Technical Crankcase vent/rocker arm oiling system 1941-50ish Chevy 216 L-6

My root problem is oil coming out of the vents in my valve cover......BUT why are there even vents in the valve cover, when there is a big old draft tube down low for crankcase ventilation?
I noticed that there is a little cover missing from the loop of tubing where the excess oil is expelled from the rocker arm oiling system. Pictures of NOS parts on eBay and other places show a sheet metal cover over the hole in the top of the overflow tube - and mine's missing. Even though my rockers are well-oiled, I have no flow from the hole - but it's coming out the end of the tube. I can see oil in the hole at idle, but it's just kinda sitting there not getting pushed out - it goes away when the engine is off. Is there any chance that when I get on the road, oil is squirting up on the underside of the valve cover, and then the flow of blowby gas is carrying it out the vents? I ran the engine fast for a bit, but that's prolly not like going down the road. Reckon why the hole is even there, when you can see the end of the tube to verify overflow?

 

Hell yes, at least 15 psi is going to the rocker system thru a tube I think you could see with the side plate off. That missing elbow on the rocker arm system directed the excess oil feeding the rocker shaft into the lifter gallery and back to the pan. That excess oil now on the top of the closest rockers is splashing a lot more oil than normal.

The design of the draft tube is at over 25 mph air passes by causing a suction on the engine. The air from the suction actually enters the engine thru those 2 rocker arm cover vents. At idle they were to help with the breathing of the crankcase. Works best when the engine was new or freshly rebuilt. Old and wore out…. not so much.

 

I think I understand - the vents let fresh air in, which sweeps the blowby out as the draft tube pulls it thru... I just wish the air going in was filtered - but I guess breather caps weren't around back then. And I can see where excessive blowby would overwhelm the draft tube's ability to pull enough gases out to pull air in.

Thanks!

 

So an interesting development arose out of an unavoidable event: I drive this car occasionally, and on one of my trips, there was an interesting noise coming from the cover on the passenger side of the engine. Not really a knock, more of a cyclical scraping racket. Scary at first. I found that the the end of the oil feed tube had disappeared from its place where it connected to the rocker arm shaft fitting. Long story short, part of the tube had broken off and fallen into the area under the side cover, and the lifters were wearing away parts of the tube as it lay across them. Interesting find: the original fittings have a 1-piece combination tube nut/ferrule. A replacement kit is available, but the shortcoming is that it includes more modern fittings with a separate ferrule and nut - and these fittings have a longer overall length! This is a problem where the tube exits the water jacket on the passenger side of the engine - because it will interfere with the cover. The cover has an extra bulge where the fitting is - and this is tight, even with the original-style fitting. Good news is that you can get the original-style tubing nut/ferrule from NAPA! I tried other parts places and hydraulic places - but the NAPA guy immediately recognized the fitting and walked right over to where they were. When I was getting ready to put the tubing and stuff back together, I decided to remove and clean up the rocker arms. There are 2 rocker shafts on these cars - but one of mine was broken into 2 pieces and the other in 3 pieces! Every break was where a bolt went thru the rocker shaft. Interestingly, the stock rocker arm shafts were made so you couldn't put them in upside down! The bolts have a shoulder on them, and the holes in the original rocker arms are larger on top than on the bottom. As a result, the bolts won't go thru all the way unless the shaft is positioned with the oil holes on the bottom (and larger bolt holes on top). HOWEVER, the replacement shafts I received had a smaller ID than original (probably to make them stronger) and the holes on top and bottom were the same size - the size of the bottom holes on the original shafts. I think it was too difficult for the shafts to have a larger hole on top, because the hole would be larger than the ID of the shaft. I had to grind away the shoulders to use the original bolts, but made sure I got the oil holes for the rockers on the bottom. Without grinding away the shoulders on the bolts (or replacing the bolts), you will find that they don't go in all the way, and you won't have many threads engaged in the head. Oh yeah - another thing to watch for is that the fitting picking up oil on the driver's side of the engine (not the one directing the oil into the water jacket) is SPECIAL and has a smaller hole in it! This limits oil flow going to the rocker arm shafts, and therefore reduces the pressure in the rocker arm lube system.

 

Just a hint: Be sure the can on the road draft tube is clean.
These things can accumulate crap inside and hinder the venting of the crank case. Hard to see and clean sometimes. We used to just put a little gas in them and burn them. Then tap the ashes out and blow them out with air. Clean as a whistle and good to go. It's been a long time but isn't there a locating pin on the bottom of the shaft bore on the head? I seem to remember one and if someone tried to tighten the cap bolts down with the pin and shaft holes misaligned it would crack the shaft. Or maybe I'm nuts.