Tower .40 Cylinder

Len,
Curt Comer and I went flying yesterday with Rollin Keszler. His Tower .40 in his RD-1 would occasionally act like it was overheating and going to die, but then would pick up and run perfectly after just flying level for a few laps. Upon checking his engine it seemed quite tight for about the last 3/8 to 1/2 inch of cylinder travel giving rise to Curtis and my specualtion that this particular engine has less cylinder taper than the OS engines we are familiar with. Rollin had previously been running this engines on all castor fuel. Could castor have built up on the piston/cylinder assembly and helped increase this tightness? Would taking the engine apart and cleaning off castor build up do the trick and free the engine up enough so that it doesn't seem to get hot and begin sagging? Somehow I think running some fuel with a portion being Klotz in this particular engine would help aleviate the problem from re-occurring.

All ABC or ABN engines should have a tighter taper on the top than on the bottom and the Tower engines definitely have more, on average, than OS. When they warm up, the top of the cylinder will expand more and then things even out. Some of these engines, especially if the cylinder has been "dropped" a bit, will be tighter than others. The tight ones simply take more break-in. It could take two to four hours of break-in. And break-in on these should be done at a leaner, just slightly rich rpm (not slobbery rich as on a Fox 35). In our CL stunt mode we run these engines richer than the RC guys do, so it is best to break them in on the prop you are going to be using.

So, assuming his engine is fully broken in and not overheating (cowl problems) then, yes, it is possible there is castor buildup. With these engines you can take them apart and use 0-0-0-0 steel wool (not 4-0, but 0000) to clean any varnish off the piston or sleeve. And, if you are having problems, a 50-50 blend of castor-synthetic with at least 20% total oil should help keep any varnish build up from occuring again. In fact, just running a few tanks of 50-50 mix might be enough to clean it without disassembling it.

This reminds me off what my Tower .40 was doing just yesterday. I just got it (actually a friend bought it for me and had it retimed for stunt, as well as put an hour run time on it (whatta a nice fella!!)) and put it on my stunter.
It was being a little difficult to needle, wanting to run in a fast 2-cycle, occassionally dropping to a fast four. On the last run of the day on it, it was running in a fast four-, almost 2-cycle run on a 10-5 Master Airscrew prop with Sig Champion 10% nitro, 20% oil (50-50 mix).
A few laps into the flight I did 1 loop and the engine quit. Not wanting to damage the engine I decided to stop for the day (it was really starting to get windy). It is very tight at TDC.
I would estimate it has roughly 1 hour total run time on it. Is this enough? Also, I used a ST NVA that I turned down to the size of an OS NVA. Could this be affecting it?

FYI, venturi was measured at .271 ID.

Thanks for all input.

Steve

I don't know how your engine is set up, so cannot judge the correctness of your venturi size. I set the ones I do at around .278 to .282 with the OS needle (.135 diameter). Stock FP engines have had venturis all over the place, but the current lot uses a .265. Some people in trying to make the run like a Fox ("If it runs like a Fox and sounds like a Fox, it probably has the power of a Fox") will use even smaller venturis and kill the power further with additional head gaskets. I would guess that you are pretty close on that, however.

As to tightness, a good fit will take longer than one hour to wear in, although you should certainly be able to fly it before then. But I would suggest that you just continue to fly it, making sure you don't go overlean as this is what can hurt the engine. Flying in warm weather won't hurt it as the hotter it gets the more the cylinder should expand away from the cylinder. Your fuel should be OK as well. Don't know about the prop. Again, I don't know about the set up, but it does seem a little small (diameter).

On the ST needles, if you wish to keep the same choke area without boring out the venturi more you can also simply file each side of the needle where it goes through the bore. Take about ten thousandths off each side making it oval in shape and you will retain the strength better. This is also something you can do by hand and don't need a lathe for the operation.

(Just thought I would use one of those guys.)

Thanks for the info. I will certainly give it a try this weekend. I also have some Sig Champion fuel that's all castor - making it 10% Nitro, 20% Oil. This is what I've been wanting to break-in the engine on. The other just happened to be handy at the time.
From what I understand, only the timing was changed on this engine; no ports were changed or blocked. Don't know if that helps, but my flying partner Tom Farmer has the same rework on his FP40 and it runs like a proper stunt engine should. Guess I'm just gonna have to start playing with props.

Steve Berry
AMA 628018

(Well, I finally decided to jump in!) What Jim stated is true and when I returned home after the flying session that day, I disassembled the engine and cleaned everything. I also felt the crank was still slightly tight in the case, so I mounted the crank between centers in my Taig lathe (BTW, this lathe is a really nice addition to any shop!)and used 1200 paper and solvent to carefully polish the journal. Now everything is very, very nice, with just a bit of pinch at TDC, and I can't wait for our next flying session. So, if anybody has a fairly new engine that suddenly changes habits, they might consider a thorough cleaning of all the parts before fretting about the other possibilities.

RK

Hmmmm, ArKay? I know a handle that would be better than that one! How about RocketMaster? Since you are!

Ramjett
ArKay, you hve just passed the Fox 35 maintenance exam and are now a world class mekanick.