How do you guys fixture the engine in the milling machine to "true up" the mounting lugs on a Fox 35 Stunt?  Is the engine disassembled to do this?  Is a special fixture made to hold the crankcase?

Pictures of your setup and process would be very much appreciated.

Thanks in advance!

   Yeah, I think they all have a shallow draft angle to help get them out of the dies. When I had access to a mill, I used to just clamp them lightly but securely between the front bearing and the back plate surface with a soft block or aluminum plate. Then when you do the cutting, take it just a few thou at a time going from front to back, it doesn't take much. If you have a good set of files, and a steady hand and good eye, you can file it with out much effort. Just have a good mounting surface the right size to check how the engine sits on it. Most are not that far off, but the occasional one has some sort of error in it and when you tighten it down on a mount it flexes the case. If you throw in a mount that is not level and parallel, you are in bigger trouble. People were not aware of this back in the day. That is one reason why so many f the old engines you find have so little time on them and need breaking in. Not everyone was successful at this hobby back then and failed to stick with it or seek help, and no internet back then. If I ever get my garage squared away, I want to add a small table top mill just for proposes like these and for repairing welded up welded mounts.
  Type at you later,
   Dan McEntee

Lotta work to just to put a washer under the front holes.

The problem becomes what do you use for datums? If I was doing it I would build a fixture that first used a vertical plate with a hole that I would insert the end of the shaft through and tighten down on the thrust washer. That establishes x and y along the thrust line.

Next I would make some sort of block with a Destaco clamp to align z axis.  Use a mag mount dial indicator on the quill and check the flatness and coplanarity.

I think I would find that since Duke machined the pads all in the same setup they would be flat and parallel anyway, and all this would do is make sure they are aligned with the thrust line.

Once everything was good I'd take a skin cut, but you have to be careful since you want the mounting pads even with the shaft centerline.


Now that I'm done, it sounds like a great opportunity to screw up a perfectly good engine. Unless you're having a lot of premature failures of the crank bushing you won't see any performance change.

(Not to mention that unless what you're mounting to is flat and true you're only pissin' in the wind anyway.)

Chuck

    But they were not machined at the factory, at least not always. They were cast with a shallow draft angle to help get them released from the dies.  If the engine is dis-assembled, you can used the head surface to locate it level and clamp in a machine vise from front to back like I mentioned before.   Clamp just tight enough to hold steady and not distort things. Sweeping the mounts back front to back with a dial indicator can verify that they are level. Then the kiss cut you mentioned is all that is needed to make them flat. I've done them this way several times along with hand filing them when necessary May not be for everyone but has worked well for me.
    Type at you later,
    Dan McEntee

Thanks!  I'll look for it....

I've done this many times. You bolt a block of metal to the top of the engine then clamp that in a vise.  Get your best compromise so you're not taking too much metal from one spot. The lugs are off in two planes twisted and bent so to speak. Use a milling bit with a 1/16th radius tip. The thrust line won't be off any amount that matters. There's a thread with pics in the speed section I did a while back on a Fox speed engine.


Motorman

Yup...I noticed.

Of course anything works, but I wouldn’tuse the cylinder bore as a reference. You know, in quality engines the shaft/bore perpendicularity is never 90°. In engines like Fox, it can be whatever for other kinds of (bad) quality reasons.
So the most logical way to clock it straight is to use shaft bore as I wrote earlier.
Certainly, if it’s easier, you can use the cylinder axis for mounting the workpiece but just don’t trust it’ll be straight like that. L

One of the reasons the Fox is probably so variable is because they did not machine the mounting lugs flat.
Without that as a reference point, all subsequent setups are suspect.

No doubt done to save cost of that single operation, but would have easily paid that back with better quality.

It’s allways a good practise to do things that make sense. In engine work it means that materials are good & correct, angles are correct, round things are round and flat things are flat, for example.
If that’s not the case you may have it working, but as Henry Nelson put it, you’ll never know if you made the right thing wrong or the wrong thing right😂
L

      So, then, TIG-weld the entire case into a solid block, then machine away everything that doesn't look like an Aero-Tiger 36?  And make a new crank, a new piston/liner, a new head, and re-use only the conrod, wristpin, and screws. Oops, don't use the screws either.

   At some point, you have to wave the white flag and go plunk down $50 for a used 25LA. I am, however, well aware that this doesn't fit with "our" general approach to life.

     Brett

   Yeah, but this is an engine for a Fox .35 Speed Event model, so he has to use a Fox.35 stunt engine.  But whether it's for this event, or just a sport model, it's this draft angle that can get some people into some run problems. One of the reasons we picked this engine for this entry level speed event, is that there wasn't a whole lot you could do with the Fox .35 as far as machine work.  Duke designed the thing to as light as possible and there is just no metal there to even drill out the venturi very much to be of a whole lot of use and still hold a spray bar! It's amazing how much the case can flex. Hell! If you aren't careful tightening the head bolts you can put the engine in a bind!  The event was intended for guys to bring any model that had a Fox .35 in it, even a purpose built airplane, and try their hand at speed. I have observed on many, many models that I have helped people with that the motor mounts are not even and flat. Even if the Fox mounts were machined at the factory as soon as you bolt the engine down you put the piston in a bind, and it can get worse with the mounts as they come and I'm sure you are aware of that. It is easy enough to file them flat though if you have a good hard mounting surface that you can check them against as you work, and it's worth the 20 to 30 minutes it may take to do it because it will help make for more consistent runs.   When I mount a Fox in any model I check to see how it sits. If it doesn't look and feel right I fix the mounts in the model first if needed and then file the engine if I haven't already.
    type at you later,
    Dan McEntee

So Dan, if I understand what you're saying - you purposely chose a stock, low tech, low performance motor to give everyone a level playing field and now you're espousing that people need to take their engines to a machine shop and modify them to be more competitive?

Seems to me a much better strategy given that Fox .35s are dirt cheap would be to buy a dozen of them and pick the one that runs best. I'm willing to bet that will give a better end result than milling the mounts.

YMMV, but I'd pick a good one and then spend my effort on the propeller, where I can likely make significant gains.

I can see it now, FoxBerg racing with a carbon fiber Shoestring and a balanced and blueprinted engine, flying monoline.

Chuck

     Uh, no, you do NOT understand what I'm saying. Doesn't sound like you read my post at all!  I don't know how you could surmise what you posted out of what I wrote?

   Type at you later,
     Dan McEntee

Robert,
GREAT solution without milling!

Here's my setup. 3/8" thick by 1 3/4" wide aluminum plate with 6 qty .116" dia holes on a 1.095" dia bolt circle.  Clamped to my mini mill table.  Solid, no chatter setup.  I really didn't like clamping in a vise such a thin walled casting.  Took me 20 minutes to make the fixture, 5 minutes to draw the pattern in CAD.

Seems like everyone is looking for complicated solutions to an easy problem.

I don't run Foxes.  If I did, I would simply take a couple swipes with a file on the maple motor mounts.

My building sequence starts by bolting the engine to maple mounts.  Then adding firewall, etc. while the engine is still bolted down.  That guarantees perfect fit between engine lugs and motor mounts.

I'd be interested in knowing what the gage pin Ø is and then mic the shaft.  Do that to several motors.

I'd also walk the crank over to the Studer, true it and drop it about .0015" and then take it over to the laser and put some spiral grooves on it. But then again, I have access to FUN toys to play with.

I used what pin was tight, not sure it was the same one for each one i did.

      It won't be...  Gauge pins are usually in .0001 increments, there might be 20 different ones that could fit.

     Brett

Gage pins are in .001 increments.   Either sets of +.0002 or -.0002

Gage pins are in .001 increments.   Either sets of +.0002 or -.0002

You can buy them in almost any size you want and they are usually .0002" but it depends on the class of the gage pin how many decimals they go out. I've seen them to 20 millionths.

Not working for NASA,  + or - .001 is what I have.  Any improvement is better than what fox sent out the door as good.

After flying with a Fox, why would anyone change to a different engine?  The Fox is the ultimate stunt engine, so what could be better?

So what happens when you simply use another Fox on the day?

Hi Brett, I agree but at least you can reference the internal bore and at the very least you get a homogenous dead flat mount as opposed to two discreet lugs that without accurate milling could be anywhere.

Hi Dan,
              Another benefit with using a radial mount is that the thrust line is easily changed on both the pitch and yaw axis.
Sometimes with speed models flying tangent to the circle is too much out thrust and it's good to have the ability to adjust this.
Cheers, Chris.

Has anyone ever proven the theory that the case distorts at normal lug bolt down torques?

I flew Fox 35s for decades never giving this a thought. Just built the Ringmaster or whatever, bolted on a broke in engine, and went flying

I don't fly anymore so not motivated.  But seems easy to bolt a plate to top of cylinder and mount a dial gauge indicator and see when torqueing what, if any, increased distance is detected

I think Roberts J&B weld method is simplest for any engine.....if it is needed

Did you read the reasoning above? It’s all been explained a couple of times.
I really don’t care about Foxes but what we discuss here is something that makes life easier when done right.
You can make it work also using methods that make no sense but then again, you’ll never know what you did right or wrong.
I like things that make sense.
Also, at crankcase temperature, JB-Weld is not really harder and more stable than the wooden motor mount. L