Inliners International Forums General Discussion Engines GMC 270 in armoured car question

That tube directs oil from the rocker shaft back down into the engine. The cylinder head rails on the 270 are very low - actually flush with mounting surface of the rocker shaft brackets. I'm thinking that tube is there to help take some of the oil load off of the valve cover gasket and maybe the rear valve(s), by directing excess oil down into the bottom of the engine. It also raises the pressure in the rocker shaft slightly, as the oil has to go up before it can exit the shaft. (Not, BTW, 100% sure if the above was what the designers had in mind when the put that tube in there, or if there is another reason.)

I am holding one of the tubes in my hand right now, and there does not appear to be a restrictor in the tube. Since the tube ID is very close to the OD of the corresponding hole in the rocker shaft, I doubt there is much internal restriction.

The only restrictions in the rocker shaft lubrication system that I am aware of are located in the tube to pipe adapter fitting on the front of the cylinder head, and in the brass splitter block screwed into the main oil gallery that feeds the rocker shaft, oil filter, and oil pressure gauge.

The 270 has six support brackets for the rocker shaft. The front and rear are the same PN, and have a vertical oil passage running up the center of the bracket. The center four brackets are the same, and do not have a passage. The two front/rear and four center brackets are obviously not interchangeable with each other.

Oil flow to the rocker arms comes in the front of the cylinder head, up the center of the front bracket and into the rocker shaft. It then fills the rocker shaft and lubricates the rocker arms through the holes in the rocker shaft located at each rocker arm. Once the rocker shaft fills with oil, it comes out the top of the rocker shaft through the rear bracket, into the tube, and back into the engine.

As mentioned previously, there is a screw on the top of the front bracket, that both locates the rocker shaft in the bracket, and seals the hole in the top of the shaft to keep oil from coming out.

The tube end slides into the threaded hole on top of the rear bracket Since the brackets are interchangeable, this threaded hole would have been used for the retaining screw, if the bracket was used in the front. The retainer tab on the tube goes under one of the bracket bolts. There is a gasket that goes between the tube and the bracket.

Assuming the Staghound's oiling system is the same as any other 270, during pre-oil, you should start to see oil seeping up through the holes in the tops of all 12 rocker arms, along with a fairly steady stream coming from the tube. You should not see any oil coming from the center brackets, since there are no oil passages in them - nor are there any in the cylinder head to feed them, even if they were there.

Hope this helps.

Best Regards.....