1968 L 79 PCV valve

[Jeff P.]

I need an education on this. I was told I have a repo 2nd edition at a recent Regional. Can someone explain an original to a repo please. Thanks in advance Jeff.

[Joe L.]

Re: 1968 L 79 PCV valve

I need an education on this. I was told I have a repo 2nd edition at a recent Regional. Can someone explain an original to a repo please. Thanks in advance Jeff.

Jeff-----


What is the AC "CV" number of the valve you are using?

[Jeff P.]

Re: 1968 L 79 PCV valve

Thanks for reaching out Joe. CV 769 C IMG_4367.jpgIMG_4361.jpg

[Gary B.]

Re: 1968 L 79 PCV valve

Jeff,

This only posting by Joe might help you:

https://www.forums.ncrs.org/showthre...413#post362413

I searched for 769C and came up with a half dozen threads.

Gary

[Gary B.]

Re: 1968 L 79 PCV valve

Jeff,

This only posting by Joe might help you:

https://www.forums.ncrs.org/showthre...413#post362413

I searched for 769C and came up with a half dozen threads.

Gary

[Mark D.]

Re: 1968 L 79 PCV valve

Check out the latest edition of the 68/69 TIM&JG. It'll probably even show a picture.

[Gary B.]

Re: 1968 L 79 PCV valve

The Current 68-69 TIM&JG says..... All vehicles are equipped with an AC positive crankcase ventilation (PCV) valve #CV736C.
Jeff, Joe... What 68-69 engines used 769 PCV Valves?
Don't have any 68-69 small blocks handy to look at...
Gary B

[Joe L.]

Re: 1968 L 79 PCV valve

Thanks for reaching out Joe. CV 769 C [ATTACH=CONFIG]122738[/ATTACH][ATTACH=CONFIG]122739[/ATTACH]

Jeff-----


There's a very curious thing regarding PCV valves for 1968 Corvette small blocks-----GM never catalogued a valve in SERVICE for those applications! The PRODUCTION valve was GM #6424251. However, with respect to PCV valves, PRODUCTION GM part numbers are virtually never the same as SERVICE part numbers even though the valves, themselves, are usually identical and of the same AC "CV" number. To make matters worse, the PRODUCTION part number of 6424251 was unique to 1968 so we can't "extrapolate" with information from other model years.

However, AC, the GM division that actually manufactured the PCV valves, did catalog a valve in SERVICE for 1968 Corvette small blocks. That valve was AC CV-764C also known as GM #6487284. The strange thing about this valve is that I have seen known NOS examples that have two vastly different configurations-----one having a threaded base and another having a non-threaded base which installs into a rubber grommet. The 1968 Corvette version would have had to be the non-threaded style.

I have no information that the AC CV-769C was ever originally installed on a 1968 Corvette. From what I have been able to determine and as far as Corvettes go, the CV-769C was used only for 1973 Corvettes with L-48 or LS4. In PRODUCTION it was known as GM #6487830 and in SERVICE as GM #6487532.

I should also mention that it's very possible that the configuration of the grommet-mount CV-764C somewhat changed over time. Early versions might have had the "machined nipple" configuration as seen in early versions of the CV-736C and CV-746C used for many 1969 and later.

One more thing: why the difference in GM part number for PRODUCTION and SERVICE versions of the same "CV" number valve? Well, I certainly do not know for sure. However, AC Division was "famous" for doing this with many of the parts they manufactured. I've always believed it was due to the fact that the parts sent to PRODUCTION were packaged in "bulk" whereas the identical SERVICE parts were packaged in boxes.

[Joe L.]

Re: 1968 L 79 PCV valve

The Current 68-69 TIM&JG says..... All vehicles are equipped with an AC positive crankcase ventilation (PCV) valve #CV736C.
Jeff, Joe... What 68-69 engines used 769 PCV Valves?
Don't have any 68-69 small blocks handy to look at...
Gary B

Gary------


I too used to think that the CV-736C was used on 1968 Corvette small blocks until some research I did on this question. Now, I'm pretty sure they did not. From what I have found, the CV-764C was used for 1968 Corvette small blocks. 1969-72 Corvettes with base engine, L-46, LS-4, and LS-5 were equipped with the CV-736C. LT-1 and L-88 were equipped with the CV-746C.

I cannot find that any 1968-72 Corvette was originally equipped with a CV-769 PCV valve.

Determining the original PCV valve after 56 years by empirical means is, in my opinion, pretty much a "lost cause". These valves are often replaced as a matter of regular service and the vast majority of cars probably lost their original valves a LONG time ago. I would not be the least bit surprised if the common replacement was the CV-736C. That may even have lead to the modern day belief that the CV-736C was the original valve used.

[Jeff P.]

Re: 1968 L 79 PCV valve

Always learning. Thank you all.

[Joe L.]

Re: 1968 L 79 PCV valve

All-----

As I pondered this more, I came up with another possibility. It's possible that the original, 1968 small block PCV valve was actually a predecessor to the CV-764C. For several reasons, that makes sense to me. However, try as I may, I cannot come up with what that valve was. I do know that the PRODUCTION-utilized GM #6424251 was used only for 1968 and no other model year.

[Gary B.]

Re: 1968 L 79 PCV valve

Joe. Do you know what difference is between the 736 vs the 746? Is it the amount of vacuum it takes to open it?
Just curious, if it's a holley or a quadrajet difference. Hydraulic vs solid lifters?
Just wondering...
Gary B

[Joe L.]

Re: 1968 L 79 PCV valve

Joe. Do you know what difference is between the 736 vs the 746? Is it the amount of vacuum it takes to open it?
Just curious, if it's a holley or a quadrajet difference. Hydraulic vs solid lifters?
Just wondering...
Gary B

Gary-----


Yes, the installed PCV valve is generally related to the vacuum characteristics of the engine. Special high performance and heavy duty engines typically have camshafts with lift and overlap resulting in those engines producing generally lower vacuum. Of course, these same engines of the day typically are equipped with mechanical lifter camshafts and Holley carburetors. Engines with lower vacuum such as the L-71, LT-1, and L-88 were generally equipped with the CV-746C.

[Terry M.]

Re: 1968 L 79 PCV valve

Determining the original PCV valve after 56 years by empirical means is, in my opinion, pretty much a "lost cause". These valves are often replaced as a matter of regular service and the vast majority of cars probably lost their original valves a LONG time ago. I would not be the least bit surprised if the common replacement was the CV-736C. That may even have lead to the modern day belief that the CV-736C was the original valve used.

Joe
I beg to disagree with you regarding empirical evidence. The best evidence of how the cars were built are the cars themselves. Those of us who judge these cars occasionally have the opportunity to see low-milage, generally unmolested, Corvettes. It is fairly easy for an astute observer to identify replacement or even R & R PCV valves. If a number (pick a number) of Corvettes with the same engine type all have the same numbered and configured item one can draw the obvious conclusion. Any changes will show up as outliers. The documentation, if one can muddle through it (and you do an outstanding job of that. I am sure it is not easy.) all, is just dandy when it supports what we find on the cars; but when it doesn't I will "hang my hat" on what the cars tell us every time. However one has to get "out in the field" (That is the judging field.) to see these unique Corvettes. Generally they are not at the drive-in or mall car show -- although every once in a while I am rewarded with one of these "diamonds in the rough." The ensuing conversation can be very rewarding for both the owner and I.

The line workers didn't have the AIM sheet at their work Station. It was the job of QC Department to verify that production met the specifications. At French Lick we were fortunate to have a former St Louis worker who spent several years in QC tell us about the reality of the assembly line. He told the same stories as George Barlos & Phil Hawkens (both career QC workers at St. Louis Corvette) told me and half a dozen others for years. The workers pit the parts that were at hand on the cars. The line operators only called attention to what might have been the wrong part if it didn't fiy.

[Joe L.]

Re: 1968 L 79 PCV valve

Joe
I beg to disagree with you regarding empirical evidence. The best evidence of how the cars were built are the cars themselves. Those of us who judge these cars occasionally have the opportunity to see low-milage, generally unmolested, Corvettes. It is fairly easy for an astute observer to identify replacement or even R & R PCV valves. If a number (pick a number) of Corvettes with the same engine type all have the same numbered and configured item one can draw the obvious conclusion. Any changes will show up as outliers. The documentation, if one can muddle through it (and you do an outstanding job of that. I am sure it is not easy.) all, is just dandy when it supports what we find on the cars; but when it doesn't I will "hang my hat" on what the cars tell us every time. However one has to get "out in the field" (That is the judging field.) to see these unique Corvettes. Generally they are not at the drive-in or mall car show -- although every once in a while I am rewarded with one of these "diamonds in the rough." The ensuing conversation can be very rewarding for both the owner and I.

The line workers didn't have the AIM sheet at their work Station. It was the job of QC Department to verify that production met the specifications. At French Lick we were fortunate to have a former St Louis worker who spent several years in QC tell us about the reality of the assembly line. He told the same stories as George Barlos & Phil Hawkens (both career QC workers at St. Louis Corvette) told me and half a dozen others for years. The workers pit the parts that were at hand on the cars. The line operators only called attention to what might have been the wrong part if it didn't fiy.

Terry-----

I agree that empirical evidence is usually the best for determining originally installed components; I use it myself all the time. However, I just do not agree that it's reliable in the case of often replaced maintenance parts like PCV valves, oil filters, air filters, gasoline filters and the like.

I'm certain that 1969 small blocks were originally equipped with a CV-736C because that's what my car was originally equipped with. I'm reasonably sure that most 1970-72 small blocks were also equipped with the CV-736C because the AIM's for those years show the same PRODUCTION part number as for 1969. However, I'm not so sure about 1968 because the 1968 AIM shows a different PRODUCTION part number for the PCV valve.

Since GM never catalogued a valve for 1968 small blocks, it could very well be that parts countermen and repair shops ended up having to assume that the 1968 valve was the same as 1969.

As I mentioned previously, the AC catalog lists the CV-764C as the correct valve for 1968 Corvette small blocks. However, the very same AC catalog specifies the 1969 small block valve as the CV-736C.

Is it possible that 1968 small blocks were, in fact, originally equipped with the CV-736C PCV valve? Yes, it's possible. However, what I described above and previously makes me HIGHLY suspicious. And, like I say, even if I were to observe many, apparently low mile, original 1968 cars equipped with CV-736C valves, I'd still be highly suspicious that those were the valves originally supplied with the cars.

Note the attached photo of an NOS AC-CV-764C PCV valve. While many AC PCV valves are of different configuration, note how this valve, curiously, has the exact same configuration as the CV-736C and would install perfectly in a 1968 Corvette small block.




Screenshot 2024-06-20 at 00-13-08 AC DELCO# CV764-C NOS OEM PCV VALVE GM# 6487284 eBay.jpg