Broken rocker arm
Thread Starter
Heel & Toe
Joined: Nov 2013
Posts: 23
Likes: 2
Broken rocker arm
Broke a rocker arm on a 1970 LT1. Fresh motor but I had throttle stick open once and bounce motor off of rev limiter or maybe it was just valve float. Should I replace all of the rocker arms, the push rod does not appear to be damaged.
Thanks,
Phil
Thanks,
Phil
Team Owner
Joined: Mar 2002
Posts: 44,675
Likes: 1,830
From: Huntsville AL
This was from Joe Lucia at the NCRS discussion forum
I was in the parts gathering mode for an upcoming L82 rebuild on my 1973 that has guide plates so I needed the "NON" guided rocker to go back as OEM delivered. No problem I will just order 3 new ones, that is where I hit the wall. GM no longer makes the non guided rocker and the Sealed Power P865R was nowhere to be found. I found 4 NOS GM 3974290 rockers on eBay for $72.79 and bought those.
If your 70 LT1 is OEM original than you should have guide plates and need the non guided rockers that also should be the GM 3974290 rockers.
Dennis-------
Those 3 rocker arms need to be replaced.
The rocker arms originally installed on your engine were GM #3974290 and those are what you have pictured. The GM #3974290 was discontinued in July, 1989 and replaced by the GM #10089648 which remain available. The latter are a guided type rocker arm with impressed rails on the valve end which prevent lateral movement of the rocker, effectively centering it on the valve stem. However, these should not be used on engines with guide plates GM #3973418 which your L-82 should have. Actually, the guided rockers eliminate the need for guideplates. 1988-92 Corvettes used the 10089648 in PRODUCTION in combination with guide plates GM #10111771 which were mainly an assembly aid. These were eliminated after 1992.
By the way, the GM #10089648 are extremely high quality rocker arms but they are quite expensive with a current GM list price of about $35/each.
You can look for NOS GM #3974290 or you can obtain aftermarket rockers. Most of these are the non-guided type and virtually identical to your originals save for the "O" embossment. Sealed Power P865R is such a rocker arm.
Those 3 rocker arms need to be replaced.
The rocker arms originally installed on your engine were GM #3974290 and those are what you have pictured. The GM #3974290 was discontinued in July, 1989 and replaced by the GM #10089648 which remain available. The latter are a guided type rocker arm with impressed rails on the valve end which prevent lateral movement of the rocker, effectively centering it on the valve stem. However, these should not be used on engines with guide plates GM #3973418 which your L-82 should have. Actually, the guided rockers eliminate the need for guideplates. 1988-92 Corvettes used the 10089648 in PRODUCTION in combination with guide plates GM #10111771 which were mainly an assembly aid. These were eliminated after 1992.
By the way, the GM #10089648 are extremely high quality rocker arms but they are quite expensive with a current GM list price of about $35/each.
You can look for NOS GM #3974290 or you can obtain aftermarket rockers. Most of these are the non-guided type and virtually identical to your originals save for the "O" embossment. Sealed Power P865R is such a rocker arm.
If your 70 LT1 is OEM original than you should have guide plates and need the non guided rockers that also should be the GM 3974290 rockers.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
At first I thought piston / valve strike.
But then, after looking at the pushrod oiling hole, I would guess that rocker was running dry. Really dry.
Actually, none of them seem very oily. Not much oil in the head. Couple drops at the end of each rocker. But the rocker ball area looks dry also. And unless you cleaned the broken rocker with a shop towel, where's the oil?
So I would question if the lifters are truly getting enough oil and if not, then perhaps the lifter oil galley is plugged.
I would install another rocker. I would observe this cylinder bank at idle with valve cover off.
Then check the other head running
But then, after looking at the pushrod oiling hole, I would guess that rocker was running dry. Really dry.
Actually, none of them seem very oily. Not much oil in the head. Couple drops at the end of each rocker. But the rocker ball area looks dry also. And unless you cleaned the broken rocker with a shop towel, where's the oil?
So I would question if the lifters are truly getting enough oil and if not, then perhaps the lifter oil galley is plugged.
I would install another rocker. I would observe this cylinder bank at idle with valve cover off.
Then check the other head running
Last edited by HeadsU.P.; Jun 25, 2021 at 09:25 PM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
You can replace one with a good used one if you want to go that route. Let me know if you need one. I probaly have a bag of them out in the shop. A whole new set of nitro carb rockers is an option too.
If it was mine I would grab a good used one and put it back into service. That is what the book says to do.
Comp makes some nice cast steel roller tip rockers that I use on nearly everything these days. You might think about those. I could not say anything bad about them as long as you do not go over 350# of spring pressure over the nose. You are nowhere near that.
If it was mine I would grab a good used one and put it back into service. That is what the book says to do.
Comp makes some nice cast steel roller tip rockers that I use on nearly everything these days. You might think about those. I could not say anything bad about them as long as you do not go over 350# of spring pressure over the nose. You are nowhere near that.
Team Owner
Joined: Mar 2002
Posts: 44,675
Likes: 1,830
From: Huntsville AL
Some more Joe Lucia GM rocker arm data to go with your morning coffee.
There were 5 different rocker arms used on Chevrolet small blocks over the 1955-95 period. Each was also supercessive to the former for SERVICE. These are as follows:
1955-57----GM #3837175----I don't know what was stamped on these;(does anyone know for my edification?)
1958-63-----GM #3746241-----I don't know what was stamped on these; (does anyone know for my edification?)
1964-69-----GM #3843359-----These were usually stamped "O" but some late 60's examples were stamped with a "triangle";
1970-86-----GM #3974290-----Stamped with a "V"
1987-95-----GM #10089648-----Guided type rocker
1955-57----GM #3837175----I don't know what was stamped on these;(does anyone know for my edification?)
1958-63-----GM #3746241-----I don't know what was stamped on these; (does anyone know for my edification?)
1964-69-----GM #3843359-----These were usually stamped "O" but some late 60's examples were stamped with a "triangle";
1970-86-----GM #3974290-----Stamped with a "V"
1987-95-----GM #10089648-----Guided type rocker
Well, thanks to some wonderful assistance from another member I think I have some more of this puzzle figured out.
First of all, I now am virtually certain that the stamping on the end of the rocker arm is Chevrolet-assigned, specific to the rocker arm part number and has nothing to do with identification of the manufacturer.
Next is the fact there's something we have been missing here which we never figured out before even though we should have been able to do so from P&A Catalog information. I'm ashamed of myself. However, the 64-68 rocker arm, GM #3843359, is described simply as "arm, valve rocker". The GM #3974290 is described as arm, valve rocker (with ball)". So, that represents an obvious difference between the 3843359 and the 3974290 and, possibly, the ONLY difference between the two.
As a matter of fact and as it turns out, that was the only difference between the two parts from 1970 until 1974. The 3974290 was a UNIT which consisted of the 3843359 rocker arm + rocker ball and nut. Of course, the 3843359 rocker was apparently the one with the "o" embossment, so 3974290 available until 1974 had the "o" embossment.
In 1974 the components of the 3974290 UNIT changed per revision to the specifications for the unit. The 3843359 rocker was dropped and replaced by rocker arm GM #355940 which was not available separately. The 355940 rocker arm was specified to include the stamped letter "V" for identification. So, 3974290 during and after 1974 should have included a rocker arm with the "V" embossment.
First of all, I now am virtually certain that the stamping on the end of the rocker arm is Chevrolet-assigned, specific to the rocker arm part number and has nothing to do with identification of the manufacturer.
Next is the fact there's something we have been missing here which we never figured out before even though we should have been able to do so from P&A Catalog information. I'm ashamed of myself. However, the 64-68 rocker arm, GM #3843359, is described simply as "arm, valve rocker". The GM #3974290 is described as arm, valve rocker (with ball)". So, that represents an obvious difference between the 3843359 and the 3974290 and, possibly, the ONLY difference between the two.
As a matter of fact and as it turns out, that was the only difference between the two parts from 1970 until 1974. The 3974290 was a UNIT which consisted of the 3843359 rocker arm + rocker ball and nut. Of course, the 3843359 rocker was apparently the one with the "o" embossment, so 3974290 available until 1974 had the "o" embossment.
In 1974 the components of the 3974290 UNIT changed per revision to the specifications for the unit. The 3843359 rocker was dropped and replaced by rocker arm GM #355940 which was not available separately. The 355940 rocker arm was specified to include the stamped letter "V" for identification. So, 3974290 during and after 1974 should have included a rocker arm with the "V" embossment.
Race Director
Joined: Jul 2018
Posts: 10,589
Likes: 8,411
From: San Antonio, TX/Mahopac, NY
2026 Restomod of the Year Winner
2024 Corvette of the Year Finalist - Modified
2024 Corvette of the Year Finalist - Unmodified
2023 C7 of the Year Winner - Modified
Looking at your pictures, two things come to mind. Is that a bit of rust on the valve spring retainer? Are there others that look like that one? Makes me wonder if that pushrod oil hole or the corresponding hole in the rocker pushrod pocket were open. Definitely replace that pushrod and verify the new one is the exact same length. See if you can see through the damaged push rod oil hole or run a piece of mig wire through it. It just looks like that rocker was not getting all the oil it required, but it could be fine and it is just the pictures.
The second item is the end of the pushrod. You have solid lifters, so the only give is the valve lash adjustment. It is possible this one was slightly tight and the only one damaged, but I would remove all the rockers and inspect both the push rods and rocker arms for damage or excess wear. Frankly, you are going to need to check/adjust lash on all 16 anyway so this extra step is not too time consuming and gives you piece of mind.
Good luck!
The second item is the end of the pushrod. You have solid lifters, so the only give is the valve lash adjustment. It is possible this one was slightly tight and the only one damaged, but I would remove all the rockers and inspect both the push rods and rocker arms for damage or excess wear. Frankly, you are going to need to check/adjust lash on all 16 anyway so this extra step is not too time consuming and gives you piece of mind.
Good luck!
Last edited by Factoid; Jun 26, 2021 at 08:46 AM.
Thread Starter
Heel & Toe
Joined: Nov 2013
Posts: 23
Likes: 2
Thanks everyone one for all the information. There seems to be a film of oil on everything, I don't know what is considered normal. One thing I noticed is some the rockers have a very large gap to valve stem. I'll need to check all of those and adjust if necessary. Been a very long time since I adjusted valve lash on an engine but I'm sure I can find a good youtube video on how to do this. I'll pull all the rockers and pushrods and inspect, replace bad parts, adjust valves and go have some fun.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Rockers will not fail from being too tight but rockers can fail from being too loose. If loose, the valve train will be shocked at full lift ramp speed not the designed slower clearance ramp speed.
Lack of lube at the rocker can punch out the pushrod cup socket. Pushrod tip IS damaged but the arm is what failed first. This one is snapped apart.
My money is on some mechanical interference or just the fatigue of being too loose for too long.
Lack of lube at the rocker can punch out the pushrod cup socket. Pushrod tip IS damaged but the arm is what failed first. This one is snapped apart.
My money is on some mechanical interference or just the fatigue of being too loose for too long.
Team Owner
Joined: Mar 2002
Posts: 44,675
Likes: 1,830
From: Huntsville AL
Thanks everyone one for all the information. There seems to be a film of oil on everything, I don't know what is considered normal. One thing I noticed is some the rockers have a very large gap to valve stem. I'll need to check all of those and adjust if necessary. Been a very long time since I adjusted valve lash on an engine but I'm sure I can find a good youtube video on how to do this. I'll pull all the rockers and pushrods and inspect, replace bad parts, adjust valves and go have some fun.
DONT USE THIS METHOD FOR SOLID LIFTERS
Last edited by Mr D.; Jun 26, 2021 at 06:06 PM.
Team Owner
Joined: Mar 2002
Posts: 44,675
Likes: 1,830
From: Huntsville AL
Here is one for the "For what it's worth department"
After I cleaned up my used set of rockers, I discovered they were all the "triangle" configuration. Because I had access to a few NOS "o" rockers, I decided to take them to a nearby metalurgical lab and let them tell me if there were any differences in the two configurations.
Bottom line is there is a significant difference!
The Rockwell Scale A average value for the GM # 3843359" "triangle" tips was 36.7 where as Rockwell Scale A average value for the GM # 3974290 "o" tips was 70.7. For those in the know here, that is just north of 180 ksi. This territory is considered "ultra good stuff" by structural engineers. This supports the comments above from others regarding the "o" tip rockers.
The "290" rockers also have a polished tip right out of the box. I'll presume the polished tips reduce friction on the valve stem tip and make valve lash settings a little easiler. Regardless, I suspected they were special and now I know why...
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Just did this today on my 73, easy button. Used a remote start trigger to bump the engine around.
https://www.youtube.com/watch?v=5EGlb_VpTAw
https://www.youtube.com/watch?v=5EGlb_VpTAw
Race Director
Joined: Mar 2018
Posts: 10,707
Likes: 2,167
From: Golden Colorado
I would take them all off for close inspection. Check all push rod holes and blow out with air. Reassemble and run engine with valve cover off. There should be oil going everywhere, not just a film. What is your oil pressure at idle with engine warm? If oil is not flying everywhere you have a bigger problem.
Good Luck.
Good Luck.