1965 L78 and 1966 L72 factory connecting rods, dimple or not?

[Steven S.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

All------


One correction to my above posts: the 67-68 L-88 rod, GM #3909846, was NOT a floating pin type rod. So, therefore, it would have been functionally identical and machined identical to the 65-69 SHP rod, GM #3856240. Perhaps GM substituted the 3909846 for the 3856240 in some PRODUCTION SHP 427's.

Only the 1969 L-88 and ZL-1 used the floating pin style rods.

Joe, if GM did substitute them for the 3856240 rods, which is a perfectly reasonable explaination, then it was much more widespread than just some production SHP 427's. In all you passenger car circles, it's considered a fact that your L78's were equiped with 3/8 dimple rods.

Perhaps I missed it, but what was the actual differences then between the 3909846 and the 3856240 (aside from dimples, ridges, etc.)?

[Richard L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

I have a new over the counter L88 short block, cast date H197, block 321. The rods are 3/8" bolts, dimple rods and floating pins. See attached photos, note the original piston markings from the factory as this engine is new.

L88 pioston 1_1.jpgL88 piston 4_2.jpgL88 piston 5_3.jpg

[Richard F.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

I am looking at the 1976 revision of the Corvette Parts Catalog. Section 0.603 lists the 6240 rod as 65-69 SHP (ident. rib on con. rod cap) (blue) at $25.00 and the 9846 rod as 67-68 w/H.D. (pink) at $45.00 Section 0.639 Lists RETAINER, Piston Pin for 67-69 w/H.D.,AL. Cyl and case (427) at $.66.

[Steven S.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Joe, I did some checking and all references to a 3909846 rod are calling it a 3/8" bolt floating pin type.

Pat, I just looked over the rods out of this engine and ironically half of them are what you described and the 1965 design with one rib/bump on the cap, and the other half are the 1966 design.

Steve

[Joe L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Perhaps I missed it, but what was the actual differences then between the 3909846 and the 3856240 (aside from dimples, ridges, etc.)?

Steve-----


I was wondering that myself. However, my information that the 3909846 rod was not a floating pin-style rod is incorrect based on the information provided by Richard Lordo. So, I guess we're back to my original supposition that GM may have, at some point, used the forging used for the 3909846 rod to manufacture the 3856240 rod with the small end machined differently (i.e. the 3909846 for floating pins and the 3856240 for pressed pins).

Anyone out there have any KNOWN ORIGINAL 1965 L-78 rods not currently installed in an engine?

[Joe L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

I am looking at the 1976 revision of the Corvette Parts Catalog. Section 0.603 lists the 6240 rod as 65-69 SHP (ident. rib on con. rod cap) (blue) at $25.00 and the 9846 rod as 67-68 w/H.D. (pink) at $45.00 Section 0.639 Lists RETAINER, Piston Pin for 67-69 w/H.D.,AL. Cyl and case (427) at $.66.

Richard------

One curious thing here: the Group 0.639 retainer, GM #3942423, would have been a 1968 model year-released part number and it had no predecessor part number.

[Patrick B.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Steve-----


There may be some confusion as to what is meant by a "dimple". In general parlance and the way I use the term, the "dimple" refers to a small "bump" seen in the depressed section of the I-beam an inch, or so, below the small end.

The SHP 3/8" bolt rod was GM #3856240. This rod was used for all 1965-69 L-78, L-72, and L-71. As far as I know, it did not have any "dimples" as described above. It was discontinued without supercession in September, 1992.

The 3/8" bolt rod used for 1966-67 L-36 was GM #3856239. As far as I know, it had no "dimples" as described above. It was discontinued from SERVICE in February, 1969 and replaced by GM #3933174.

The 3/8" bolt rod used for 1968-69 L-36 and L-68 as well as 1970-74 LS-5 and LS-4 was GM #3933174. This rod had no "dimples" as described above. It was discontinued in November, 1981 and replaced by 3/8" bolt rod GM #14015328. The latter rod had no "dimples"

The GM #14015328 was discontinued in July, 1984 and replaced by 3/8" bolt rod GM #14075624. The latter rod had no "dimples".

The GM #14075624 was discontinued in July, 1994 and replaced by 3/8" bolt rod GM #10212764. The latter rod had no "dimples".

So, if there were 3/8" bolt rods that had "dimples", I don't know what they were. I don't ever recall seeing any but it is possible I missed something along the way.

One more thing I should add: 1967-68 Corvettes with L-88 used 3/8" bolt rod GM #3909846. I don't know anything about this rod. I suppose it's possible it had "dimples".

Unfortunately, rods do not have part numbers or forging numbers to help correlate them to the catalog number. Its seems like the marking of the 6240 SHP rods changed over the years. I have a collection of BB parts, including a 67 Chapparrel Aluminum 427 block, and I have seen lots of BB parts. All the 68-69 SHP rods I have seen look like the 68 L88 rods in Richards pictures with dimples on the side of the beams and two bumps on the caps, except they are for pressed pins. The difference between them and 9846 rods may be pressed vs floating pin, heat treatment or rod bolts . Interestingly, my 67 L-71 and Steve's 66 L-72 show the evolution in marking skeemes. MY 67 engine has one bank of 68 style rods appearing in March 67 and the other bank of 66-67 style rods with 2 bumps on the caps but no dimples, while Steves engine has one bank of 66-67 style rods and another bank of 65 style SHP rods with only one bump on the cap. GM must have kept the weight of the SHP rods constant to have used different styles in the same engine. Since all of my rods were factory numbered with the even ones of one style and the odd numbered ones of the other style they are consistent with having been original, and steve's engine is a second example of this practice at the factory.


.

[Bill W.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Joe , I have the rods and pistons from 65 396 # 16120 . It does not have dimples or ribs on the rod or the cap . It also is old enough it had bolts holding the windage tray instead of nuts .

[Wayne M.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Here's another tidbit, thanks to P&A30 catalogs, Gr 0.603.

Rev May 1965: 3856239 --- exc. Sp.H/Per (396) ie. The low-horse, one digit less than the 6240.
Rev Oct 1965: 3856239 --- Pass. Corvette (exc Sp. H/Per) 396, 427 (Ident rib on con. rod cap)
3856240 --- Pass. Corvette w/ Sp. H./Per (396, 427) -- no cap rib(s) comment

Rev Oct 1966 Same as Oct '65 (ie. the 6239 rod cap has the rib; the 6240 doesn't)

Rev Oct 1967 Same as Oct '65 (ie. the 6239 rod cap has the rib; the 6240 doesn't)

Rev July 1969 the low horse 3856239 has now been replaced by 3933174 (on 2-69). Only comment is "(orange)".
The 3856240 now has the comment "(ident rib on con. rod cap) (blue)"

So, seems to say that at the time of the service replacement of the low horse rod 3856239 by 3933174, the ident. rib on the cap was switched to the high horse rod cap ????

Attached pic is of a '65 low horse 396 (all '65-'66 396's used the 6223 cast crank). No ribs on rod caps. No dimples on the upper rod. Don't know the history, but motor has 0.030 TRW pistons, and one rod cap has weight added, so it's definitely been apart .

P.S. the "dimple" should really be called a "pimple".

[Steven S.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Gee thanks Wayne, and here I thought we were on the road to having this figured out (kidding). That does throw some doubt into the mix, however I've seen errors in those old parts manuals before, not to say that this is one of them.

I agree, dimple isn't accurate, however "pimple rod" just doesn't have the same ring.

[Joe L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Gee thanks Wayne, and here I thought we were on the road to having this figured out (kidding). That does throw some doubt into the mix, however I've seen errors in those old parts manuals before, not to say that this is one of them.

I agree, dimple isn't accurate, however "pimple rod" just doesn't have the same ring.

Steven and Wayne------

Sometimes the information shown in non-bold type of the "part description" column of the P&A Catalogs is "anachronistic"

[Joe L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Here's another tidbit, thanks to P&A30 catalogs, Gr 0.603.

Rev May 1965: 3856239 --- exc. Sp.H/Per (396) ie. The low-horse, one digit less than the 6240.
Rev Oct 1965: 3856239 --- Pass. Corvette (exc Sp. H/Per) 396, 427 (Ident rib on con. rod cap)
3856240 --- Pass. Corvette w/ Sp. H./Per (396, 427) -- no cap rib(s) comment

Rev Oct 1966 Same as Oct '65 (ie. the 6239 rod cap has the rib; the 6240 doesn't)

Rev Oct 1967 Same as Oct '65 (ie. the 6239 rod cap has the rib; the 6240 doesn't)

Rev July 1969 the low horse 3856239 has now been replaced by 3933174 (on 2-69). Only comment is "(orange)".
The 3856240 now has the comment "(ident rib on con. rod cap) (blue)"

So, seems to say that at the time of the service replacement of the low horse rod 3856239 by 3933174, the ident. rib on the cap was switched to the high horse rod cap ????

Attached pic is of a '65 low horse 396 (all '65-'66 396's used the 6223 cast crank). No ribs on rod caps. No dimples on the upper rod. Don't know the history, but motor has 0.030 TRW pistons, and one rod cap has weight added, so it's definitely been apart .

P.S. the "dimple" should really be called a "pimple".

Wayne-----


Here are some photos of NOS examples of the 3933174 rod. By the appearance of the GM parts labels on the boxes, these are quite old:



GM #3933174(1).JPGGM #3933174(2).JPGGM #3933174(3).JPGGM #3933174(4).JPG

[Joe L.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

And more:


GM #3933174(a).JPGGM #3933174(b).JPGGM #3933174(c).JPGGM #3933174(d).JPG

[Patrick B.]

Re: 1965 L78 and 1966 L72 factory connecting rods, dimple or not?

Wayne's picture is correct for a hydraulic cam big block. The rods with no bumps are very common and came in every non-SHP big block regardless of the confusing notes in the parts book.