What is diameter of push rod in 68/390hp?

[Terry F.]

Just wondering what the original diameter is for the push rod in 68/427 390hp. Also, any opinions on push rods? I like single piece design. I would like to know about materials to look for. Full harded steel, etc. My goal is light and strong. I like original stuff but if I can improve on design without any noticable visual difference, I like to improve things. Weight and strength are most important to me. Thanks, Terry

[Terry F.]

Re: What is diameter of push rod in 68/390hp?

I am just trying to get things together before I take everything apart. Thanks, Terry

[Clem Z.]

68 should be 3/8" dia. make sure you buy

push rods that are hardened or the guide plates will wear them thru. some after market ones are only hardened on one end so if you install them up side down they will wear thru.

[Joe L.]

Re: What is diameter of push rod in 68/390hp?

Terry-----

I think I've gone over this one with you previously. In any event, here goes again (or, for the first time if I haven't gone over it previously):

1968 Corvettes with L-36 or L-68 originally used 5/16" OD, "one piece" pushrods. These pushrods were of GM #3904376, intake, and 3904377, exhaust. These same pushrods were used for L1967 L-36/L-68 as well as all 1969 L-36/L-68.

Pushrod weight varies all over the place. It depends upon pushrod OD, pushrod tube wall thickness, and pushrod material. The latter is a minor factor since most steel alloys weigh about the same and, for a mass the size of a pushrod, the differences are pretty negligible.

I have pictured below NOS examples of several GM pushrods. I'm using exhaust pushrods since they're shorter and fit in the picture better for a close-up. Obviously, the intake pushrod that's part of the "set" is identical except for length. The uppermost is an original GM #3904377 exhaust pushrod. You will note that it is a "one piece" design, very similar to small block pushrods. This tip design is considered a "180 degree" design.

The second one down is a GM #346282 pushrod. It was used for all 1973-74 Corvette big block applications and replaced the 3904377 for SERVICE in 1975. You will note that it's a "welded-ball" design. Welded ball provides a "270 degree" pushrod tip. It also provides a perfectly round interface with the rocker arm and valve lifter seat.

The third pushrod down is a GM 3/8" pushrod. I provide it for purpose of size comparison only. This is a SERVICE-only pushrod (and, the most handy 3/8" big block exhaust pushrod I have).

The lower pushrod is is a GM 7/16" pushrod. I provide it for purpose of size comparison only. This is a SERVICE-only pushrod (and, the most handy 7/16" big block exhaust pushrod I have).

ALL GM PRODUCTION pushrods for big blocks, regardless of size and regardless of application, are manufactured from 1010 alloy steel. To be specific, this includes L-36 through ZL-1, LS-4 through 8.1L.

(Note: the lower 2 pushrods are NOT bent as they appear in the photo. They're brand new GM. There was some sort of "aberation" in the photo.)

[Joe L.]

Correction

I stated that "I used the exhaust pushrods because they are shorter". That is incorrect. Big block exhaust pushrods are LONGER than the intake pushrods; I don't know what I was thinking when I said that. I guess I really used the exhaust pushrods for the comparison because they were easier to get my hands on than the intake pushrods.

[Terry F.]

Re: What is diameter of push rod in 68/390hp?

Thanks for being patient with me. The reason I ask is because my engine appears to have all 3/8 inch push rods in it?? Sort of makes me wonder what is going on. They are all single piece design like that one in your picture. So, I am scratching my head a bit. Thanks for all the information. I will have to pop off my valve covers and take a peek a gain but I am sure they had that tapered head and were single piece. I am certain they were not any larger but definitely not 5/16. Thanks, Terry

[Terry F.]

Re: 68 should be 3/8" dia. make sure you buy

Clem, I am confused. Mine are 3/8 inch and the well informed Mr. Luchia is stating 5/16. I would suspect that 3/8 inch is a stiffer rod. Do you have a specific supplier that you could recomend?? Thanks, tErry

[Terry F.]

Re: What is diameter of push rod in 68/390hp?

Joe, what size push rod diameter was used in 67 covette 390hp? Thanks, Terry

[Clem Z.]

joe were the 65/66/67 ones 3/8"as i

was sure they changed the dia of the hyd lifter engines pushrods sometime during the BBC run from 3/8" to 5/16" but i thought it was much later than 68

[Joe L.]

Re: What is diameter of push rod in 68/390hp?

Terry----

Part of the problem here stems from the fact that GM NEVER cataloged a pushrod for 1968-69 L-36 and L-68 applications. However, we do know this:

E1967 L-36/L-68 used a 3/8 OD pushrod; L1967 L-36/L-68 used a 5/16" pushrod. All 1967 L-71 used the same 3/8" OD pushrod as early L-36/L-68;

1968-69 L-71 used a 3/8" pushrod which was of a "1 piece" design. Actually, it was not a true, 1 piece design inasmuch as it had pressed-in tips, but it APPEARS as a "1 piece" design.

1967-69 L-88 used a 7/16" OD pushrod. 1967-68 were of "welded ball" tips; 1969 used a "1 piece" design tip (actually, a "pressed-in" tip)

The above were, basically, all of the pushrods that were used in the 1967-69 period. So, the 1968-69 L-36/L-68 used either the 5/16" OD pushrods as were used for late 1967 L-36/L-68 OR they used the 3/8" pushrods used for 67-69 L-71.

"Somewhere along the way", it seems that I came upon some information that said that the 68-69 L-36/L-68 used the 5/16" pushrods. However, that may be incorrect. The strange thing is that the 3/8" pushrods for 1968-69 are clearly defined in all the 68+ P&A Catalogs at for "with SHP" applications (i.e. L-71). If they were for all applications except L-88, then I would have expected the catalog to just say "except HD".

It would be interesting to hear from some folks with known-original 1968-69 L-36/L-68 engines who've been into them. The problem, of course, is that push-rods are an often-changed sort of piece. So, it's hard to know what's original and what's not.

[Joe L.]

Re: What is diameter of push rod in 68/390hp?

Terry-----

Early used 3/8"; late used 5/16". However, once again, there are "miscues". Some Chevrolet parts references say that the 5/16" was used early and the 3/8" was used late.

[Joe L.]

Re: joe were the 65/66/67 ones 3/8"as i

clem-----

73-74 definitely used the 5/16" pushrod. However, 5/16" pushrods were also used earlier, too.

1970-72 all definitely used 3/8" pushrods.

The GM #3904376 and 3904377 pushrods were definitely used SOMETIME during the 1967 model year for L-36/L-68 engines and those pushrods are definitely 5/16". Based upon a little further research that I've done since my first post, they may or may not have been used for 1968-69 L-36/L-68. I think we'll need some "empirical" information to get to the bottom of this.

[Clem Z.]

i remember there was a change from working

on them back then but that was a long time ago so a lot has gotten lost in the fog since then

[Joe L.]

Re: 68 should be 3/8" dia. make sure you buy

Terry-----

Per a little further research and my posts below, I'm not so sure now that 68-69 L-36/L-68 did originally use 5/16" pushrods.

The whole deal with pushrod selection is that it's a "trade-off" sort of thing. The larger OD and thicker wall you get, the heavier the pushrod is going to be. Strength is good; weight is bad. So, one needs to settle on the right COMPROMISE for the particular application. Strength that's not needed for the application won't do you any good and, since it comes at the cost of weight, it will do harm.

For most street applications, a 3/8" OD pushrod, 0.080 wall thickness, and 4130 material will be the absolute maximum that one would need and represents a good balance between weight and strength. Comp Cams "Hi-Tech" line of pushrods fall into this category.

Quite frankly, though, a 3/8" OD, 1010 steel, and .060" wall thickness with welded ball ends will have all the strength you need them to have and will be a bit lighter. Also, keep in mind that welded ball is a PRODUCTION-validated design and is VERY reliable.

[Clem Z.]

here is the deal on the 5/16" push rods

the only 67 390-400 HP 427's that used the small 5/16" push rods were those built before 1 Sept 1966 for the 67 model run. After 1 Sept, they went back to the conventional 3/8", just like all the previous 65-66 engines and the 67 435. It was somewhat of an experiment and only the first batches of 67 engines built in Aug of 66 had the small 5/16" push rods.