In the last few months, or so, I've been doing a little research on the above-referenced topic. I've discovered some interesting things, and I thought I'd present my findings.
First of all, to understand this topic, one needs to understand the THREE different models of spin-on-type oil filters used on 60's Chevrolets. Most folks with Corvettes think that the 1968 model year was the first model year which used the spin-on type filters. For Corvettes, that's true. However, spin-on type filters were used much earlier on other Chevrolet models.
The first use of the spin-on filter for a Chevrolet product was the the 1962 Chevy II L-6 engine. In 1963, the use spread to L-6 engines used in most other Chevrolet 6 cylinder applications. For 1964, the first use on a V-8 occurred with the Chevy II 283 cid engine. In order to accomodate chassis requirements, these engines used a special Chevy II-only block with a raised oil filter pad, too.
In 1968, the use spread across all Chevrolet lines for both L-6 and V-8. The only exceptions were certain taxi and police applications which continued to use a version of the old cannister style filter for a few more years.
The oil filter used in PRODUCTION and specified in SERVICE for the 1962 Chevy II L-6 application, most 1963-67 L-6 applications, and 1964-67 Chevy II with V-8 applications was the AC PF-11, GM #5578052. This is a filter which is about the same diameter as the AC PF-25 but with a length of 5-1/2", about the same as the much later PF-35. Its configuration is somewhat different than later filters inasmuch as it has a different type crimp seal at the top and lacks the "flutes" at the bottom for the oil filter wrench. Instead, it has different sorts of "indentions" at the bottom, the purpose for which I have no idea. All versions of it that I've seen are white background, with blue printing and a completely red "AC" logo. There is no embossment on the bottom of the filter. Thanks to contacts in the Chevy II world, I have seen versions of it manufactured in 1962, 1963, 1964, 1965, 1966, and 1967. ALL are the same.
In September of 1968, the PF-11 was discontinued from SERVICE and replaced by the PF-29, GM #6437946. The PF-29 is an oil filter which is the same diameter as the PF-25. It uses the same style crimp seal as the PF-25. However, it is 5-1/2" long, just like the PF-11 and the later PF-35. It has the same style "flutes" at the bottom as we are accustomed to seeing on early versions of the PF-25. I have several NOS examples of it manufactured in 1968 [none for sale]. All are white background with blue printing, an all-red "AC" logo, and no embossments on the bottom.
In January, 1970 the PF-29 was discontinued from SERVICE and replaced by the PF-25. However, BOTH the PF-29 and the PF-25 were released and existed PRIOR to the time that they replaced the filters that they superceded. The dates given above are the dates that the last GMSPO inventory of the earlier filters "went out the door".
The PF-11 and PF-29 were, basically, "first generation", paper-element technology filters with respect to the filtering medium used within them. The PF-25 was a filter which used "second generation" filtering technology.
After considerable research I have reached the conclusion that the PF-29 was the oil filter used in PRODUCTION for ALL 1968 applications which used the spin-on type filter, INCLUDING CORVETTE. At this point, I am virtually 100% certain of this. Here's why:
1) The 1968 Corvette AIM specifies the oil filter used as GM #6437955. This is the PRODUCTION part number for the PF-29 oil filter. The 1969 Corvette AIM also shows this part number as the initial part number for 1969 applications, so there's virtually no chance that a later revision to the 1968 AIM included a supercession that we don't know about;
2) The PF-25, which we have all regarded as being the filter used for 1968 applications, DID NOT EVEN EXIST at the beginning of the 1968 model year in August, 1967. As a matter of fact, it DID NOT EXIST as of the END of the 1968 model year in August, 1968. I can find no record of its RELEASE or EXISTENCE as of as late as October 1, 1968 which was well into the 1969 model year.
3) The only possible PRODUCTION application for the PF-25 prior to 1968 was for the Chevy II applications. Initially, even though I could find no record of its release or existence, I thought that it might have been used in this application, due to its shorter length, in conjunction with the raised oil filter pad. With the help of several Chevy II afficionados and experts, I have determined that the PF-25 was never used on a PRODUCTION 1964-67 Chevy II application or a 1968 Chevy II application. The oil filter used in PRODUCTION for all 1964-67 Chevy IIs was the PF-11. The oil filter used in PRODUCTION for the 1968 Chevy II was the PF-29. In September, 1968, the PF-29 became the SERVICE oil filter for all 1962-67 Chevy II applications.
Of course, it's virtually impossible to verify the above based upon observation of original cars. In fact, that's how I believe that the BELIEF that the PF-25 was used on 1968s occurred in the first place-----there are no cars extant which are fitted with their original oil filters so everyone just ASSUMED that the PF-25 had been used all along. It hasn't, though.
My current challenge is not to do further research on the 1968 oil filter situation. I'm VERY, VERY confident that the PF-29 was originally used for all 1968s, so further reserach would be like O.J. Simpson's search for the "real killers". My current challenge is to determine when in the 1969 model year that change from the PF-29 to the PF-25 occurred. As I mentioned, the 1969 AIM specifies the GM #6437955 as the oil filter initially used for 1969 applications. This is the PRODUCTION part number for the PF-29 oil filter. Dated 8-9-68 is a revision showing a change from the 6437955 to the GM #6437055. This is the PRODUCTION part number for the PF-25 oil filter, known in SERVICE as GM #6438261. The GM #6437055 then continued to be used in PRODUCTION right through the end of C3 production in 1982.
The 8-9-68 date would likely translate into a production-line change no earlier than about October 1. As I say, though, I can find no information that the PF-25 had been released or existed by that date, so it was likely sometime after that that the change actually occurred. I know that by October 1, 1969, the PF-25 was specified as the SERVICE oil filter for all 68-69 Corvettes, so it had to have happened sometime between October 1, 1968 and October 1, 1969, even though the PF-25 did not officially, formally, and finally replace the PF-29 for SERVICE until January, 1970. Trying to "zero in" on the PRODUCTION changeover date will be a daunting task, though. However, I'm reasonably confident that at least very early 1969s were equipped with the PF-29 and NOT the PF-25.
I have one "ace-in-the-whole" that might help here. As I've mentioned previously, I have a friend who is the original owner of his October-1968-built L-46 coupe. It so happens that this guy is a real "pack rat" and he SAVED his original oil filter when he changed the oil for the first time. He can't locate it right now (an affliction that all pack-rats inevitably suffer from) but he says that he KNOWS that he still has it somewhere. He told me that his recollection is that the original filter was longer than the first replacement filter he installed AND he has a memory like a STEEL TRAP which he has demonstrated to my amazement over the years. I'm after him DAILY to find the filter, though.
Regardless of how this 1969 research goes, I remain absolutely confident that the 1968 oil filter was the PF-29. In fact, I'm so confident and I think that the evidence is so compelling that I think that we ought to change the 1968-69 Judging Guide at the next edition and recognize the PF-29 filter as the only correct filter for 1968 Corvettes.
In case anyone's wondering how the PF-35 figures into this whole thing, the answer is that it doesn't. The PF-35 was released in 1970 and didn't replace any other filter. It's first and only use is for certain MD and HD truck applications beginning in 1970. Even though it's almost exactly the same size as the earlier PF-11 and PF-29 filters, it DID NOT replace those filters; the shorter PF-25 replaced those filters. The initial PF-35 used the same "second generation" filtering media as the PF-25, though; just more of it.
First of all, to understand this topic, one needs to understand the THREE different models of spin-on-type oil filters used on 60's Chevrolets. Most folks with Corvettes think that the 1968 model year was the first model year which used the spin-on type filters. For Corvettes, that's true. However, spin-on type filters were used much earlier on other Chevrolet models.
The first use of the spin-on filter for a Chevrolet product was the the 1962 Chevy II L-6 engine. In 1963, the use spread to L-6 engines used in most other Chevrolet 6 cylinder applications. For 1964, the first use on a V-8 occurred with the Chevy II 283 cid engine. In order to accomodate chassis requirements, these engines used a special Chevy II-only block with a raised oil filter pad, too.
In 1968, the use spread across all Chevrolet lines for both L-6 and V-8. The only exceptions were certain taxi and police applications which continued to use a version of the old cannister style filter for a few more years.
The oil filter used in PRODUCTION and specified in SERVICE for the 1962 Chevy II L-6 application, most 1963-67 L-6 applications, and 1964-67 Chevy II with V-8 applications was the AC PF-11, GM #5578052. This is a filter which is about the same diameter as the AC PF-25 but with a length of 5-1/2", about the same as the much later PF-35. Its configuration is somewhat different than later filters inasmuch as it has a different type crimp seal at the top and lacks the "flutes" at the bottom for the oil filter wrench. Instead, it has different sorts of "indentions" at the bottom, the purpose for which I have no idea. All versions of it that I've seen are white background, with blue printing and a completely red "AC" logo. There is no embossment on the bottom of the filter. Thanks to contacts in the Chevy II world, I have seen versions of it manufactured in 1962, 1963, 1964, 1965, 1966, and 1967. ALL are the same.
In September of 1968, the PF-11 was discontinued from SERVICE and replaced by the PF-29, GM #6437946. The PF-29 is an oil filter which is the same diameter as the PF-25. It uses the same style crimp seal as the PF-25. However, it is 5-1/2" long, just like the PF-11 and the later PF-35. It has the same style "flutes" at the bottom as we are accustomed to seeing on early versions of the PF-25. I have several NOS examples of it manufactured in 1968 [none for sale]. All are white background with blue printing, an all-red "AC" logo, and no embossments on the bottom.
In January, 1970 the PF-29 was discontinued from SERVICE and replaced by the PF-25. However, BOTH the PF-29 and the PF-25 were released and existed PRIOR to the time that they replaced the filters that they superceded. The dates given above are the dates that the last GMSPO inventory of the earlier filters "went out the door".
The PF-11 and PF-29 were, basically, "first generation", paper-element technology filters with respect to the filtering medium used within them. The PF-25 was a filter which used "second generation" filtering technology.
After considerable research I have reached the conclusion that the PF-29 was the oil filter used in PRODUCTION for ALL 1968 applications which used the spin-on type filter, INCLUDING CORVETTE. At this point, I am virtually 100% certain of this. Here's why:
1) The 1968 Corvette AIM specifies the oil filter used as GM #6437955. This is the PRODUCTION part number for the PF-29 oil filter. The 1969 Corvette AIM also shows this part number as the initial part number for 1969 applications, so there's virtually no chance that a later revision to the 1968 AIM included a supercession that we don't know about;
2) The PF-25, which we have all regarded as being the filter used for 1968 applications, DID NOT EVEN EXIST at the beginning of the 1968 model year in August, 1967. As a matter of fact, it DID NOT EXIST as of the END of the 1968 model year in August, 1968. I can find no record of its RELEASE or EXISTENCE as of as late as October 1, 1968 which was well into the 1969 model year.
3) The only possible PRODUCTION application for the PF-25 prior to 1968 was for the Chevy II applications. Initially, even though I could find no record of its release or existence, I thought that it might have been used in this application, due to its shorter length, in conjunction with the raised oil filter pad. With the help of several Chevy II afficionados and experts, I have determined that the PF-25 was never used on a PRODUCTION 1964-67 Chevy II application or a 1968 Chevy II application. The oil filter used in PRODUCTION for all 1964-67 Chevy IIs was the PF-11. The oil filter used in PRODUCTION for the 1968 Chevy II was the PF-29. In September, 1968, the PF-29 became the SERVICE oil filter for all 1962-67 Chevy II applications.
Of course, it's virtually impossible to verify the above based upon observation of original cars. In fact, that's how I believe that the BELIEF that the PF-25 was used on 1968s occurred in the first place-----there are no cars extant which are fitted with their original oil filters so everyone just ASSUMED that the PF-25 had been used all along. It hasn't, though.
My current challenge is not to do further research on the 1968 oil filter situation. I'm VERY, VERY confident that the PF-29 was originally used for all 1968s, so further reserach would be like O.J. Simpson's search for the "real killers". My current challenge is to determine when in the 1969 model year that change from the PF-29 to the PF-25 occurred. As I mentioned, the 1969 AIM specifies the GM #6437955 as the oil filter initially used for 1969 applications. This is the PRODUCTION part number for the PF-29 oil filter. Dated 8-9-68 is a revision showing a change from the 6437955 to the GM #6437055. This is the PRODUCTION part number for the PF-25 oil filter, known in SERVICE as GM #6438261. The GM #6437055 then continued to be used in PRODUCTION right through the end of C3 production in 1982.
The 8-9-68 date would likely translate into a production-line change no earlier than about October 1. As I say, though, I can find no information that the PF-25 had been released or existed by that date, so it was likely sometime after that that the change actually occurred. I know that by October 1, 1969, the PF-25 was specified as the SERVICE oil filter for all 68-69 Corvettes, so it had to have happened sometime between October 1, 1968 and October 1, 1969, even though the PF-25 did not officially, formally, and finally replace the PF-29 for SERVICE until January, 1970. Trying to "zero in" on the PRODUCTION changeover date will be a daunting task, though. However, I'm reasonably confident that at least very early 1969s were equipped with the PF-29 and NOT the PF-25.
I have one "ace-in-the-whole" that might help here. As I've mentioned previously, I have a friend who is the original owner of his October-1968-built L-46 coupe. It so happens that this guy is a real "pack rat" and he SAVED his original oil filter when he changed the oil for the first time. He can't locate it right now (an affliction that all pack-rats inevitably suffer from) but he says that he KNOWS that he still has it somewhere. He told me that his recollection is that the original filter was longer than the first replacement filter he installed AND he has a memory like a STEEL TRAP which he has demonstrated to my amazement over the years. I'm after him DAILY to find the filter, though.
Regardless of how this 1969 research goes, I remain absolutely confident that the 1968 oil filter was the PF-29. In fact, I'm so confident and I think that the evidence is so compelling that I think that we ought to change the 1968-69 Judging Guide at the next edition and recognize the PF-29 filter as the only correct filter for 1968 Corvettes.
In case anyone's wondering how the PF-35 figures into this whole thing, the answer is that it doesn't. The PF-35 was released in 1970 and didn't replace any other filter. It's first and only use is for certain MD and HD truck applications beginning in 1970. Even though it's almost exactly the same size as the earlier PF-11 and PF-29 filters, it DID NOT replace those filters; the shorter PF-25 replaced those filters. The initial PF-35 used the same "second generation" filtering media as the PF-25, though; just more of it.
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