PCV 736C Sighting in a 1973 Corvette

[Tom R.]

As TIMJG coordinators quickly learn, once that latest revision hits the streets, then team members hear from fellow members with original Corvettes and how they may differ from the printed word.

The 1973-74 3rd edition, out in February of this year, posted that two PCV valves were called out in 1973, 746C for L82s and the 769C for L48s/LS4s. In 1974, all engine options used 774C. The 3rd revision did not address the use of 736C but the 1970-72 5th edition did, calling it out for Rochester carb applications, apparently regardless of engine option.

A fellow NCRS member with a March 13th built 1973 L82 M40 contacted Team Leader Pearce that his Corvette, purchased new, has the 736C valve. I'm curious if other 1973 owners with known original Corvettes have the 736C valve. Any insight on use would equally be appreciated.

[Tom R.]

Re: PCV 736C Sighting in a 1973 Corvette

Another 73 owner, Jimmy Patitucci, pointed me to this forum discussion thread posted by Joe Lucia in 1999. He describes the evolution and markings of the 736C valve. I've quoted the section that relates to the photo above while the full link follows.

As many of you know, the AC CV-736C PCV valve had rather wide usage in Corvettes during the 1967-1972 time period. It was used on many 1967 427-435 engines and also most 68-72 engines with the exception, primarily, of 70-72 LT-1 and 1969 L-88/ZL-1 engines. The configuration of all original CV-736C valves was the same and, for a considerable period of time thereafter, service replacements were also identical to the originals. However, the configuration did change somewhat over the years since. Understand, though, all valves, even to the present day service replacements, are basically the same general configuration, with small detail differences. Herewith, a synopsis of the different configuration detail changes:

1967-1981

During these years of production of the valve, the cylindrical section of the valve(which inserts into the valve cover rubber grommet) has the stamped inscription "AC SPARK PLUG" AND "CV-736C". Around the very bottom of the cylindrical section are four, approximately equally spaced manufacturing marks which appear as indented "dots". Looking at the valve from the bottom through the hole in the case, the bottom of the internal valve has the raised letter "D".

https://www.forums.ncrs.org/showthre...light=PCV+736C

[Joe L.]

Re: PCV 736C Sighting in a 1973 Corvette

Another 73 owner, Jimmy Patitucci, pointed me to this forum discussion thread posted by Joe Lucia in 1999. He describes the evolution and markings of the 736C valve. I've quoted the section that relates to the photo above while the full link follows.


https://www.forums.ncrs.org/showthre...light=PCV+736C

Tom-----


I've checked this out and right from 1973 GM specified the AC 746C, GM SERVICE part number 6484525, for L-82 and the AC 769C, GM SERVICE part number 6487532, for L-48 and LS-4.

PRODUCTION part numbers for many AC Division parts, including PCV valves, were different than SERVICE part numbers although the parts are virtually always identical. The PRODUCTION part number for the 1973 L-48/LS-4 PCV valve was GM #6487530. The PRODUCTION part number for the 736C valve used for 1972 and earlier was GM #6484541. I cannot find that this number was ever used for 1973 but that does not mean it was not. As a matter of fact, the valve pictured above is the very early style valve and I have little doubt it was the one originally installed. PCV valves of this configuration are actually quite valuable.

[Jimmy P.]

Re: PCV 736C Sighting in a 1973 Corvette

Thanks Joe,
I guess we'll need to add this info back into a future Revision update in the form of a footnote. The older Second edition of the 73-74 Judging Manual did list the 736C as the PCV valve used for for 73 vehicles. It DID NOT specify anything different for the L82 & LS4 engines even though the 73 AIM does list 769C #6487530 for the 454 & L48's and the 746C #6484699 for the L82. Could this have been just a carry over from the 72? One would think, but not way into March of the 73 production run Funny thing is that this never surfaced from any one on our revision team. The sampling of cars we had to study admitantly, was minimal however most were original one owner cars. We had original 73 L82's with the 746C PCV.
We had an original Sept. 72 build L82 with the 746C in it. Now this original March built 73 L82 has the 736C in it. Go figure! Nothing is ever simple black and white with these cars, we've certainly found that out when working on the latest Revision. I must have read every TDB archival thread dealing with these valves starting back when I was trying to get things right for my '73. Most, if not all of that information lead me to believe that the 736C stopped in 72 and wasn't used in 73. However, I installed the 736C in my car when I had it judged just because the manual said it was the correct one.
On the other hand it's nice to see that the latest 73-74 Revision is being purchased & used as intended and owners are bringing these anomalies to our attention....

[Ronald L.]

Re: PCV 736C Sighting in a 1973 Corvette

What does the the 73 AIM say?

It's been 45 years, even one owner, can the memory be that exacting unless you have a receipt?

If they started 73 MY with the 746 valve, and you had a 73 dated service parts book, and SERVICE indicated the 736, lots of them sure would have gotten into cars, as back then it was oil & air filter and pcv valve every 3500 miles or so.

I don't see that you can rationalize a sample size of one in March that they were interchangeable.
I have lots of these in dated boxes from this period of time, just do not see it happening unless there is MORE data.

Now for "THE DATA"---
The 1973 AIM on 3-20-1972, a year prior, was signed off that the PCV valve for the L48 AND LS4 was 6487530 and 6484699 for the L82.

If neither of those part numbers are the the 736 valve, the I do not see how anyone can rationalize the 736 valve in a car 7 months after Start of Production.


On the valve in the picture - Please send the dimension from the rolled flange to the bottom, it does not look TFP for 1973...

[Joe L.]

Re: PCV 736C Sighting in a 1973 Corvette

What does the the 73 AIM say?

It's been 45 years, even one owner, can the memory be that exacting unless you have a receipt?

If they started 73 MY with the 746 valve, and you had a 73 dated service parts book, and SERVICE indicated the 736, lots of them sure would have gotten into cars, as back then it was oil & air filter and pcv valve every 3500 miles or so.

I don't see that you can rationalize a sample size of one in March that they were interchangeable.
I have lots of these in dated boxes from this period of time, just do not see it happening unless there is MORE data.

Now for "THE DATA"---
The 1973 AIM on 3-20-1972, a year prior, was signed off that the PCV valve for the L48 AND LS4 was 6487530 and 6484699 for the L82.

If neither of those part numbers are the the 736 valve, the I do not see how anyone can rationalize the 736 valve in a car 7 months after Start of Production.


On the valve in the picture - Please send the dimension from the rolled flange to the bottom, it does not look TFP for 1973...

Ronald------

A January, 1973 edition of the P&A catalog specifies the GM #6487532 for 1973 Corvette, except special high performance. The 6487532 is the SERVICE part number for the AC 769C valve. However, that does not necessarily mean that the AC 769C valve was always used for PRODUCTION. Engineering approval could have been obtained to substitute the 736C for the 769C, perhaps due to a shortage of the 769C at the time.

As I previously mentioned, the configuration of the AC-736C valve shown in the original post is a very old configuration. It's unlikely that the PCV valve would have been replaced during the time this configuration valve was available in SERVICE.

[Ronald L.]

[Joe L.]

Re: PCV 736C Sighting in a 1973 Corvette

Joe
The Corvette in 1973 was very low volume as compared to metal body high volume

Production shortage and substitution records would have surfaced

Both statistically very improbable

The machines that spun this metal to its shape by fall of 1972 could make the entirety of Corvette production in a week

No shortfall
too much hopium here

Next the valve in the picture was changed in 1969. How did a part four years old get in the car? Not in the factory

More like post 1974 when they started consolidating valve usage charts in service


And you can find a PCV hoses and valve setup on eBay right now that came from a 1973 car that was taken apart for its parts ten years ago

That’s the correct valve for 1973

Ronald------

The AC 769C was not exclusive to 1973 Corvettes. The same valve was used for virtually all other 1973 Chevrolet models.

I have no information that the configuration of the 736C seen in the original photo ended with the 1969 model year.

"Production shortage and substitution records would have surfaced"? I've never seen any for anything.

[Ronald L.]

[Tom R.]

Re: PCV 736C Sighting in a 1973 Corvette

It’s just not reasonable to believe something that wasn’t made for more than three years magically fell into a late 73 production car

I’d like to see the service records somewhere along the way the original valve was changed

Ron

You must have never coordinated/work on a technical manual revision team...these production irregularities, AIM departures are more common than you think. That's why we look at the cars and not neatly write judging manuals based on AIMs. In fact, TM, shared with me in Greenville that AIM sheets were pulled and replaced continually so when we have an AIM, its like a slice in time...a slice of bologna versus the entire loaf.

Too often, enthusiasts want everything to be absolute...no gray area. The 2nd edition of the 73-74 manual called out the 736C valve but the 3rd major revision found no evidence from a very small sample of vehicles used to confirm originality. Its possible supply ran out of the 769 or workers got to the bottom of the bin and grabbed a 736, which suggests that other vehicles left the factory as well, equipped with a 736C.

I could cite other examples we found in the revision work where listed part numbers were not in sync with what was shipped on 73/74 Corvettes. Manuals are not written to be absolute but reflect the production process and variation found in known original configurations.

[Ronald L.]

Re: PCV 736C Sighting in a 1973 Corvette

Tom, you never spend your years in the OEM design, manufacturing, and supplier world. It is a lot more cut and dry. I was there in the late 70's when you guys didn't give 2 cents for a mid-year save a split window. I went to the print file and pulled the prints to restored a 1966 in the time of 79-81 to the print. There were changes in prints, and AIM, and all that stuff was very well documented. Some people listening in have those books and documents.

FYI - as it comes to pcv, Many years ago, I talked extensively to the father (mgr)-son (engineer) that were responsible for the design and production of these valves. Other products too - different people.

Back to the 73 point however, there was not ONE single revision since finished by the engineer and signed off by the leader.

That means there were no field issue requiring and engineering revision, as happened with the early 427s, nor were there any needs to modify the manufacturing process.

And you are missing the point - that valve in the picture was NOT made post 1969.
Period.

That valve was not assembly line original to ANY GM product 1970 or later. It was Neutonium physics impossible. The tools were gone, converted to the smaller design.

Sorry if it bursts the hopium bubble, however I have plenty of evidence in my collection and research.

And Tom, take the hose off that valve, post of picture... if the top is solid, those were made between April 1967 and June 1968. 5 years off production cycle???

[Tom R.]

Re: PCV 736C Sighting in a 1973 Corvette

[

Tom, you never spend your years in the OEM design, manufacturing, and supplier world. It is a lot more cut and dry. I was there in the late 70's when you guys didn't give 2 cents for a mid-year save a split window. I went to the print file and pulled the prints to restored a 1966 in the time of 79-81 to the print. There were changes in prints, and AIM, and all that stuff was very well documented. Some people listening in have those books and documents.

Your post above is written in second person...either write in first person or third person. As written, it assumes I was there and cared.

And you are missing the point - that valve in the picture was NOT made post 1969.

Period.

Ok, so it was not produced after 1969! But that does not mean it was not in the supply chain. My point!

[Keith K.]

Re: PCV 736C Sighting in a 1973 Corvette

Tom -

Ronald sounds convincing to me. Realizing how often these were changed at routine service stops, it would be difficult to imagine an original surviving this long unless there was a concerted effort to save it. For grins I checked my '67 L71 - it has a 736C as it is supposed to, but I'm sure that it was replaced along the way.

Keith

[Joe L.]

Re: PCV 736C Sighting in a 1973 Corvette

Tom -

Ronald sounds convincing to me. Realizing how often these were changed at routine service stops, it would be difficult to imagine an original surviving this long unless there was a concerted effort to save it. For grins I checked my '67 L71 - it has a 736C as it is supposed to, but I'm sure that it was replaced along the way.

Keith

Keith------


If it's not of the configuration seen in the first photo, then you KNOW it was replaced. Want to replace your valve with an original configuration valve like the one pictured? Be prepared to spend some BIG $$$$$.

[Joe L.]

Re: PCV 736C Sighting in a 1973 Corvette

Tom, you never spend your years in the OEM design, manufacturing, and supplier world. It is a lot more cut and dry. I was there in the late 70's when you guys didn't give 2 cents for a mid-year save a split window. I went to the print file and pulled the prints to restored a 1966 in the time of 79-81 to the print. There were changes in prints, and AIM, and all that stuff was very well documented. Some people listening in have those books and documents.

FYI - as it comes to pcv, Many years ago, I talked extensively to the father (mgr)-son (engineer) that were responsible for the design and production of these valves. Other products too - different people.

Back to the 73 point however, there was not ONE single revision since finished by the engineer and signed off by the leader.

That means there were no field issue requiring and engineering revision, as happened with the early 427s, nor were there any needs to modify the manufacturing process.

And you are missing the point - that valve in the picture was NOT made post 1969.
Period.

That valve was not assembly line original to ANY GM product 1970 or later. It was Neutonium physics impossible. The tools were gone, converted to the smaller design.

Sorry if it bursts the hopium bubble, however I have plenty of evidence in my collection and research.

And Tom, take the hose off that valve, post of picture... if the top is solid, those were made between April 1967 and June 1968. 5 years off production cycle???

Ronald------


If the 736C valve was not manufactured in the configuration shown after 1969, then, assuming the valve pictured was a SERVICE replacement, how did the valve pictured get installed on this car sometime well after 1973 (when the car needed a replacement valve)? Could it have been really old stock that some parts store or dealer (incorrectly) supplied at the time? If so, the same sort of scenario could have played out in PRODUCTION.