All right guys, I went to the hobby shop to buy the FAI fuel,,, but ended up walking out with head shim material instead. I bought a package of .001, .003, .005 and .008 brass sheet. (the head insert is already brass so I'm not worried about galvanic corrosion).

Ultimately this will be a better solution for me since all I have to carry my planes to the field is an 11 year old honda accord, so two planes, a field box and refreshements (cokes not beers) pretty much fill the car already and having to deal with 2 different gallons of gas could be done but is definetly not what I prefer.

I plan to take measurements of the head tonight, and go to my buddies place to CNC route out a bunch of different shims. I'll start with teh .003, and progress from there, if I don't get anywhere doing this, then I'm only out $3.50 and I'll go get the FAI fuel.

Hope to have some data this weekend.

I think most people use a standard gasket which is about .01 inches. But you can fine tune it, highest RPM should be the best. Though you might want to shim it up a bit more if you expect to use even higher nitro in the future.

Well, that pretty well takes care of that....

Later.

Jim

0.003 shim, found that there is already a factory installed shim measuring .008 or so.

Attached is the temp graph.

it means you've got better results?

Looks like you already got a 20 degree plus drop, not bad.
Now was this with the stock or aftermarket muffler? Really not all that important but may be worth checking.
Still you have seen a major drop (to me) in temperature just from the this change, that's progress...

From my calculations, I dropped 70, was 250, now 180. Got three flights yesterday without issue, so for now it seems fixed.

It seems like a big reduction, so I'm not really sure that the the t'couple was making adequate contact with the head. If if I feel motivated I'll try this again with it better attached.

Yeah, this was with the bisson muffler, which to be honest I don't really like because its REALLY LOUD.

I flew the plane yesterday with a 14-4 and the guys were asking me if the muffler fell off... it was quite unbelievable. Someday I'll get something quiter but not now. Its much quieter with the 13-5, and also about 500 less RPM.

The power is still pretty poor, before the first overheat issue it seemed much better, but it could be my imagination.

Also, it now hesitates when throttling up, and idles much poorer than before the overheat issue, but since I changed the ring, I probably didn't align the cyclinder properly but I did the best I could (if its off,,, I can't tell by looking)

At least its now flyable. I think I'll upgrade next year to either an OS75 or the ST 90.

Glad to hear you got it back in the air.
I was just looking at the peak temps when I came up with that value not the average.
You will probably have to fuss around with the low and high mixture for a bit as you have changed the compression and it might take a while to find the sweet spot again.
As far as the muffler they can be fun to find one that will work as well in all aspects as you want. But I do like the "look" of a in cowl muffler rather than something that looks like a water heater hanging off the plane.

Just nice to hear you got it in the air and no dead stick...

Thats true, but why are almost all in cowl mufflers designed and sometimes made so poorly, loud, no backpressure, and expensive? All at the same time.

I have often wondered about that myself...
I have found that some of the in cowl setups are actually considerably lighter than the stock setups, and a few can be tweaked a bit to get the most performance out of them.
I am thinking about a piped system in my next build, just for something different.

It's all about priorities.

If you want it to 'look' a certain way and are willing to settle for less power/performance to achieve this look, use a normal Pitts type/in cowl muffler. If you want to maximize power/performance with what I and many consider a small sacrifice to looks, use a tuned muffler/pipe that will increase power/performance. These tuned exhaust systems can, in many situations, be wholly or partially hidden inside the fuselage or in a tunnel down the belly. Jett Engineering makes the Turbo Jett. A tuned Pitts style/in cowl muffler.

Terry in LP