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

Hi, newbie here,
I'm rebuilding a GMC 270 in a WWII Staghound armoured car. My first time working on this type of engine. (There are two of them in this vehicle, coupled up to Hydramatic transmissions.)
The manual talks about a rocker shaft overflow tube coming out of the rear rocker shaft pedestal. The engine I'm working doesn't have this, it's missing. The other engine, does, but the tubing is 1/4"" maybe even 5/16", and it's obviously hand made, with gobby brazing, so I don't trust it as being original.

The supply tubing from the adapter screwed into the front end of the main oil gallery is 3/16" and it runs up to a y-fitting screwed into the head, which also supplies the oil filter.
When I run the oil pump with a drill (room temp 15W40), with no overflow tubing installed in the rear pedestal, and 55 psi in the main gallery, I get oil pissing out of the rear overflow hole in the rocker shaft and no oil coming out of the rear 3 shaft pedestals. So the rear overflow needs some kind of restriction to build pressure in the rocker shaft, especially as the 3/16" tubing on the supply sideis is also supplying the oil filter.
I'm thinking 3/16" tubing from the overflow to the nearest pushrod drain hole.
What was installed originally on these engines on this,overflow?
Thanks,

Malcolm

I don't know if this will be any help. I just went out and looked at some stuff in the almost junk pile. On a rocker set with the stamped steel rockers all six stands have 3 holes in the bottom, two for bolts and one that could align with an oil galley hole in the head. All have bosses on the top but only 2 are drilled. The front one is drilled and has a brass screw head plug in it. The 4th one is drilled and appears to have had a tube pressed onto it. It is open now. Looking at a head only the front stand has an oil hole under it. The only place the rockers can get oil is from the front stand. How it gets to the others I don't know or why #4 would need a way to get rid of oil. Oil would have to flow along the shaft to reach the others. Maybe I'm not seeing something. It's been a long time since I had to think about this.

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.....

Just re-read your post. So, you are saying that the oil filter is fed by the same supply line as the cylinder head?

Is that the production configuration ? The one that I am familiar with, such as on my CCKW, has the splitter block as mentioned in my previous post. The cylinder head is fed with a dedicated 3/16" line and the oil filter is fed with a separate 1/4" line.

It does not seem like you would get much oil flow through the rocker shaft if there was an alternate path through the filter...

Also, will try to post an image of the aforementioned tube. The diameter is 5/16".

Best Regards...

So the shaft assembly I was looking at seems to have the #4 and #6 stands reversed as the tube should come from #6 and mine from #4. No wonder I took it off.

Those rocker arms would have seen a lot of high pressure oiling with that configuration!

Probably an old racer trick.