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

[Steven S.]

I recently have taken apart a december 1966 L72 engine that appears to have retained many original components. To my surprise there were no "dimples" on the rods, however, the caps were unlike any standard BB rod cap I've seen. The caps appear identical to the caps found on the dimple rods with what I would describe as a rib on either side of the cap at about the six o-clock position when viewed installed on the rod. I did some internet searches and found that some suggest that the actual dimple did not show up until 1967 (or perhaps late 1966). I read through some of my books and found in a 6th edition Chevrolet power catalog this;
"Chevrolet offers a high-performance connecting rod with 3/8-inch fasteners as part number 3856240. This rod was used in 1965-69 high-performance 396 and 427ci big-blocks. It is Magnaflux inspected at the factory, and finished for pressed piston pins. It can be identified by a rib on it's cap and a blue color code."
Any insight from those who have taken apart original L78 and L72's?

Steve

[Joe L.]

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

I recently have taken apart a december 1966 L72 engine that appears to have retained many original components. To my surprise there were no "dimples" on the rods, however, the caps were unlike any standard BB rod cap I've seen. The caps appear identical to the caps found on the dimple rods with what I would describe as a rib on either side of the cap at about the six o-clock position when viewed installed on the rod. I did some internet searches and found that some suggest that the actual dimple did not show up until 1967 (or perhaps late 1966). I read through some of my books and found in a 6th edition Chevrolet power catalog this;
"Chevrolet offers a high-performance connecting rod with 3/8-inch fasteners as part number 3856240. This rod was used in 1965-69 high-performance 396 and 427ci big-blocks. It is Magnaflux inspected at the factory, and finished for pressed piston pins. It can be identified by a rib on it's cap and a blue color code."
Any insight from those who have taken apart original L78 and L72's?

Steve

Steve-----


The rod you are describing is the GM #3856240 as mentioned in the Chevrolet Power Manual. This rod had the ribbed cap. The only "dimple" rods I am aware of had 7/16" rod bolts (as well as the ribbed cap). There were several different versions of the 7/16" bolt rods. The most common are the 1969 L-88/ZL-1 rod which was designed for floating piston pins and had smooth-machined, boron steel bolts and the LS-6 rod which was designed for press-fit piston pins and had knurled shank rod bolts. The LS-6 rod also had a somewhat narrower beam section than the 1969 L-88/ZL-1 rod.

[Steven S.]

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

Joe,
I know we have discussed this before, but if you were to do some web searches on the subject I believe you will find enough info from auctions and prior discussions on the topic of 3/8 dimple rods to support their existence. I can only assume that it's a later variation of that 3856240 rod.

[Wayne M.]

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

Steve, Joe -- FWIW, the photography in the 1965 Chevrolet Interim Shop Manual ST-71 (ie. mid-season) introduction of the Mark IV big block, seems to show rods with no dimples, and no small rib on the front and rear sides of the connecting rod caps. The engine in the pictures obviously has 4-bolt mains with X-drilled [main-to-rod journal] crankshafts.

Many of the same photographs were probably re-used in the 1966 Overhaul Manual (ST-73), but one (Fig. 58) is new.

P.S. Steve -- did you mean to say a December 1965 L72 ?

[Steven S.]

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

Wayne, I sure did mean December 1965, my bad.

[Richard F.]

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

The rods that came out of my 68 have 3/8 bolts and a dimple. The only ribs I see are on the cap at both parting lines, at what I would call 3 and 9 o'clock of the big end opening. These rods require piston circlips.

[Joe L.]

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

Joe,
I know we have discussed this before, but if you were to do some web searches on the subject I believe you will find enough info from auctions and prior discussions on the topic of 3/8 dimple rods to support their existence. I can only assume that it's a later variation of that 3856240 rod.

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".

[Joe L.]

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

The rods that came out of my 68 have 3/8 bolts and a dimple. The only ribs I see are on the cap at both parting lines, at what I would call 3 and 9 o'clock of the big end opening. These rods require piston circlips.

Richard-----

All 1968-68 L-36 and L-68 originally used rod GM #3933174. This rod definitely had no "dimples". Also, if by "circlips" you mean piston pin retainers, no Corvette big block except L-88/ZL-1 ever originally used floating pin rods requiring piston pin retainers.

[Patrick B.]

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

The bottom of the rod caps on hydraulic lifter big blocks have a ground surface that is rectangular like on a small block, but the heavier duty rods on solid lifter big blocks have indicators. On 66 up HD rods both 3/8 and later 7/16 bolt rods, there are bumps on the long sides of the ground rectangle. In addition, all 68 and later HD rods have dimples on the sides of the beam. I believe these dimples started during the 67 model year. My original 67 L-71 with a build date T0309JE has all rods with bumps on the caps, but the rods on one bank also have dimples like 68 rods. All the rods are properly numbered, appear original in every way. I also had some rods from an original 65 L-78, and the caps had a bump on only one side of the ground rectangle rather two sides. I am in Florida now, but I can take some pictures when I get home next week.

[Richard F.]

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

I am attaching pics of the rods I am talking about. They came out of a 68 L88 motor. The floating pin pistons that they were mated to are .060 over, so I can't guarantee they are the original rods, but they are floaters and they are 3/8 and they do have dimples. The rods a little out of focus and greasy.

[Joe L.]

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

I am attaching pics of the rods I am talking about. They came out of a 68 L88 motor. The floating pin pistons that they were mated to are .060 over, so I can't guarantee they are the original rods, but they are floaters and they are 3/8 and they do have dimples. The rods a little out of focus and greasy.

Richard-----


1967-68 L-88 used rod GM #3909846. All I know about this rod is that it used 3/8" rod bolts. Apparently, though, it is the 3/8" bolt rod that had dimples.

It's also very possible that GM may have, at some point, used this forging to manufacture the 3856240 rods, the difference being machining for press fit rather than floating pins.

[Richard F.]

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

Joe, I didn't get very good pics. If there is some detail you would like to see, I'd be happy to try again.

[Steven S.]

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

Thank you to all who have replied, here is a picture of the "rib" as I was describing it or the "bump" as Pat described it. This is on a 66 rod and the feature is on both sides of the cap. Pat, thank you so much for the clarification, you confirmed what I pieced together from various discussion, but I wasn't aware of the 1965 variation.

[Joe L.]

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 L.]

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

Joe, I didn't get very good pics. If there is some detail you would like to see, I'd be happy to try again.

Richard------


They're OK. They show the details we're concerned with here.