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February 16, 2014

Rocker Arms

The rocker gear was great fun to work on.  It's very machine-like, with all those levers and joints, and it's one place where small changes can make a significant difference in the performance of the engine.  The rockers' job is to transfer information fr
om the cam lobes to the valves.

In taking everything apart, there was a lot of evidence of heavy wear.  the rocker shaft was deeply scored under some of the rockers, and many of the rockers showed heavy wear, too.

One interesting thing was that the rocker shaft showed significant wear on its top side where it is held by some of the pillars.  The only way I can explain this is that the shaft was twisting quite a bit.

Well, the shaft was definitely a gonner, so I got a new one.

But then I started thinking about the rockers themselves, since they certainly couldn't be used in their current condition.  I'd much rather see bronze bushed joints in this application, and there are sources for bushed rockers.  There is also at least one outfit that will bush existing rockers, and this got me thinking whether this was something I could do.  I think the bushed rockers run around $16-$18 each, so it's not really a huge-ticket item, but the challenge really appealed to me.  

Bushing a hole is not rocket science--you drill the hole out to rough size, then ream it to a few thousandths smaller than the bush OD, then press the bush in. This situation was going to be a little more complicated, though.  For one thing, I was a little worried about removing much material from the bottom of the rocker, where it is pretty thin--only about 0.150".  I thought boring the rocker for a 0.062" wall bush was taking too much.  I was much more comfortable with a bush with a 0.031" wall thickness, but couldn't find the right size commercially.  I finally decided to open up the rocker bore by 0.062" (remove 0.031" all around), press in a 0.062 wall bush, then bore and ream the bush.  

The other thing that makes this job a little more interesting is work holding.  I had to figure a way to securely hold an odd-shaped piece so it can't shift or rotate.  I have a small mill, but the process I ended up with could also be done on a decent drill press.  

I made a little fixture that I held in the mill vice.  The fixture had two clamps on it, one for each side or the rocker.  Luckily, these rockers have one flat side.  There was also a stud that one arm of the rocker would rest against to keep it from turning.

The rocker was drilled and reamed oversize, the bush was pressed in, and the bush was drilled and reamed, all without moving or removing the rocker from the fixture.  This keeps everything concentric.

Then the bushes were chamfered.  I had to move the rocker for this to do both sides.

The results looked pretty good:

Then had to drill the two oil holes in each piece.  The holes are different sizes.

I really considered this a success.  After I got everything set up, the bushing only took a couple of hours.

My back-patting was cut short when I realized that these rockers still looked like used items as evidenced by the deeply worn patches on the shoes (the parts that contact the valve stems).

This took considerably more thought than the bushes.  The rocker shoes appear to be circular arcs with about a 5/16" radius.  After a lot of doodling,  I made up this little jig that holds the rocker bore vertical, and allows it to rotate around the center of the shoe arc.  The ball screw controls how much material is removed.  The whole jig is clamped to the table of my little 1 inch belt sander so that as the fixture is rotated, the shoe is resurfaced.  I only took off enough material so that the wear marks just disappeared.  From the point where the shoe was first barely touching the belt to where the deepest of the worn areas was gone only took about 1/4 turn of the 24-pitch adjustment screw, indicating that at most, about 0.015" was removed.

I thought the finish looked a little rough (120 grit on the belt), so I honed the surfaces with a fine stone.

These rockers were quite a lot of work, and the drills and reamers added up to maybe half of what new bushed rockers would have cost.  But, on the other hand, for the $100 or so, I got the rockers AND the tools, not to mention some pleasant shop time, some good experience, and a story to tell.

Next up is trying to get a handle on the valve geometry.

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