Valve timing 4 stroke glow

[Swede]

All domestic (USA) shipping. Grrr... you guys get ALL the good stuff!

Seller FREDSUSA has a copy for $7 + $13 shipping, although it'll take a few weeks. Believe me, anyone who loves radial engines will also love this book. It was printed in 1939, at the outbreak of WW2, and discusses the maintenance of basically all of the major Western-made radial engines, like the Wright series, the Wasp, Wasp Jr, Continental, Kinner, etc.

Lots of superb information. I'm surprised there are so many copies available, so cheap.

Good luck with your project. FWIW, my deHavilland Cirrus uses tool steel cams and tappets, very basic and plain, and there is very little wear. My larger engine has a 4340 steel ring cam hardened to RC46, and the tappets are very hard A2 steel, with a hemispherical surface, no rollers. The cam surface has nothing more than a shiny band, NO wear or defacement otherwise, and the engine has been run for well over 100 hours on a stand.

 

[petertha]

FWIW, my deHavilland Cirrus uses tool steel cams and tappets, very basic and plain, and there is very little wear. My larger engine has a 4340 steel ring cam hardened to RC46, and the tappets are very hard A2 steel, with a hemispherical surface, no rollers.

I've always wondered about this. Your larger engine, I assume the Hodgson? From what I am able figure out of that one, the tappet end is hemi-shaped as you say & rides over a similar lift profile of 0.078"? So after extended running one would expect either 1) a flat facet wearing on the end of the tappet hemi profile 2) a groove wearing on the cam profile... based on their relative hardness. You say no wear, thats awesome.

So on simpler cam profiles with (base circle > flat > nose radius) profiles, I believe like the Cirrus, flat bottom tappets would seem to have a few disadvantages: the potential for the edge digging into the cam as its rising, almost like a lathe tool. Potential for timing jumps as its transitions from flat to edge vs rounded tappets that kind of ramp a bit smoother into the cam profile. It obviously 'works' as you say. Maybe rounding those little buggers is not as easy as simply drawing them & thts more the issue? ;D How did you do yours BTW?

Also including the missing pic of the housing method I was considering to retain my curve bottom tappets running in aliggnment to the cam, loosely modeled after the real engine (sans rollers). It looks like a lot of extra fussy work that could hopefully be simplified with hemi-end tappets?

 

[Swede]

So on simpler cam profiles with (base circle > flat > nose radius) profiles, I believe like the Cirrus, flat bottom tappets would seem to have a few disadvantages: the potential for the edge digging into the cam as its rising, almost like a lathe tool. Potential for timing jumps as its transitions from flat to edge vs rounded tappets that kind of ramp a bit smoother into the cam profile. It obviously 'works' as you say. Maybe rounding those little buggers is not as easy as simply drawing them & thts more the issue? How did you do yours BTW?

When I say "no wear" obviously the correct answer is actually very slight "acceptable" wear. Less wear than I anticipated.

On the Cirrus, the diameter of the tappet bases is quite large relative to the cam. Here are the pictures. Note the cam shows some shine on portions of the nose, but as this cam was cut with an end mill and profiled by hand, there were bound to be high sections which wear first, and as it wears, surface contact improves. I think with any model IC engine, there's going to be some run-in time where the valve clearance will need to be adjusted more than once, to take up this initial wear.

The tappet bases don't even show much of a shine.

The Hodgson - the tappets had a full hemisphere made of A2 steel. Each of these now has a polished, flattened profile on the tip that measures perhaps 0.020" across. A better choice than a hemisphere profile might be having the tappets chamfered first, say 45 degrees, and then simply smoothing the remaining small diameter at the tip somewhat, so a larger surface rides the cam rather than a theoretical point.

Also, I think it's important to make the cam harder than the tappets, causing more wear on the tappets vs. the cam. Tappets are a lot easier to replace in the future than cams.

In retrospect, I wish I had engineered roller tappets using tiny instrument ball races, with the tappets keyed to their bushings. 1/8" square stock would do the trick, with each set of tappet bushings being one piece and pressed into the crankcase.

I freely admit to having only a basic knowledge of cams, cam profiles, and the performance changes that can result. I was far more into materials, how they wear, and getting basic, reliable performance was the goal from day one. After digesting a half-dozen cam articles on how to wring the very last joule of energy out of an IC engine, I was a bit discouraged, until I followed the deHavilland Cirrus 1/6th scale build in Strictly IC; saw that he milled the cams, polished by hand, and I understood that if all I wanted was a runner, not a race engine, it's not all that critical.

 

[gbritnell]

If the cam and tappets are both hardened and the tappets have a hemispherical shape then the wear is going to be much, much less than if one or the other is unhardened. I have always made my flat tappet cams from unhardened drill rod (W-1) and hardened the tappets/lifters. On some engines the oiling is by splash while others are pressure fed. I have checked the cams after many hours of running and the cams are polished on the lift part of the lobe but not on the flank diameter (valve clearance). On my V-twin and now on the radial I have opted for roller lifters and use a similar setup for both engines. The lifters and the rollers are unhardened drill rod and the cam is likewise.
If you design a lifter with a hemispherical shape then the valve timing specs are going to change relative to a flat type lifter. (tangent points). I wouldn't recommend a 45 degree chamfer as that would introduce a sharp edge (albeit small) where the chamfer meets the flat and here again it would change the valve timing because of the tangent points.
Attached are several pictures of the radial I'm building with the lifters, crankcase and cam ring. The lifters have a small flat milled on them and the lifter boss is extended outward to accept a small set screw that has the tip ground flat. Upon final assembly the set screws will be adjusted for minimal clearance and Loctited in place. I will use a low setting Loctite for future removal if necessary to work on the lifter.
gbritnell

 

[Swede]

Excellent information!

I wouldn't recommend a 45 degree chamfer as that would introduce a sharp edge (albeit small) where the chamfer meets the flat and here again it would change the valve timing because of the tangent points.

In my mind, I'm picturing a tappet chamfer of 45 degrees, leaving a round tip diameter of say 0.032" to 0.050"... now, I'm going to mount the tappet in a drill press or lathe, and with a cratex abrasive in a dremel, apply the cratex to the revolving tappet tip. This will smooth and round off the circumference of that end circle.

Basically, a full hemisphere is a point that rides on the cam ring. On the base circle portions, there is little or no wear, no real pressure... they are essentially floating. As the cam lobe approaches, there'll be initial contact slightly off tappet axis, switching to the other side after cam peak. As the tappets rotate in their bushings, the overall wear will create the small, shiny circular spot that I've seen on my own hemispherical tappets.

But in the end, a hardened hemisphere works fine. It's just a thought.

 

[petertha]

... now on the radial I have opted for roller lifters ...
...lifters have a small flat milled on them and the lifter boss is extended outward to accept a small set screw that has the tip ground flat.

Very nice George. The FS Kinner roller lifters are actually very similar to what you have made there. One key difference is they are orientated as a sliding pair, clustered right adjacent to each other. Your very neat set screw idea riding on the flats works perfect on the cam ring style engine because exhaust & inlet each have their own holes in the crank case & their own respective set screw sets. The FS Kinner 'houses' the roller lifter pair assembly in yet another part that retains them by their outer flats. And they slide on one another by inner flats. My cad picture 211544 shows a crude equivalent. I'll try & scan it from the manual just for interest, but I'm sure you already know. Replicating that is where its starting to look 'newbie-challenging'. But never say never, right? I just realized I've come full circle without a single metal shaving to show for it!

Can you provide a few dimensions for reference (lifter body dia, roller ball dia, axle dia/material). Another thing Im coming to realize is model cam assemblies, at least mine, is quite big by comparison. If you scaled the FS down it would be watch jewelry so that in iteslef requires some compromises.

 

[petertha]

On the Cirrus, the diameter of the tappet bases is quite large relative to the cam. Here are the pictures.

Thanks for the pics. For my engine, that typical inline engine cam spacing is part of the problem. The wider the cams + longer the intermediate segment + blank shaft allowance for 3 widths of timing gears.. means the the back half of the crankcase starts to get quite long & look a bit weird. Its all about compromises so Im doing what I can.

But maybe I missed something important. Is there some sort of cross pin in the tappet that keeps them retained for rotating freely in their hole?

 

[petertha]

Kinner roller lifter assembly.

 

[gbritnell]

Hi Peter,
Here's a PDF of the drawing sheet with my roller lifters.
George

View attachment ENGINE RADIAL SHT 4e.pdf

 

[Swede]

Thanks for the pics. For my engine, that typical inline engine cam spacing is part of the problem. The wider the cams + longer the intermediate segment + blank shaft allowance for 3 widths of timing gears.. means the the back half of the crankcase starts to get quite long & look a bit weird. Its all about compromises so Im doing what I can.

But maybe I missed something important. Is there some sort of cross pin in the tappet that keeps them retained for rotating freely in their hole?

On my Cirrus, the tappets are free to rotate. As to how much they actually do, I have never checked.

The pin in the cam is a cheat... the cam is darned small, and rather than mill a keyway, which would potentially enter (and mar) the rear journal area, I simply drilled and pressed in a hardened pin. The cam gear had its keyway slot created by the common technique of using the lathe, and grinding a tiny bit with a hook - the bit is run in, fed, and retracted, like a shaper. Commercial keyway broaches aren't common in that small size.

I've had zero problems using this pin rather than a true key. If this is done, I think it helps quite a bit if the gear slot is shallow, so that when the gear is fitted, you get a decent pressing action of the top of the pin with the bottom of the slot. Tight is good.

Another cheat to key a gear to a shaft is to score the shaft lightly in two opposite sides, fit the gear, then drill undersized for a hardened pin. The drill follows the scoring, and you end up with two holes, each occupying 1/2 of the gear and the shaft. This is light duty stuff, but works fine for tiny setups. Any forces strong enough to strip these round keys is going to be also strong enough to cause other damage like stripped gear teeth.

 

[petertha]

...the tappets are free to rotate....
...the pin in the cam is a cheat...

I'm glad I pursued this because I was obviously completely discombobulated. I thought that picture was showing the tappet resting on the end of the crankshaft just to prop it up for photography because its tiny. the color of that end segment looked different than the adjacent cam area. You mentioned the tappets were free to rotate which made me wonder why then the cross pin. Nooooww I get it!

Well that conversational detour saved me yet another question as to good ways to keep timing gears retained! Double bonus. Thanks for that.