice 60 mile trip yesterday, I noticed an oil leak in a new location. It appears to be coming out of the center intake port and running down the block. I wiped under the valve cover, this is dry. Where would this oil be coming from, how is the best way to repair it? It was not there before the trip downtown. Motor carries +40 psi oil pressure, vent tube is functional.

Long shot but if you can’t see the leak maybe oil sending unit if you have a electric oil gauge.

About the only thing between the valve cover gasket and the part of the block with the stain is the cylinder head gasket. I can't think of any oil passages on that side of the head, but you might try a head retorque anyway. It can't hurt, and it might help.
Jerry

Can you see the leak from the top?

I can't see it from the top. I will post pics as I go thru removing the valve cover and complete the re-torque. The stock oil pressure gauge is connected. This work will be 2x weekends out. Great directions and options. Thanks for the help !!!!

Hmmm. Keep up posted.... but listen. I was outside after reading and seeing the pic of your oil leak earlier today. At the corner inward fence post, image an "L" shape, i hang a bug light occasionally. Well, i hose clamped a piece of round stock i bent to pertrude up and outward, with a hook shape to hang the buglight, from the fence post. Long story short. The round stock is starting to rust after a few years. But the rust mark running down from the fencepost, appears to ride the hoseclamp then comes out partially around it and looks like it is actually leaking rust from a different area on the hoseclamp. I suspect there is a very minuet vavle cover leak from gasket then trickling around the exhaust manifold, which is super hot on a longer ride, thinning the two or three drops looking like river running down da block. As previously mentioned. Look at the valve cover gasket.
On a side note. You would make James Bond look bad, if there was enough oil in the exhaust passage to leak out of the manifold to trickle down the block, to create an external leak. Clouds would block the road behind you.

You sure it's oil, my 250 has a spot like that in about the same location. I suspect mine is unburnt fuel leaking from the intake gasket. I never smell oil burning, but occasionally smell fuel after shut down.

I guess that is why Chevrolet painted truck engines Light Black (some call it Dark Gray), so that you wouldn't notice minor leaks for which engines of that era were known to have.

Joe - I smelled gas after the Sunday drive, couldn't trace it. I haven't got to re-torque of the head yet. No smoke behind me, Bond's secret move is beyond me. I'll try the simple check of the intake gasket bolt torque after I get the oil breather tube cleaned out and re-installed.

I checked the intake and exhaust manifold bolts, they are what I suspect are very loose. A couple of them look like I'll never get a torque wrench on them; does anyone have a good rule to use to tighten them? I think that it will be to get a good high stool to work from and use a 9/16" box end wrench and snug each one to the same 'feeling' and count flats turned ; assuming that all started out the same. I plan to check the head bolt torque out sometime in the next 2 days.

I’ve had the same thing on my gmc engines. Unburned gummy fuel sludge. Clean it off before it eats the paint off your block 😊

Buy a torque wrench adapter. How to use one.

The only way to torque those inner bolts on the manifolds is by using a crows foot and an offset calculation table. I used to do that allot during my career in the Nuclear industry. I just recently installed mine and I don’t own a set of crows feet so I used the torque wrench on the outer ones and guesstimated on the inners.

You can get a rough idea of what 30 lbs torque feels like by using a bolt and nut, the same diameter and pitch as the manifold bolts and clamp the bolt in the vice so the thread is sticking out the top and the head is up against the bottom of the jaws so it can’t be pulled thru. Add a washer and nut, then torque to 30 ft. Lbs. Now mark the nut in relation to the vice and take the wrench your going to use to tighten the manifold bolts and break the nut free. Now tighten the nut back to your marks. Do this a few times and you’ll to get an idea of what 30 ft. lbs. feels like.

Most people just tighten them with a wrench!

Just asking here but you did use the alignment rings when you installed the intake exhaust manifolds correct? If it were me, I would probably unbolt the intake/exhaust, clean up the fuel leak marks on the block, little paint touch up if necessary, and reinstall using a Rimflex gasket. The Rimflex is a one piece gasket which makes installing it a little easier, and it is a little more forgiving then the stock gaskets. As for torqueing the bolts down with a torque wrench, I have never done that in the 40 plus years I have been messing with these motors for the reason you discovered. So I use a long wrench and tighten them down firmly. After a week or two of use, I retighten, and will generally go back a third time just to be sure.

Most likely you are referring to a Remflex manifold gasket?

A lot of vendors are selling that same name/number gasket that looks like this?
The description looks right: "Remflex exhaust header gasket sets feature 100 percent flexible graphite construction that's effective up to 3,000 degrees F, far exceeding any vehicle's exhaust system temperature. They will rebound 30 percent when tightened, creating an optimum seal that eliminates the need for re-torquing. The unique Remflex material has been proven for over three decades in industrial applications, where temperatures routinely exceed 2,000 degrees F. They're designed to crush up to 50 percent, filling uneven header flanges for maximum sealing and eliminating the potential for exhaust leaks. Choose the gasket style and dimensions for your specific exhaust headers."

They also make/market pieces of materials.
These might be handy with "odd GM-truck" applications, like updraft carburetors' manifolds on COEs?

I ordered bulk Rimflex for my 503, it was a failure. The mat’l is so thick, it won’t hold still to get a straight cut. It must be stamped out when they make a whole one.

Ed

Thanks for the review/warning, Ed

If I had to use it, after reading your warning, I'd probably "sandwich" the material between two pieces of thin/dense plywood/hardboard:
- then, draw the pattern on the board
- then, cut with a fine-toothed jig (sabre/sabre) saw

I have never used the bulk rimflex material but I have used the rimflex intake/exhaust gasket and it works really well. On one of my motors I run a really old set of clifford long tube headers. I probably purchased these headers in 81 or 82. They always warped a bit and always leaked. I would use muffler paste on the ends to help quell the leaks. Once I used the rim flex, nada. I was sold. I have used them on probably 10 motors so far and frankly would never go back to the original style gaskets.

That being said, there is one draw back. If you have to remove the intake/exhaust, the rim flex is a little messier to remove then the stock gaskets which generally just fall off. But that is part of why they work so well.

I've made crowfoot-type wrenches out of Harbor Freight Chinese box end wrenches and an acetyline torch. Bend the wrench to the shape you want and weld an extension onto the other end to plug in the torque wrench. Getting to some of the stovebolt cylinder head bolts for a retorque without disturbing the valve adjustment is a lot easier with a wrench like that. Snap-On used to make that tool, but I haven't seen one in years.
Jerry

If you do use a crows foot here’s a link on how to calculate the proper torque. [LINK]

Here is a picture of what HR is talking about. In these cases no offset calculation is necessary.

With crows foot you can use the flat method for torque

Please elaborate.

Ok after taking in all of the details posted, I have a method in planning to attack the soot in tail pipe, black dribble under the intake manifold and gas smell thru-out the garage with the oil bath air cleaner off...
finish cleanup of the filter unit of the oil bath, let it dry out.
unscrew and remove the cap off the top of the carb; clean out any gas debris in the bowl, jets and float. Hopefully the gasket will survive dis-assembly and re-assemble.
Clean out the glass bowl of the gas filter and gas filter.
This should take care of gas supply issues
torque the manifold bolts using the 'feel method with wrench' after setting the ones closest to the middle with torque wrench, may see what this feels like with the torque wrench and the crows foot head. (I feel really dumb, not thinking of using the Snap-on set that I found at a yard sale in a dirt-filled box)
Check torque of head bolts using the normal torque wrench, flat crows foot/calculated method and weld-on end/bent wrench method from HR
I'm going to use the oil bath air cleaner for the test runs, it should be clean, dry and ready for use by the time I get all of the other tasks completed. Plan to use straight 30 weight oil to the fill line.
I'm going to install new plugs and replace the coil. The plug wires look great, the coil is strangely covered in oil.
I have three rainy days ahead here in North Carolina, I should have it all ready to go when the sun comes back out again.

I was able to complete:
- head bolt torque check; they were spot on
- intake / exhaust bolt check. Set to specs in manual, used crows foot setup
- installed new plugs, used 14mm x 1.25SAE tap to clean out the threads. The old plugs were a mess, gap was not right, some were really short on the arc, all were really sooty.
Set new R43 AC plugs to .035 and put a small drop of anti seize on the start of threads, torqued them to 25 Ft/lbs
- Cleaned out base of air filter, filled with a pint of 50 weight oil (per manual suggestion, i'm glad I checked it)
- Installed new coil
- Was able to remove the deposit under the intake port, it will be interesting to see if it re-appears
-Decided not to dig into the carb just yet
- cleaned off all oil deposit on underside of chassis and body from the engine vent tube
- oil filter cannister is now painted medium blue (sourced at Ace Hardware) AC waterless decal did NOT stay adhered to the lid, I sprayed the lid Chevy orange, let it season out a week at 73 degrees (inside house), lid was warm when I applied the decal. The ringed decal stayed on about 10 minutes when i took it out to the garage. It felt like no adhesive on it.
Engine started very quickly, ran very smooth. Road test to occur tomorrow.