Even in the case of California emissions packages, the F44 RPO designation allowed a 1/2 ton truck to avoid the requirement of catalytic converter(s). This was done by tweaking a regular 1/2 ton up to a point where the GVWR was above the limit that would require the use of a cat (6000 lbs) Minor upgrades were made to the springs, brakes and tires to accomplish this increased load capacity.
When the F44 package was added, the 1/2 ton truck was then essentially a 3/4 ton truck. These upgraded 1/2 tonners were commonly called Big 10/Heavy Half.
This practice began with advent of cat con requirements (for all vehicles with a GVWR below 6000) in 1975. It didn't fool anyone for very long. California raised the minimum GVWR for no cats from 6K to 8.5K in 1978 - the rest of the country lasted until 1979.
So if I am reading everything correctly, your truck (F44 RPO and 1977 MY) was never equipped with any catalytic converter(s) as supplied from the factory. Consequently it is now - and always has been - in compliance with CA emissions requirements.
A dead giveaway is the design of the transmission crossmember support. The supports on 1/2 ton trucks from that era will be different depending on whether cats were installed or not (don't ask me what the difference was - I just learned all this tonight).
What probably
cannot be used - to determine if a truck came with an original exhaust that featured cats - is the other emissions components. There were too many variations and work-arounds across all the engine/trans/altitude packages to use them as an indication.
Even up till 1986 (like my truck for example) on HD emissions packages, it was common to see an A.I.R. injection system complete with 2 air pumps and exhaust manifold injection - but the truck never needed cats.
Some Big 10 history:
The Big 10 was promoted as a heavy duty two wheel drive half ton for the 1975 through 1980 model years. Although it offered truck buyers somewhat more load carrying ability than the standard C10 half tons, its real advantage- -and the motive behind its conception- -was that it provided buyers with the opportunity to purchase a half ton truck that was unencumbered by the dreaded catalytic converter, which was first introduced to a skeptical American car buying public just in time for the 1975 model year.
For several years prior to that time, all trucks with gross vehicle weight ratings of 6,000 lbs and below were forced to comply with the strict light duty emission standards that also affected passenger cars. The EPA's selection of the 6,000 lb. threshold likely reflected its awareness that that number had become the traditional de facto dividing line between half ton and three quarter ton trucks. And since the vast majority of trucks purchased by consumers were half tons, limiting the emission constraints to them probably seemed to be a reasonable compromise to the government, while avoiding potential protests about cost and implementation by the truck manufacturers- -and their commercial customers- -that might have occurred had the laws been expanded to the heavier vehicles.
Before the advent of the catalytic converter, the distinction between light and heavy duty emission controlled trucks was largely ignored by consumers. But the converter introduction and it's nearly universal application in 1975 light duty emission vehicles changed things dramatically. Unlike the EGR and evaporative canister devices that preceded it, the converter had an exclusive appetite for expensive unleaded fuel which outraged truck buyers- -and especially fleet purchasers- -in a nation that still had access to relatively cheaper leaded fuel.
Recognizing a new marketing opportunity, or necessity, light truck makers made relatively minor spring, tire, and brake modifications to their existing half ton models to push the GVWR just over the 6,000 lb. threshold. Ford lead the way by introducing the heavy half ton F150 as an alternative to their traditional half ton F100, while Chevrolet introduced the F44 Heavy Duty Chassis package as an option for their C10 pickup.
These new-for-1975 models moved into the heavy duty emission classification enjoyed by three quarter ton and heavier rated models. This allowed the F44 equipped C10, later dubbed the Big 10 for market visibility, to comply with government emissions regulations using only a PCV valve, heat stove, and relatively loose controlled combustion system (CCS) tuning. In contrast, the 1975 light duty emission certified standard capacity C10 required a PCV valve, heat stove, stricter controlled combustion system (CCS) tuning, EGR, evaporative canisters, a vacuum activated early fuel evaporation (EFE heat riser) valve, special outside air ducting to the air cleaner, and a catalytic converter. But by 1979 the EPA- -following legislation enacted in California a year earlier- -raised the light duty/heavy duty emission dividing line from 6,000 to 8,500 lbs. GVWR which brought all heavy duty half ton and almost all three quarter ton pickups into the light duty emission fold. Following this legislation, consumer appreciation for the heavy duty half ton concept- -so gelded- -largely faded away.
All the above information comes from some guy who appears to know what he is talking about. It is just a tiny part of his answer to the questions in the post shown below - which is almost identical to your situation :
What is a Big 10 or Heavy half?
i just purchased a 77 half ton pickup and the 4th digit in the vin is a 4. the gross weight for this truck is 6050 lbs. this makes it a Big 10. this is a calif. truck but no cat converter. also the intake manifold(v-8) dosent have an egr valve. is this correct? or do all trucks regardless of weight require an egr valve? what are emissions for the heavier trucks? thanks
Link to the entire explanation:
http://67-72chevytrucks.com/vboard/showthread.php?t=155294
