Diesel opinions needed

THat video is what led me to think that this medallion might be a good candidate for a diesel head.

I have done quite a lot of experimentation converting Cox engines to diesel. The Queen Bee conversion that Kris as demonstrated is the only really successful one that I have seen based on a head conversion.

I have found that there is a fundemantal problem with the head conversions. Although they work, they are not efficient because of the Venturi size. The Venturi opening needs reducing. This would be difficult to achieve on a Medallion with the delrin Venturi.

On the Bees I use tubing to reduce the diameter and on Tee Dee's I have remade the Venturi. The Medallions with the NVA passing through the Venturi will be a little difficult.

Nothing is impossible and if you are happy with and under performing Diesel engine then convert. There is an article in Model Engine News on a Tee Dee 09 conversion.

When I find it I will edit this post. Here it is the article

Model Engine Gallery Page 17


This pair of Cox TD 09 Diesels is the work of Jon Fletcher (OZ). Jon lapped the bores to reduce taper to that acceptable for a diesel. That meant a new piston and conrod assembly. His first attempt used press fit spacers either side of the conrod up inside the piston but after bursting a piston on assembly, Jon changed to wire circlips made from 0.012" diameter piano wire as gudgeon pin retainers. Influenced by Alan Allbon designs, Jon set the pin low in the piston to give a good compression seal. The gudgeon pin is 0.1250" dia. drilled out at either end. The total reciprocating weight actually ended up less than that of the TD 09's hardened piston and ball joint little end. Jon found he had to fabricate some copper sealing washers out of copper bar 0.008" thick to get a leak free joint between button insert and cylinder internal face. He also had to sleeve the venturis down from 0.133" dia. to 0.106" as the big glow venturi required the needle to be 6-1/2 turns open and raw fuel could be seen going down the inlet when running flat out. Unsurprisingly, the engines had an enormous thirst. With the smaller venturi, the needle position is about 3 turns open.
The engines are very easy to start and go almost as hard as the glow versions n standard diesel mix as the glow version does running 30% nitro. After running in and using contest diesel fuel, Jon expects they will equal or exceed the glows performance.

Since fit90 is new to diesels I would guess that he does not want to do all the rework listed above. I believe he was asking if a screw-on diesel conversion head would work. I have not tried it since I have many .09 purpose built diesels waiting for a plane, but I believe RJL makes a conversion head.

That said, I also remember that Mr. Davis (DDD) experimented with the TD .09 and said they run too hot as a diesel thus he opted to NOT make a head for them.

My suggestion would be to get purpose-built diesels and run the Cox engines as glow. If you do the conversions be prepared to lose both the engines and the price of the conversion.

I have only one conversion...the TD .049 from DDD back when they came out plus a DDD reedie crankshaft that I have not used. The conversion works. I wen through a couple of Teflon disks getting it adjusted.

George

Actually, the medallion .09 uses an .05 r/c carb. I was hoping that since the carb is a bit on the small side for a glow .09 then maybe it would be a lot closer to what a diesel .09 might like. Also, on the tanked bee engine I used a Texaco back plate. And, the biggest news of all, I just ordered an MP Jet .04 "classic" diesel engine. It is approximately .037 cubic inches and looks like a mills. I am also looking forward to trying the queen bee mod like Kris has. It would really thrill me beyond description if I could get it to perform any where near as well as his. Thanks again for all the help. I am really digging the diesel engines.

fit90 wrote: ...And, the biggest news of all, I just ordered an MP Jet .04 "classic" diesel engine.  It is approximately .037 cubic inches and looks like a mills.  I am also looking forward to trying the queen bee mod like Kris has.  It would really thrill me beyond description if I could get it to perform any where near as well as his.  Thanks again for all the help.  I am really digging the diesel engines.

 
I believe the "Classic" is a "Letmo" diesel converted to beam mounting. I have the radial mounted Letmo replica and, although it runs well, it is a PITA to mount because of the rear intake and tank. The Classic mounts, as you said, like a Mills. Since the Classic is a long stroke it will take a wide range of props. Can't remember whether I used a 7x3 or an 8x3 to break the Letmo in.

Oh, another difference between the Classic and Letmo is that the Letmo has a keyed prop (yes, you have to drill a small hole in the prop hub). I THINK they changed that on the Classic.

Who has the Classic? I got my Letmo from Ed Carlson.

George

I ordered it on line at icare-icarus.com. It was on sale for $77.00. I remember seeing one from Mr. Carlson that was called the Letmo. This looks like that one but I don't know enough to know the differences.

MP Jet did indeed make a Letmo replica, that was just a smaller model (.6 cc) and it had the original rear mounting and keyed prop etc.

I don't know how many series that they made but series II was a run of 500 engines, each marked with a number, I only have one of them and it's number #476;

The picture of the one I ordered has mounting beams as part of the crank case casting. The description called it "classic diesel". When it comes I will definitely show it off. It will probably be covered in finger prints from me fondling it, though.

Yes, it looks like they improved a little on the Letmo design and made into a more user friendly beam mount. Then they could not really use the name Letmo anymore so they called it "classic" instead, I guess.

Will be interesting to see what other differences that they have made, the keyed prop is not needed either I suppose.

"Valic" has a done a little comparison and test run as well;

Great video! Now I am really looking forward to getting the engine. It looks to be both nicely made and easy to run. I hope it is.