I have two fox 36x models here.  Of the two, which is better suited for a combat model. I will only be sport flying it, not for competition.
But what are the differences? Is one much better than the other?

One on the right - think Andre's correct.  A 35x - straight up plug
One on the left-looks like a 36x needle bearing

Upon closer examination of the picture, I think Frank is correct that the plug is straight up and not angled incorrectly. You have to look real close (I had to use reading glasses!), but the plug does appear straight. If I can see a good pic of the sleeve, I can tell for sure if its a 36X or a 29X, but with the straight plug, I suspect it's a very early 36X in a 35X case.

As for the 35X: Using sport fuel (like 10% nitro, castor lube), it will easily outlast most of us.  You only get into issues with the crank when you start re-timing and pumping the nitro to it.

I just purchased 3 very lightly used 35X engines and I will have no qualms whatsoever about putting them on appropriate vintage combat airplanes and running them on 10% w/castor and open venturi's.

As for your 36X:

* IF it has no web in the exhaust, it was manufactured from about 1964 or so up until about 1970. IF it has a web in the exhaust, it was manufactured after 1970, as I recall.

Aside from the "Tigre vs Fox" preferences, a decent Fox 36X will run and run and run if used reasonably (leave it at stock specs) and taken care of (keep castor going through it and don't get crazy with the nitro content).

I have a stock 36X on a Demon, and pulling .018" lines and on 10% nitro, will easily bust 100 MPH no sweat.

Enjoy!

Here's a pic of the liner and head

Hi Dane:

The cylinder liner is an .800 bore, therefore it has a 36X piston/sleeve in it.  For sure not a 29X.

It will be fine in a Super Satan and with a Fox 36X restrictor in place on pressure w/10% nitro it ought to run about 90 MPH.

Around 1960, Fox produced the needle bearing Combat Special for $19.95.
In 1977 he introduced the Combat Mark III.
Nothing in between was worth trying to start.

Around 1960, Fox produced the needle bearing Combat Special for $19.95.
In 1977 he introduced the Combat Mark III.
Nothing in between was worth trying to start.

Paul :
Ouch. Golly, gee whiz, holy smokes. That's quite a spread of Foxes that you put in your trash heap.
  The performance of the Fox 36xBB ( circa 1964-72) .  Well, at the Vintage Combat Festival this past September, there wasn't a ST within 15 MPH of 120.  No doubt ,There maybe some somewhere. We had half dozen or so old Foxes crack 115.
 BTW just to show that I'm open- minded, I have a C35( beautiful machine work that tickles my Italian pride) mounted on a Voodoo and it performs nicely. A G35 would no doubt be faster, but heavier.
It's all fun.
Frank

Spoken like a true Supertigre fan.  

TRUE.

Although after a few years good ST's became hard to find as well.  When Duke gave away Combat Mark III's at the 1977 AMA Nats it was a whole new world.  At that point the only people competing in combat were those who could MAKE ST parts.  The Mark III was like Colt 45 compared to a musket.

Yeah, there were a few good Foxes in between, but I didn't have enough money buy a bunch of engines hunting for a few good ones.  Sort of like looking for an engine for a Fox 35 Stunt event.