Saturday, January 16, 2016

Miller Overhead On The Curtiss OX-5

Many people operating the OX-5 opt for the Miller overhead instead of the stock setup.  Developed by Leslie Miller, they made valve train components and others like high-compression pistons for the OX-5.  I've heard some say that the Miller overhead offers little advantage over stock as far as valve guide wear.  While that may be true, it does offer another advantage - the components are much beefier.  
Rocker arm replacement version

Miller offered 2 versions:  This one shown to the right was a rocker arm replacement along with the "positive action" mechanism. This supposedly offered more positive opening and closing of the intake valve especially at high RPM.  The rest of the components were stock.













Complete Miller conversion





This version was the complete conversion.  All of the components were made more heavy-duty and a grease zerk allowed greasing instead of oiling most of the moving parts.
















Rocker arm comparison

Stock vs. Miller comparison
Here you can see a comparison of the rocker arms of the different versions.  The top rocker arm is stock - it has a radiused tappet that rides on the exhaust valve.  

The middle arm is the Miller replacement for stock.  It adds the roller, but all components still have to be oiled. 

The last one is for the full Miller conversion.  It has a grease tube (soldered on top)  that allows the yoke for the exhaust push rod to be greased.  



This clearly shows the difference in the rest of the components.  The stock are on top, the Miller below - much more heavy-duty.













This being my first OX-5 I'll be operating, I can't make any comments at this time on whether the Miller is worth it, but it makes sense to me that the roller on the rocker arm should offer less friction and less side load on the valve.  I don't think you can argue either that the more heavy-duty components are also an advantage.  With my engine being so low time, my stock setup was fine, but when I came across a Miller setup, I thought it was worth the conversion.  The only components I will not be using is the "positive action" portion.  I didn't have those parts, and it's questionable that there is any advantage.

ASSEMBLY

Miller overhead components

Intake tappet on left after polishing out flat spot
These pictures pretty much tell the story, but I'll point out a few misc. items along the way.
Pre-assembly done

Safety for brass plugs

Prior to soldering

Soldered and safetied

Exhaust push rod tube yoke is safetied by soldering

Cleaning out threads on intake yoke











The intake tappet that mounts on the "H" tends to get a flat spot wore on it.  I found this easy to polish out.


































The shafts on the intake yoke are kept in with a brass plug.  Miller had an unusual way of safetying this - a wire that looks like a staple is soldered on the plug and wraps over the edges of the yoke.  The original had a semi-round profile.  I found that a cotter pin had the same shape so I cut and used those.













































The exhaust push rod yoke is safetied in a similar manner - soldered but no "staple".










The intake valve pull-down tube screws into the bottom of the yoke.  Running an 11/16-24 tap to clean out the threads makes assembly easier as well as adjusting the intake valves.






Spanner wrench for adjusting intake valves

Installation complete
 Another helpful item when adjusting the intake valves is a spanner wrench to turn the pull-down tube.  Here's a picture of one I made, it fits in existing holes in the tube.













So here they are installed.  If you look carefully, though, the rollers on the exhaust end are not installed.  New ones had to be made, and I wanted to make sure what diameter to make them. 

If you look at other sets of Miller components, you'll see different diameter rollers and other things liked shortened push rods.  I believe these are all attempts to change the geometry of the rocker arm so the proper valve setting (.010") at the exhaust valve can be obtained.  With the Miller rocker arm, you loose one degree of adjustability:  the stock arm can be adjusted at both the push rod and at the tappet.  The Miller one can only be adjusted at the push rod.

Why does the geometry change?  The most common thing is eliminating the cylinder hold-down spider.  Many people eliminate this to get rid of the 4 long cylinder studs - I don't think my engine came from the factory with this installed.  The thickness of the gasket under the water outlet/rocker arm support can change the geometry.

You can make up some of this difference with the adjustment of the exhaust push rod, but as you keep threading it up (to increase the clearance of the roller at the exhaust valve) it eventually hits the rocker arm.  I think this is why sometimes you'll find shortened push rods.  Since I had to make new rollers anyway - I could just make them to the proper diameter.

Old and new rollers

Complete set of new rollers








Here you can see the new rollers I machined.  These were made at .720" diameter which gives the adjustability I need.  I made them, as well as the shaft they ride on, from A2 tool steel.  I still have to heat-treat both the rollers and shafts as they are hard.





















Once I heat-treat the rollers, I can start finishing up things on the gear end of the engine.  So I'll keep plugging away - check back soon!




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