.020 alleged high compression head

In that case, why would Cox bother putting a knurled circle on some .020 glowplugs. Was it to seduce potential .020 Tee Dee buyers? Or just to meet the TEE DEE tradition?

On Ebay, there is one for sale, believed to be original, with a knurled glowplug:

http://www.ebay.com/itm/COX-020-TEE-DEE-GLO-MODEL-AIRPLANE-ENGINE-CONTROL-LINE-WITH-BOX-/390743434369

They did not only come on finished engines, but were also sold as spares. A mystery to me.

Lieven

IIRC, my Pee Wee's purchased in the 1960's and 1970's all came with the knurled inner ring on the head. Cox possibly removed the knurling as this was an additional machining operation as a cost cutting measure for the later engines and parts.

Cox .020's were revolutionary when the came out. The Pee Wee in the 1950's was more powerful than some of the .049's of the time. No wonder why modelers went after the Cox products. The .020 Tee Dee was just that much more powerful. Ken Willard used the Tee Dee in an RC Schoolgirl bipe in the RCM magazine that became a Top Flite all balsa kit. The Bipe has a 32" span, shows a Cox Medallion .049 with throttle setup, but Ken wrote it up powered by the Tee Dee. It was powerful enough to tote along a .049 plane.

I am really sorry to dredge up an old thread but was curious about a knurled 020 head that came into my posession. I opened up two results from google about it, this thread, and another on rcgroups started by Bernie back in 2011 when he introduced his current production 020 heads. Bernie seems to be of the opinion that there were high compression 020 heads prior to the 90s and apparently modelled his new production 020 heads on them whereas this thread concluded that there was never such a thing. I am confused to say the least.

ThermalSniffer wrote:I am really sorry to dredge up an old thread but was curious about a knurled 020 head that came into my posession. I opened up two results from google about it, this thread, and another on rcgroups started by Bernie back in 2011 when he introduced his current production 020 heads. Bernie seems to be of the opinion that there were high compression 020 heads prior to the 90s and apparently modelled his new production 020 heads on them whereas this thread concluded that there was never such a thing. I am confused to say the least.

No need to apologize really. We're still discovering things about Cox engines/parts that we didn't know. I do tend to agree with Ron Cribbs' comment concerning the same combustion-chamber area between the differing part numbers. The larger .049 glow-heads can fairly-easily be identified when placed side by side. The area where the glow-head seals down on the copper gasket, has a wider band of material which reduces the combustion-area. With my "limited" machining experience; I would gather that the conical-area was (and still is..) formed by a ball-(radius)-nose end-mill in the machining process. A higher-compression head would merely require less of a depth-setting than standard.. or removing less material from the inside of the head. I doubt that it's any more technical/scientific than that.

I will say that the conversion-heads that use a standard 1/4-32" glow-plug don't deliver the performance of an original "standard-compression" Cox glow-head design. I think the reason for this is; that a standard/low compression glow-head blank is generally used for the conversion. When this head is drilled/tapped for a standard 1/4-32 plug; some compression is lost.. through the threaded area and whatever integrity exists with the gasket used for that plug. The  Glo-Bee clip-ring "insert-type" plugs avoid this. I feel that this is why they work as well as the Cox glow-plugs. It's just my theory though. I'd like to see Cox adapter heads made from high-compression "blanks".. which might yield better performance. One could then experiment with more/less Copper head-gaskets to tweak their performance with different fuel-blends/props.

Roddie, two reasons why adapter heads don't work well. First reason is the threads. Threads don't seal and you lose compression around the threaded portion of the plug. Second, the volume within the plug where the filament is retained. This is why turbo plugs and Nelson plugs work, they don't use a copper washer to seal the base but rather a taper at the bottom which seals it. The other is the filament is contained within a very narrow hole further reducing it's chances to take up volume. While you might see a performance gain using a different style head, the chances of the above outweigh the head design.