Seems to me the 29 looked like the 36, bigger than the 35 over all. The 35 has a three bolt backplate, I think the 29 and 36 had four bolt backplates. I had a 29, but it was 30 some years ago, so my memory may be foggy.

What Brett said.
Many years ago on Stuka Stunt, there was a discussion on different versions of 29's. There were straight plugs and several slanted plug versions. Maybe you can find that topic in their archives

I had a 29 that made just as much power as a stock 35. It had a slanted glow plug.

      Hi:

      i was asking about the Fox engines circa 1954. 

      I have no intentions in making a .29 into a .35 but I was just curious as to what the differences were.

      Thanks for the replies.

                                                                                               Frank

I had a Fox 29 that looked exactly like the 35.  It did have the same power, i.e. the plane could fly as fast, but the 29 wouldn't turn the same diameter prop.  I always had to go with a 9 in. prop.  it would just overheat on a 10 in. prop.  When I finally learned how to measure the port timings it looked like the 29 had more exhaust timing and a wider timing on the crankshaft port- timed for higher rpm.  Which makes sense because it was aimed at contest free flight where they needed high rpm and a low pitch prop.

Frank,

Duke Fox was pretty ingenious about using essentialy the same major pieces for various displacements. His small-case .40BB schneurles show family resemblance to some of the old square intake Combat Specials, for example.

The 4-bolt backplate baffle-piston .19 and .25 were nearly identical, except the .19 had a smaller bore. Not sure, but they may have had the same stroke. The stroke/bore ratio in the .19 may have been "square," i.e., equal bore and stroke. That made the .25 "over-square" - bore greater than stroke. The only difference between parts may have been a thinner cylinder wall and a matching piston for the .25.

The briefly offered .19BB schneurle looked a lot like the current .25BB schneurle, which has a longer stroke. The engine layout has a lower crankcase, and a finned spacer between it and the head. The .19's fin stack is shorter than the .25's. As I recall, the .25BB is "square" at 0.680" bore and stroke. The .19's shorter stroke would have made it "over-square."

Some Fox variants on a basic setup have a belly-band added around the lower case, apparently to fit a longer stroke...

Port timing is another factor. A shorter stroke may allow higher-RPM timing... Although the .25BB schneurle is also able to rev pretty high.

The slant-plug, baffle piston .29 and .36 from the '70's was a totally different design from the Stunt 35 and the 4-bolt backdoor baffle piston .19 and .25. Heavier. Mounting dimensions were quite different. Noticeably more power, and a reputation as shakers until well run-in.

The Stunt 35 is an example of how Fox continued using the same major pieces, with minor mods as needs appeared. Fifteen or twenty years back, I "overhauled" an early 4-bolt head, 3-bolt backplate Stunt 35 for an acquaintance in Connecticut. "Modern" - or at least currently produced - moving parts all fit. The shaft, piston and rod were different only in crankpin and piston pin diameters, and the prop driver. Replacement shaft with prop driver spline needed its matching drive washer: the original was a tapered pair - always fun to get the prop tight where you could flip it for starting, and where it would stop flat so you didn't bust one every landing.

Had to use the 4-bolt head - they just aren't available any more.

The 4 bolt 29's used for speed and "B Team Racing" were Fox 29R's. Much hotter than the plain 29's.

W

I don't recall a line rule that ever allowed smaller lines for .29's than .35's. It was only recently that .012's became allowed for .25's, after all.

I had a Fox .29 Stunt when I was in HS, that I bought in a Pawnshop in Oak Harbor (always check Pawnshops near military installations!) for $6, near new. I don't recall the reduced diameter area on the liner as shown in the picture above. I much preferred my Fox .201, which was really just a .19 with .201 stamped on the side.  Steve

I have one of the 29s and haven't run it yet, but also had the 36 version of this engine and used it quite a bit. They're decent sport engines that require a proper breakin, then can be used on a sport model. Note that they are not "stunt" engines in that they don't give the 4-2-4 run of a Fox 35, for example. They also vibrate quite a bit. Good luck finding a muffler for it, btw. Fox made open and closed front silencers for this series, but they haven't been available in many years. If you do find one, hopefully it'll be the closed front muffler, as my open front unit didn't muffle much at all, even after I packed it with copper wool.

I also bought one if the 19s many years ago and still have it. Again, it's an iron piston engine that requires breakin. I flew mine on a Goldberg Junior Satan and it worked well. Eventually added a Veco carb via a homemade adapter and used it on a Sturdy Birdy I built for my son 20 years ago. It worked well on that, too.

Bob in NEPA

I hogged out the venturi on my 36 with a Dremel tool until it was square, and about as open as it could get without destroying it. Flew it on a VooDoo with a Tornado 8-8 prop, lots of nitro (Rogo 58%, I think) and no streamer and my buddy clocked it at 118mph. To this day, I suspect he timed it for 6 laps instead of 7. I dunno. I eventually got a new crankcase for it and transferred the parts. I ran it many times after that and gave it away on a Cosmic Wind a couple of years ago.

Quote from: David_Ruff on February 08, 2014, 08:39:22 AM
I will ask my dumb question here about the 4-2 break. What causes the break?  Is it the leaning of fuel mixture?  If so, how does that happen?  In a perfect world the fuel mixture should not change with a fixed system?

Hi David ,in a nutshell, it is a combo of things, load and mixture, the fuel head does change, as well as load when the airplane is in maneuvers, to see for yourself, run an engine on the bench, set it just in between a 4 and 2 cycle, raise only the tank up higher, then lower the tank only lower than the engine, you will see the engine go richer and leaner. you just changed the mixture.

Randy