jschenck

My Feedback: (23)

Does anyone know where I might be able to get a head button for my new Fox-74 made per Flightline's spec's? I have a NIB late '80s 74 with the TN card that I'd like to not burn up! I already bought some castor oil that I will add to the 5% nitro wildcat blended 18% oil fuel I will use for this engine.

Flightline Fox-74 modifications

Thanks for any help!

Sport_Pilot

Just about any medium sized town should have machine shops willing to make one from their drawing. Check out the yellow pages and call some up and ask.

ajcoholic

My Feedback: (10)

jschenck,

I can make you one. Email me your address and I can make it up and mail it to you.

[email protected]

One thing, in the drawings the part that is to go into the cylinder sleeve shows a measurement of 1.0 - 1.05 inches. That must be a misprint and should read 1.000 to 1.005, as a 50 thou tolerance is unlikely, even 5 thou is large when talking about engines. Is there any way you can accurately measure the inside diam of your sleeve?

I do lots of engine building and machining and like old engines too - I want to help you out with this.

Andrew Coholic

jschenck

My Feedback: (23)

sport_pilot -
I was concerned about calling a machine shop up on this since the labor cost for making just one of these may make the button more expensive than the engine! I may be wrong about this, but I thought I'd ask here to see if there was already someone setup to machine these, thus reducing the cost per unit - just my thoughts so far, perhaps I'm off on this.

ajcoholic -
I sent you an email. I will see if I can get clarification on the measurements. I don't have a micrometer to measure the engine. I will see if I can round one up.

Thanks again guys ...

Sport_Pilot

IMO the .05 tolerance is correct. That would be a .025 tolerance between the surface of the sleeve and the head button. This is not a critical area. Not nearly as much as the depth or thickness of the button.

Pepe J

My Feedback: (2)

The rule of thumb in situations like this....

If it fits in the I.D. shoot for the bottom of the tolerance. If it fits on the O.D. shoot for the top of the tolerance.
If a part is made to the drawing and there is a problem with the fits, the blame goes to the draftsman/engineer and you still get your pay check.

Nice of you to volunteer AJ.... And that Perry Pump issue if you recall? I resolved it. The pump was Cah-Cah. Thanks.

Happy Fly'n....

funflyer99

While you are at it you should consider having a carburetor adapter made so you can use a well engineered carb from OS or similar. I had 2 Fox 74's and both were fitted with the new head buttons witch allowed me to use more nitro and have a better idle and transition. Then I machined a few carb adapters of different sizes to use different carburetors and that made all the difference in the world. I used both the carburetor from an OS 46 and 91 with great results. I can't remember off hand what the exact numbers were but, both carbs let the engines idle at about 500 RPM lower than the Fox carbs. They would also idle for several minutes and then transition to full throttle without hesitation. The top end gained a few hundred RPM with the 46 carb and about 1500 RPM with the 91. I also used the OS carbs with the stock Fox head buttons and guess what? The engines ran equally as good. Just my opinion, but if I were to invest any time and money it would be the carb.

Todd.

Sport_Pilot

You can buy a carb adaptor from Conley Precision aka Perry. Depends on the carb. I think Fox has an improved twin needle carb also, or the mod that Flightlines shows on their site.

Re-read the initial post. He has the twin needle carb!

ajcoholic

My Feedback: (10)

25 thou clearance all around the button is a HUGE amount! I never have seen an engine in which the head button was not a nice slip fit into the sleeve. He will send me the sleeve and I will fit it properly.

I doubt that there is 50 thou difference in the sleeve diameter either, that would be a significant increase/decrease in displacement. Tolerances are in the order of +/- 5 ten thou maybe, but 50 thou? Thats allmost a 1/6th of an inch!!

AJC

jschenck

My Feedback: (23)

The carb is the old 'pre air-bleed'' style. I may end up swapping the carb out if I can't get a reliable tune for it. From what Flightline wrote, some are good some are not. Depends on the machining of the needles.

Does anyone have thoughts on just burning straight FAI fuel? I'd have to mail-order it since on one carries it here. Seems to me that might help keep the heat issue under control.

Sport_Pilot

Most engines designed for straight FAI fuel run very well on it. Easy to mix yourself just 80% methanol and 20% castor oil. You can use synthetic oil instead, but then it's not really FAI fuel. Most engines that are made for FAI fuel will tolerate 5%. Some may run better on it. A trick for those that run slightly hot on 5% is to mix 5% and FAI fuel to get 2.5% nitro. This makes the idle adjustment more reliable as the idle mixture adjustment will be narrow, unless the carb has a very fine idle mixture needle. Because of this an air bleed type carb is easier to adjust with FAI fuel, I suspect this is why Fox went to that type.

Sport_Pilot

About the head to sleeve fit. Yes most engines have a slip fit. I recall having to pry the head off of one. Varnish build up between the head and sleeve did that. But some, mostly older engines, (back when castor was the only oil?) have been very loose. .05 would not have been that uncommon.

fliteline-RCU

Regarding the fit of the head button in the liner, .05 IS a misprint!
For best performance the head button should be a a fairly snug fit in the liner. We'll update the diagram shortly.

Thanks for pointing this out guys. Surprised that this wasn't picked up sooner.

Regards,
George Ronnenkamp
and Lyn Murray
Fliteline Solutions

ajcoholic

My Feedback: (10)

I am a collector and user of vintage engines. I have had 100's apart and make repro parts. I have yet to have an engine that had more than a few thousandths clearance in between the head/head button and the sleeve. Let me know of a particular engine that has this much clearance - I wont believe it untill I see it with my own eyes...

Andrew

Sport_Pilot

Not sure it is that much. But I'm thinking the old Fox Stunt .35 was pretty wide. Also the McCoy .35. Not .05 but then they were smaller engines.

ajcoholic

My Feedback: (10)

I dont want to sound picky, but I dont think some of you guys realise how much 50 thou is - like I said about 1/6th of an inch!

The McCoy 29 and 35 redheads were about 2 thou clearance for the head to fit into the sleeve. Restored 25 of them last year.

Just to give you an idea of the tolerances, the average lapped bushing supported crank has 1 1/2 to 2 thousandths clearance. That is how much we are talking about. Most heads that have to be "picked" off that are snug have maybe .0005 clearance. Thats a light press fit.

Just trying to help out, thats all - again I am not the expert but just my experiences with these things.

Andrew

Sport_Pilot

I know exactly what .05" is. But this is a far cry from a bushed crank. There is no reason to be that tight, except to increase compression. A low compression engine like a Fox Stunt doesn't need a tight fit between these surfaces. In fact I have seen heads made without any depressions into the sleeve at all.

jschenck

My Feedback: (23)

Wow - what a piece of craftsmanship! I really like the way these Fox engines feel.

If I am reading my caliper right, the inside/top of my cyl measures to be .997 inches. The head button measures to be *Exactly* 1.000 *iff* (if and only if, for you non computer types !) I am reading my veiner caliper correctly.

I can tell everyone that the button does not rattle around inside the cyl. sleeve! It is not as tight a fit as the guys at flightline suggest as it slides on and off easily, but it does not rattle either.

I can't wait to fly this engine! Now I need to build that Stinger-60 for it! But first, a new head button and a few gallons of FAI with a few oz of castor added for flavor! mmmm...

I can't figure out why Fox doesn't make an engine to run cool and reliably on Omega fuel... I think if they did this and --> pushed them into the dealers glass cases people would spend the money on a well made American product instead of the mass produced Asian engines.

ajcoholic

My Feedback: (10)

Ahh but thats where youre thinking is inncorrect I am afraid. Yes, many engines will run with porly fit heads and such, the key thing is they will run, but what we are aiming for is TOP performance. If you look at the best made engines, they are fit so the head or head button is a tight slip fit. What good does your squish band do if you have a cavern (in the world of engines 25 thou is just that, a cavern!) around the outer part of the squish band? Defeats the purpose.

Every little bit of extra space will impair the most efficient combustion. Even poorly fit glow plug threads will take some top rpms off. Look at a Nelson or Jett engine and see how the head buttons fit in...

JM $.002.... Andrew J. Coholic

Hobbsy

My Feedback: (102)

I have two Fox .74s and a .60. The head buttons are a very precise fit into the cylinder. I modified one head button on a slightly contrary .74 by cutting down the raised portion where the glow plug sits by slightly over 1/16 inch so the entire idle bar is in the combustion chamber. This completely eliminated the mid range hesitation and occasional dead stick.

funflyer99

If there is any place on the engine that has a .050 clearance it would be the needle valves in the carb, the needles in all my fox engines have a very loose fit and will change the mixture just by wiggling them. Actually the clearance isn't that bad but they do have plenty of slop. The 2 needle MK 10 is a better carb than the air bleed from what I've seen but still robs the 74 of its power potential. Last time I checked with Fox they really had no plans on finishing the parts reqd for the new 2 needle carb for the 45-74 while their gas engines needed the attention.

Todd.

jschenck

My Feedback: (23)

I will be paying very close attention to the needles on this carb and will do the mods suggested by the guys at Flightline. The instructions the Duke wrote up for this TN carb suggest modifying the taper to suite the exact tuning required for the engine. Specifically he states some procedures for richening or leaning the mixture for the 3/4 - 7/8 throttle position - where I indecently like to fly at a lot.

Once I have a gallon or more through the engine I may replace the carb with a Perry unit or some other type if I can get an adapter.
For now, I will put a piece of fuel tubing on the primary needle and add the gasket per Flightline's TN carb document.

ajcoholic

My Feedback: (10)

Well here is the stock Fox head and the custom one I made from the specs on the Fliteline Solutions website. Very different squishbands, and combustion chamber depths and widths.

I measured the stock head button, 0."002 clearance, mine has 0."001 for a nice slip fit.

John, I sure hope this helps you out and its fun to be part of an experiment!

Its allready back in the mail, hopefully will get to you soon.

AJC

jschenck

My Feedback: (23)

Thanks again for getting the head button made up for me!

I can't wait to get this engine on the nose of the 'Flying pickup-truck' which is currently being pulled through the air by a good running K&B .61 (Model 6550).

I'll bet the Fox .74 will just about yank the wing off this plane. As it is, the K&B pulls it straight up off the grass runway.

I know, need to do a bit of work on the hatch!

Sport_Pilot

Measured the head to cylinder wall clearance of a Fox .35 Stunt today. It was .023, not the slip fit of a modern engine. I suspect that this and the short reach of the chamber into the cylinder is why it is easy to bolt the head on slightly cocked if you don't tighten the head screws evenly.