Of all the parts on the SOHC engines, I think the rocker arms have given me the most trouble. Starting with the Dove and RAS rockers that blew up on my engine during Drag Week 2008, through the first set of T&D rockers where I fought through teething problems with the rocker arm adjusters and the rocker to shaft clearance, I can't think of any other component on the cammer that has caused as much trouble for me as the rockers. After the initial problems with the T&D rockers were resolved, they became a good reliable component, and I've run them for over 2000 street miles and lots of dyno and drag strip use without any failures. But as I started making more and more horsepower with the SOHC, other limitations of the stock rocker arm system arose.
Currently in my two SOHC engines, I only have about .015" clearance between the rocker body and the cam lobe as it spins by the big end roller. Lift at the cam is a pretty healthy .560", but because the SOHC rocker ratio is only about 1.3:1, lift at the valve is only .735" or so with this setup. Before my big SOHC started acting up last summer, it was making 960 horsepower at 6600 RPM, which is a LOT of power to get from a cam with only .735" lift. As far as I'm concerned the engine was definitely cam limited. I really wanted to use the advantages of the SOHC valvetrain to try to get lifts in the .850" range, to improve power production from the engine, but a bigger cam lobe wouldn't fit in the engine with the stock valvetrain arrangement; the lobe would hit the rocker.
A higher ratio rocker arm seemed to be the obvious solution. If I could increase the rocker ratio from 1.3:1 to 1.5:1, I could increase the valve lift all across the range by about 15%, which would be a big help in terms of power production. But there wasn't really any way to increase the rocker ratio with the fixed positions of the rocker shaft, valve, and cam. I thought about moving the location of the roller on the rocker arm, to move it closer to the rocker shaft, but this would change the point where the roller contacted the lobe, and so change the lobe centerline angle required for the cam by a fairly large amount. This meant custom cams ground from blanks with full round lobes to start with, which are expensive (around $1000 each if I recall correctly).
Outside of changing the basic design of the head, the only way that I could come up with to change the ratio was to move the rocker shaft centerline. The shafts are around .840" in diameter, and I figured that I could get away with shafts as small as .625" in diameter if necessary. The concept I came up with for this was to use some special bushings in the rocker shaft bores in the heads, which were .840" OD and .625" ID. But the ID hole would be offset towards the cam side of the rocker shaft bores. This would effectively move the centerline of the rocker shaft about .100" towards the cam. Then, I could design and build new rockers with a 1.5:1 ratio, to fit the .625" diameter rocker shaft, and still leave the cam roller wheel in the same location with respect to the camshaft.
After doing some work with my CAD program and convincing myself this was a feasible approach, I started working on the design last year. After a few weeks I came up with the rocker arm design shown below:
I designed the rockers to use the roller wheels, needle bearings, and pins from a spare set of T&D rockers that I have, because they have proven themselves to be good parts. I went with a roller tip also, and got some examples from the standard FE roller rockers that Doug at Precision Oil Pumps sells. I wanted to get away from the adjuster and just go back to the original SOHC method of using lash caps of various thicknesses to set the lash. This approach limits the cams that I can use for this high ratio rocker setup, because there's only so far you can go with lash cap thicknesses, and I have quite a few sets of SOHC cams that are ground on smaller than normal base circles and would require really thick lash caps to work. But I decided that for the engine I was going to test these rockers on, I'd stick with the biggest SOHC cams that Comp offers, so I designed the rockers to work with that cam, and the valve height arrangement in my 585" SOHC.
Last winter I got started on the machine work for one example rocker arm. I wanted to make the first one out of aluminum to make the machining easier, and to test fit the rocker into a test cylinder head, in case I had to make any adjustments to the design before I started one of the real ones out of steel. The complex shapes on the rocker made for some fairly complicated CNC programs. There were some areas where an arc in three dimensions would have been very useful, but my CNC machine can only do an X-Y arc or an X-Z arc. After programming the rocker for a while, it became clear that some fixtures would be required to hold the semi-machined rocker to get the all the machining operations correct. I decided to start without these fixtures, and machine one half of the rocker as much as possible. I got this far last winter:
Some of the machine work so far is just roughed in, to get the rocker so that it is ready to go on the fixture. At this point I had to stop on this project, because I was already running out of time on the Drag Week car last winter, and needed to focus on that. Recently I picked up this project again, in anticipation of finding the problems with my 585" SOHC and getting them resolved, so that I could try out this set of rockers. So, over the last few weeks I've designed the fixtures I need to hold the rocker in place for further machining, and got them machined over the last couple of weeks. The first piece, shown below, holds the rocker arm in three different positions:
The second photo shows a stub shaft that slides into the fixture, that the rocker arm will then slide onto. I can put a bolt through the whole assembly to keep it tight. The two posts on the left side of the photo are two fixture points for the valve roller end of the rocker. I can bolt through the valve roller hole into either of these posts to angle the rocker in two different spots, for machining at certain angles. Same thing for the two holes in the post on the right side of the fixture. All machining can be done with the rocker in one of three positions, depending on how it is bolted in place. Here's a photo of the rocker so far, positioned on this fixture:
The angled rib along the top of the rocker is another machining challenge that requires a special fixture. I would like to machine this flat with a ball end mill, using an arc in the X-Z plane. In order to do that, I had to be able to fixture the rocker arm so that the rib was along the X axis of the mill table. In order to do this I built another plate as a fixture, and put some holes in it for pins to align the first fixture. Here's a photo of the plate with two of the pins installed:
And, with the first fixture installed on the pins:
Moving the pins to different holes allows the first fixture to be rotated on the plate to a certain angle, which will line up the rib on the rocker arm with the X-axis of the mill table:
I'll get back to working on programming the machining operations for finishing this rocker over the next couple of weeks. I also have to build the rocker shafts and the offset bushings and get these installed in my test head. My first pass on the rocker shafts, also last winter, ended in failure when they warped fairly dramatically during the heat treating operation. I figured out what the problem was, and now have a solution, so I'll be starting on a new set of rocker shafts in the next couple of weeks also. This will be a fairly long running project, and I plan to get the first rocker arm done, mount it on the first shaft in the test head, and then send the whole setup out to Bill Conley to see how it behaves on his spintron machine. If it looks OK, I'll machine the rest of the rocker arms and make a whole set. If not, its back to the T&Ds! I'll update this project when I have some more info, hopefully in 2-3 weeks.
