Inliners International Forums General Discussion Engines GMC rocker arm ratio

Does anyone know what ratio the stock GMC 270/302 rocker arms are?

I think it's a "theoretical" 1.5 ratio

CNC-Dude, You bring up a good point. Usually the theoretical ratio is more than what the actual rocker arm delivers. Aftermarket rocker arm suppliers seem to have figured this out and often give you "a little extra ratio" to bring the lift-at-the-valve more into line what the theoretical ratio should have delivered.

Some stock production Ford "1.5" rockers I dyno tested actually delivered lift-at-the-valve equivalent to 1.39:1. When I switched to a Crane aluminum roller rocker lift-at-the-valve was actually 1.50 times the lobe lift. Power increased. Imagine that!!!

RR. When I first checked lift at the valve and measured the actual cam lift I cam out with a 1.41 ratio using a cam that had .025" gap cold. After working with valve lash caps and machining the aluminum base rocker stands to make sure at 1/2 cam lift the rocker arm was level I actually was able to get more.

Most of the time if you are making a "big" cam off an original, hopefully steel, the base circle is smaller and this throws off everything angluar. This gives you the lift with the newly ground cam but you still need to check the angle at zero lift and at full lift. All this is do able during assembly just takes some time.

The reason I made my own roller rockers was because I was wearing out valve guides every 2 years. The roller tip ends eliminated this and I have never replaced the guides since. I calulated 1.70 on the new rocker but came out with 1.63. I moved the rocker arm shaft .100" in my calculation. The Torrington bearings on the shaft was a pain but all worked out, Finally broke a shaft after 25 years and had 2 new ones made. Good Luck...JD

Thanks guys. I had heard from several sources that factory claimed ratios were way off. This great info confirms it.

This makes me wonder if the ratio on the Barker "Hi-Lift" rockers that were available for some engines in the '50's was actually just what the factory claimed their ratio was?

I have 2 new sets of Thomas magnesium high lift rockers for a GMC. I did put a set on #1 on a shaft once to check them out. Just putting them beside the stock ones you could see a difference on the short side...Got it yet..."short side" probably work with the stock skinny push rods but with larger diameter and a high lift cam you would need to hog out the push rod hole and I didn't want to do that. The do have a stock appearing tip and also do not come with adjusters so the stock need to fit into them...JD

Thanks jimmy six #35. This is really good stuff. I don't have any GMC high lift rockers at the moment but I do have a 235 Chevy set I can check. In one of the early McGurk books on modifying GMC & Chevy sixes he claimed that the intake rockers were interchangeable between the two series of engines. Of course the Chevy is much heavier than the Jimmy so I can see no advantage is using Chevy rockers in a 270/302.