Sorting out the 66-67 427 first design & second design PCV valves

[Ronald L.]

These 1966 and 1967 427 PCV valve usage questions have come up now and then, with no clear evidence on what part was installed when.

Here's what I have been mulling over these last couple of months since the last time this came up:

Per the asy manuals we only have one part break point, 11-18-65, and that part continued for L36 and L72 in 67.

Why - for both years in the early service parts books, as well as those from the early 70's do we have:

66 - 67 (427) 1st design
66 - 68 (427 spc hi perf 2nd design) Sp HI perf = L72

??????????????????????????????????????????????????

The AIM does not break out L36 and L72 - and only has this one record of change, so why do all the parts books carry over this 1st design and 2nd design into 1967? Moreover why carry it into the subsequent years' editions as soecific line items?

This must mean that there WAS a difference in SOMETHING that caused for these separate line items. Was it functional design or a cost reduced design - maning maufacturing process of the valve changed?

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Does this have anything to do with California emissions and A.I.R. cars?
Everything on the top of the motor dressings are different, air cleaner, all the brackets, hoses - pcv hose too - why would the PCV valve NOT be different?

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Or since K19 shows as being used with L36, the L72 broke away from that base design?
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On the L72 carb, is the vacuum port different such that it required - or they thought at the time it required non continuous valve function?

For example, does this valve close (non C function) when under hard race type acceleration?

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1st design and 2nd design can mean two things from the parts we have seen:

1. Functionality - C=continuous or Non C= non continuous.

2. Mfg technology - initially fully machined - later parts hybrid - even later parts were a fully a stamped-tube type construction.

These are my thoughts:
Perhaps L36 and L72 BECAME different and this was just missed in the AIM.

66 up to the break point 691(solid steel)
66 after the break point AND 1967 - L36 - 691C (potentially later on part solid/ part stamped)

66 after the break point and 1967 - L72 - 736C (construction _?_) where by 1969 this valve was the thin metal stamped version, 1 5/8, and later in the 70's or 80's shortened to its current configuration.

Any 66 and 67 owners out there with very original cars care to comment, others with AC Spark Plug history ???

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Just a notation on a recent 691C valves seen for sale.

It has a stamped steel bottom, text on two lines, and a box date of 3-68-7.

What we know by this is that the 691C valve continued as a service part in '68 and the manufacturing technology to make it had changes to the newer die drawn thin steel form.

Another 691C valve that has been for sale for a very long time is the correct original configuration, box date 6-66-7.

In both cases they are called 'ventilation valve'.

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Has anyone noticed when the terminology changed on the AC SP Division boxes from "Ventilation Valve" to "PCV Valve"???

From original service parts I have of various model numbers...
1968 is the earliest box, June 68, that I have with the second terminology.

That one just happens to be a 736 valve for 427 Corvette...

[Michael H.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

The CV691 was first used on 65 passenger cars with 396, exc SHP. I think the machined body may have been used at that time.

Later, the CV691 became the CV691C. (the C was added some time after SOP of the 65 models) (see Bob Jorjorian for an explanation of the C)

I think the stamped shell CV691C would have been used in production for all 1966 427 pass car and Corvette engines through Nov 65 and slightly beyond.
The 736 was used after Nov 65 and continued use for both 390 HP and 425/435 HP engines.

I wouldn't pay too much attention to the parts books. Starting with the 66 printing, there were numerious errors throughout the book that continued into the 70's and beyond.

[William L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Are these the 2 culprits??

[Michael H.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Are these the 2 culprits??

That's interesting. I thought all 736C valves had a stamped steel shell.

Here's what an August 1966 printing of the GM parts book shows for the valve.

65-66 Pass w/396 (exc Sp/H/Perf)
66 Pass & Corvette w/427 (1st design)................6421981 (CV691)

66 Pass & Corvette (w/Sp/H/Perf) (2nd design).....6423695 (CV736C)

That sounds different than the description in later GM parts books. I think those folks were confused.

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

These are two of the culprits.

Michael,

It seems the 66 427 425 HP guys usually chase the 691 valve as original.

If you look at what I posted last year, and then again what you typed, it suggests the 691 valve was on the 390 HP. And 736 on the 425 HP, your thoughts???

[Michael H.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

These are two of the culprits.

Michael,

It seems the 66 427 425 HP guys usually chase the 691 valve as original.

If you look at what I posted last year, and then again what you typed, it suggests the 691 valve was on the 390 HP. And 736 on the 425 HP, your thoughts???

The description from the 66 printing doesn't even show a 66 pass with 325 HP. Only a 66 pass with 425 HP. That means something is incorrect in their description.

Bet the 736 was used for all pass and Corvette after Nov 65.

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Even though the 691 service part was first seen in a solid for, late 65 dated boxes, and as late as mid 68 in the solid top, stamped steel bottom form?

Michael, what is your though on what separated the first design from second design?

[Michael H.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Even though the 691 service part was first seen in a solid for, late 65 dated boxes, and as late as mid 68 in the solid top, stamped steel bottom form?

Michael, what is your though on what separated the first design from second design?

I have to wonder if valves used for service were solid shell until the supply was exhausted? Those for production may have seen the stamped shell first?? I don't know.

I've always wondered what the difference was between the 691 and 736 too.

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Given we can see very limited examples of both in machined steel, and millions of the 736 in stamped steel from the 70's, 80's when the actually said AC Rochester, and the the 90's AC Delco...

Both are the same size

And they are both C = continuous

Then the difference has to be the vac pull force to open the valve?

[William L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

The description from the 66 printing doesn't even show a 66 pass with 325 HP. Only a 66 pass with 425 HP. That means something is incorrect in their description.

Bet the 736 was used for all pass and Corvette after Nov 65.

On my June 67 built 435 it has a stamped steel 736 C like the one on the right in the picture. I'm 99% sure that it's original to the car?

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Hi Bill which one is correct, 99% sure or the rolling eyes?

[William L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

Hi Bill which one is correct, 99% sure or the rolling eyes?

Ron
Flip a coin.

[Ronald L.]

Re: Sorting out the 66-67 427 first design & second design PCV valves

From dated boxes, it is pretty clear stamping was used for the lower half 1968 and later. There is not one valve I have seen out of AC made 68 and later from a solid piece or pieces of steel. Interestingly as well it seems 64 and earlier were all black oxide treated, again machined from solid steel.

On a data point of one, in a box of used PCV valves where it seems the mechanic took out the new valve, put it in the car, and then put the used valves back in the box...some nice samples of original valves from the mid 60's and among them was a 68 dated 736 box with a used 736 valve inside, constructed with the 2 piece stamped metal. 'D' on the bottom, etc.