pop23235

SB rocker arms, standard are forged "O" and SpHP are forged "A", delta, or check mark? Or do I have it backward? What are "no marking at all"?

The things I used to know!!!

Critter1

I think it's the other way around, Pop. The raised O was the HP rocker arm.

Mike Geary

The "O" rocker arms are specially processed to resist wear under severe valve train conditions. FI/365hp/Camaro Z-28 use these.

Now why did they choose "O"??

pop23235

Thanks guys. Critter, you are right I had it backward. Now my confusion. Original "O" rockers for 64 - 69 HiPo & Z are part number 3974290. That's probably not the original part number for the 64 as it is too high, but I purchased 16 of these back in ~75 and they came with a strange check mark or V with a short leg for lack of a better description. Certainly not the v I recall seeing on "common" rockers. So now I'm finally assembling the engine and see this mark, think maybe wrong part, but it's the right number. Checking what I see on Ebay, for what it's worth, used ones with this part number have the "O" and new ones have this strange V or check mark. Guess I'm wondering what do I have?

I have maybe 8 original "O" that are used, but not enough. I don't readily see a difference, but if the difference is metallurgical, it wouldn't show anyway. I also have a set with no mark whatsoever from my 63 327/300 that are definitely original. Really would like to use the new 3974290's on the 365, but would like to get my mind around these markings. Apparently there are more than "O" and "V" markings.

Mike Geary

When I rebuilt my '65 365hp motor, I was directed to the Crane "nitro carb" rockers as the closest thing to original "O" rockers. Take a look.

Critter1

The 3974290 rocker arms were first released in about 1969. They had a polished pallet end. Later 3974290's did not. The earlier "O" arms used from 63 to 69 didn't either. (can't remember the old part number, 3846xxx?) I don't know anything about the newer rocker arms with the different letter code but I suppose if GM lists them for earlier HP engines, they're probably ok.

If I remember correctly, the only rocker arms that GM sold over the counter for service in the 60's and 70's were those made for HP engines. That arm was used as a replacement for all engines.

Edit: There's a set of 8 NOS 3974290's on ebay right now for $93. https://www.ebay.com/itm/NOS-Genuine...paDbCG&vxp=mtr

pop23235

I think my old parts manager told me that— replacements were the equivalent of HD. Those on EBay have the “V” and the correct part number. Think I’ll have to move on. I can pick up some good used “O’ ones along as I find them.