Sandcast Fox .29/'60's Fox .29 Stunt

[Bill Little]

Hi All,

I have a very nice sandcast Fox.29 that is a bit down on compression.  I have a newer ('60s) Fox .29 Stunt with excellent compression.  Will the sleeve/piston interchange between them?  I want to use the sandcast engine in a Guillows Mk I Barnstormer.

Thanks!
BIG Bear
RNMM/AMM

[Perry Rose]

I put a .35 piston and sleeve in a .29. It fit and ran well. Both Foxes.

[Bill Mohrbacher]

Hi All,

I have a very nice sandcast Fox.29 that is a bit down on compression.  I have a newer ('60s) Fox .29 Stunt with excellent compression.  Will the sleeve/piston interchange between them?  I want to use the sandcast engine in a Guillows Mk I Barnstormer.

Thanks!
BIG Bear
RNMM/AMM

yes

[frank williams]

Although I only have one sandcast Fox, I found it interesting that the volume of the bypass of the sandcast Fox was much smaller than that of the diecast case.  I haven't tried it yet, but I suspect that the sandcast Fox has less of a "burp" problem.

[frank mccune]

     Hi Bill:

     Yes, you can.  I think that the .29 and the .35 cylinders have the same o.d.! I have some new p&c from a 1958 C.S. that I was hoping to put into a .35 Stunt but no go.  The o.d. on the C.S. are greater than the Stunt.
      I have a 1949 and a 1951Fox .35 Stunt engines and they are great for Stunt.  No Burp!

                                                                                                    Frank

[Bill Mohrbacher]

I did a series of article on the Stunt 35s for Engine Collectors Journal and did some work on the bypass sizes:

The BYPASS ADDENDUM

The bypass on a Fox engine is a space that connects the bottom of the case to the intake port.  It is a cavity on the left side of the case.  When the round liner is slipped into the case, the bypass space is created.  For some time, stunt fliers, first on the ISKY Bulletin Board and now on the Stuka Stunt Works BB have talked about newer Fox Stunt 35s suffering from the Fox “Burp”.  This occurs when using a side mounted engine (as on a profile model) just after the model is climbing away from the inverted position at the bottom of an outside loop.  The engine hesitates for an instant (the Fox Burp), then picks up while in the meantime the pilot has had a heart attack.   I don’t remember this problem, but I was flying with early 1960’s or older Fox 35s and didn’t usually set them for a 2-4-2 break.  Anyway, the cure has been to insert a piece or something, usually wood, into the bypass opening to reduce its cross section.     
Since I had and will have engines apart, I thought it might be good to see what the Fox bypasses have looked like over the years and document their dimensions.  Fig. 19 shows the bypass cavities of the 1951, ‘52/’53, ‘54, and ’55 engines looking from the head down.  Fig. 20 shows the tops of the bypass cavities looking in through the exhaust stacks; the dark area at the top of the cavity is really the fillet, not just a shadow.  You can see the narrow rounded top and fillet on the 1951 Permold case; a very generous fillet on the 1954 engine and a wider cavity on the 1955 engine. 

I used an ID Pin Gage to measure bypass depth and bypass width (Fig. 21).  There is a taper in the side walls of the cavity, but by holding the gage in about the middle I got close to an “average” value.  I measured diametrically across the cylinder bore into the bottom of the bypass cavity and then subtracted the cylinder bore to arrive at the bypass depth.  Then I just multiplied the width by the depth to get the bypass cross section.  I know I should calculate it by using the section of a ring, not a rectangle, but since I will use the same calculation, the results ought to be comparative.

ENGINE     BYPASS    CYL+BYPASS   CYLINDER   BYPASS   BYPASS 
      WIDTH        DEPTH          DIAM.   DEPTH   X-SECTION
1951      .514”          1.010”         .886”   .124”      .064”2
1952      .583”          1.005”         .886”   .119”      .069”2
1954      .575”          1.050”          .886”   .164”      .094”2
1955      .615”          1.025”         .886”   .139”      .086”2

OK, there are the results all documented.  There is a 25% difference between the 1955 and 1952 bypasses, but we’ll wait and see more measurements and try and find out from the fliers where the trouble occurs.  It is great to be retired and get paid by someone like the PE to do this stuff!


The BYPASS ADDENDUM (after I finished the stunt series)

Now I’ve measured bypasses from the first Permold engines through the current model, 1948-2011.  Part II of this series explains why I did this and the “Fox Burp”.  Fig. 21 shows an accepted method for stuffing the bypass and eliminating the burp (thanks to Dennis Vander Kuur). 

Looking at all the data and realizing I measured basically as cast surfaces and engines that may have come from different molds, I would conclude:

-The Permold engines, 1948 through 1951 had essentially the same bypass dimensions
-The first die cast engines, 1952-1954 had wider bypasses and the 1954 was definitely        deeper
-The 1955-2011 bypasses were wider than 52-54, but the 55-2011had about the same    bypass width. It looks like there may have been 2 different dies for the bypass depth,    one set producing .150” depths and another .130”  depths.

Now someone can run some tests and see if any of these engines really are more “burp prone” than others!

ENGINE     BYPASS    CYL+BYPASS   CYLINDER   BYPASS   BYPASS 
      WIDTH        DEPTH          DIAM.   DEPTH   X-SECTION
1948      .520”          1.014”         .875”   .139”      .072”2
1951      .514”          1.010”         .875”   .135”      .069”2
1952      .583”          1.005”         .875”   .130”      .076”2
1954      .575”          1.050”          .875”   .175”      .101”2
1955      .615”          1.025”         .875”   .150”      .092”2
1968      .620”          1.025”         .875”   .150”      .093”2
1970      .630”          1.025”         .875”   .150”      .094”2
1972      .630”          1.025”         .875”   .150”      .094”2
1973      .630”          1.025”         .875”   .150”      .094”2
1975      .630”          1.005”         .875”   .130”      .082”2
1980      .620”          1.005”         .875”   .130”      .081”2
1998      .625”          1.020”         .875”   .145”      .091”2
2011      .625”          1.010”         .875”   .135”      .084”2

[frank mccune]

     Hi Bill:

     Great work!

     I am now very confused as to why the burp happens! Lol

     One way that I have solved the burp in the past is to run my Fox .35 Stunt in a rich 2 cycle with a 9x6 prop.  It seems that with a 10x6 in a 4-2-4 cycle, I always had a severe burp problem.

                                                                              Be well,

                                                                              Frank McCune

[frank williams]

Hi Bill
Great Data ...... So '51 to '52 was the transition of four bolt to six bolt heads ...... What about the transition from 2 bolt back plate to three bolt?
Frank