I'm doing a story on a Diesel Forty and in my research, the RD6 seems to be nearly the same tractor and with overlapping production in '35 and '36. None of the texts I have at hand explain the differences.

I read about the "RD" designation possibly being for "Road District". Could that have anything to do with this? The RD series seem to have counterparts in the Cat line of the period but I don't understand the reasoning of having such similar tractors in the lineup and taking the extra expense of things like grille castings and other things.

I do see a few technical differences, though some of the info I have is anomalous. To whit: In some sources, the Diesel Forty is listed as having the D6100 triple with a 5-1/4x8 inch bore and stroke while the RD6 had the D6600 with a 5-3/4x8 inch bore and stroke. HOWEVER, I also see two 1935 Nebraska tests of the Diesel Forty (#242 & 243, 9/9/35 & 9/10/35) showing it with 5-3/4x8 inch bore and stroke. Interestingly, they are the same tractor, test 242 rated it at 44 drawbar horsepower and a rated max of 850 rpm and #243 at 52 drawbar ponies and 1000 rpm. Huh?

I don't see a test of an RD-6 (D-6 new designation from '37-40 I guess) with the 3-cylinder.

Anyway, trying to understand the Diesel Forty and it's relationship to brother/cousin RD-6.

Finally, the tractor I shot was shot at the Tri-State Portland show in 2018 and I was not able to find the owner. It's a 1935 #3G937. Does anyone know this crawler and can you put me in touch with the owner? Pic below.

Thanks for engaging the brain on my behalf.

 

"RD" was a dedication to Art Rosen, a Caterpillar engineer in the 30's and 40's, designer of the D9900 engine that was the first diesel engine placed in a Cat tractor, the RD for Rosen Diesel was in honor of his engineering work.

Not saying you are incorrect but Cat has a different take. See: www.caterpillar.com/en/news/caterpillarN...yofwhatsinaname.html

It just don't seem logical though for Cat to do that because the highway dept used that designation, I would think more so for the designer of there first diesel engine instead. Besides Cat tractors were used allot for farming, logging and factory work too.
Caterpillar | 1930s

B.C. Heacock was the only person that really knew what the "RD" stood for but was never actually mentioned. Need a time machine! lol

An take on the other part of the questions?

To my knowledge the Diesel Forty was just retagged as an RD6 to match the new nomenclature that Cat had adopted. There are not a lot of differences between the two as far as I know. We actually use the Nebraska test data from the diesel forty tests for the RD6 as well at our tractor pulls.
As far as the RD well there are lots of theories that I have heard and I will put them in order if what I think is least likely first and the last one will be the one that makes most sense to me. This has been debated among collectors many times.
a. Paying homage to Franklin Roosevelt.
b. The road district.
c. The Rudolf Diesel theory.
d. It just separated Gas tractors from diesel tractors. There was a whole line of gas tractors that were produced with an "R" R2,R3,R4,R5. In order to keep models separate and easier for customers to identify if it was a gas or diesel, they added the D to the already existing R. RD4,RD6,RD7,RD8. Similar to how they separated a gas Forty from a Diesel Forty - they just added the word diesel in the designation. They just added a D to designate that it was a diesel. This makes the most sense to me, it is probably wrong but it seems to be the simplest and most logical one I've heard.
Unfortunately I don't know that we will ever know the real reason.
Thanks
Nathan

It was mentioned that in the mid 30's, [around the RD period] "Caterpillar was the largest producers of diesel engines in the world" so I would think this would be like a celebration to Art Rosen for his work. Just my thought anyway.
www.caterpillar.com/en/company/history/a...r-of-innovation.html

Just ran across the info that may be a clue about the D6100, D6600 anomaly. The D6100 engine went into production in 1932. The D6600 came in 1935. Also, that Diesel 40 test was unit #5E501. Wasn't 5E the early prefix for the RD-6 before it went to 2H? So maybe that tractor listed by Nebraska as a "Diesel 40" was actually an RD-6, or a forerunner. Perhaps the 40 kinda gradually morphed into the RD6 after the D6600 engine appeared?