Compression

I have very positive experience with 051-s, although so far I have always bought new ones. The early ones with tapered bore show very long life...I sang those songs on one serving  in my Toucan R/C for 250+ hours, retaining a still good compression, ball-socket gap, crank-shaft fit and all other mechanical conditions.

I also run TD 049-s and would not say they work different. In my opinion the groove on the 051 piston skirt is too small to make a difference in either crank-case decompression for fuel intake - it acts in fact as an addition to the SPI - or crank-case evacuation/scavanging when the piston pumps the charge into the cylinder. I believe the groove is just for visual differentiation from the 049 pistons. Too bad the 051 cylinder/piston combos went apparently extinct on the market.

In addition, I also have a strange, new piston with 2 grooves on the skirt, that has a tad larger diameter than the 051...The 2 grooves may also serve for differentiation....what would otherwise be the point in futrther increasing the cylinder bore for higher power, and reducing the power by a double groove?

The size difference, in terms of swept volume, in these engines is only about 2%, I don't that will change much in terms of power. There are so many other things that will dominate over that from engine to another, even with the same swept volume.

The piston marking, as I have understood it, is only there in order to easily separate the two sizes. Additionally there is also the .05RC engine, without SPI. From memory the TD .051 has two grooves and the 05RC only one? Or is it the other way around...?

Kris,

the swept volume of the 050 and 051 is the same, their pistons are interchangable, and both have only 1 groove on the skirt. I have and use both types.

I read somewhere COX called the R/C version 050 simply to indicate, that is a non-SPI 051 design, necessitated by the use of exhaust muffler.

I had to double check, and yes the pistons seems to be identical for the TD .051 and the .050RC, i.e. one groove in the piston skirt...

It seems all the difference is in the cylinder then, and the .050RC has a narrower exhaust port to avoid SPI.

I thought there was an engine with two grooves in the piston as well?

Surfer_kris wrote:I had to double check, and yes the pistons seems to be identical for the TD .051 and the .050RC, i.e. one groove in the piston skirt...

It seems all the difference is in the cylinder then, and the .050RC has a narrower exhaust port to avoid SPI.

I thought there was an engine with two grooves in the piston as well?

Here is the culprit shown with the common 051 piston...It won't go into even the most worn-out 051 cylinders I have...I remember someone once guessing on CEF this belongs to the super-rare Medallion 051-s that survived a production run up to less than 300 engines?

In my search for all things .051 a few years back I actually bough 2 of the so called excess FFS .051 pistons with the double grove

from the CEF member who shall not be mentioned....OLD timers on this site will remember the guy

My stash of real .051 factory single grove pistons all mic to .4090 and the double grove one are .4095...very slightly larger

One of the 2 will fit up into each of my USED .051 cylinders but pinch tight well shy of TDC...call it too tight to run with out really lapping them in

Too lazy and no good way to measure but I suspect ALL my old stock .051 cylinders may be Taper ground and these so called excess FFS .051 pistons were meant for non taper cylinders

I thought the dual lined piston was for the killer Bee .051? Between the slit being slightly wider on one of the slits in the dual slit cylinder and the 2 lines made this distinguishable.

Fred,
I bought the double ring .051 pistons and the single ring .051 pistons from the same guy in person since he was only about 20 minutes away from me.
Both types, as measured by the best HF digital caliper, show diameters of .410 inches.
At the time I certainly knew about the .051 pistons but had never seen an .051 Killer Bee and didn't know about the double grooves. The guy didn't know what they were either.
I bought a number of .051 cylinders and a matching number of pistons and didn't realize until later that they were TD.05RC factory warranty replacement parts !!
Guess what, both types of the .051 pistons will not fit a new .051 cylinder or a used one or a new TD.05RC cylinder !!
My best SWAG is that they were incorrectly sized production parts that ended up in R&D rather than scrapped.
A number of parts that I bought were either partially finished or poorly sized.
You had to see the tremendous amount of stuff he got from the R&D Dept. and you would been speechless as I was !!
There were literally hundreds of thousands of Cox parts scattered everywhere in a nasty, poorly lit shop, on tables, in boxes, bags, and scattered all over the floor !!! I would have been content to just have the parts you could sweep up !
Once he kicked, what looked like surestart, out the way several times on the floor, so I offered to buy it for 10 bucks. When I got home and looked at it, it had a new #4 piston/cylinder, 1702 head, KB bottom end and a new gray 6x3 prop !!! Great 10 buck buy !! Looks like the R&D guys were playing around with unlimited parts at hand...
BTW , those .051 double groove pistons came out of a bag of about 50.
The .051 single groove pistons came out of a bag of 500 !!!
All in all he could have turned a couple of grand into a couple of hundred grand easy if it were not for other issues.

Pretty sure it was both. The KB and NFFS.

So there was /is a Killer Bee .051 and .049 ? It seems i have seen one of the double grooves before but just figured it was for a non-cox engine

Yes,  
Cox made both the KB .049 with the black back plate and the KB .051 had the red back plate.
The early pre Estes KB .049 had gold anodized crank cases and the good scalloped crankshaft along with the special KB cylinders.
The Estes KB .049 were natural aluminum and had regular .049 crankshafts. Still had KB cylinder and black KB back plate.
To me the KB .051 is a bit of a mystery. I bought the only one I ever saw and it had all the good  pre Estes parts, gold case, and red back plate.
I don't know if any were made after Estes and if there were any variants.
So few ever pop up at auction, that the production run must have been small.
Maybe some of our very wise CEF members will know when and how many ?

Martin Happerle stated on his website the KB051 was in production until 2004

http://www.mh-aerotools.de/airfoils/cox_frameset.htm