Rocker arm nut backing off

[Michael L.]

OK, got the '55 running! Adjusted valves while running and a few minutes later started to get small backfiring through carb. Removed valve cover and found that #2 exhaust rocker arm nut had backed off allowing rocker arm to jump off pushrod. These are supposed to be self locking right? Hopefully a new nut will cure the problem. I hope break in is OK with all the fumbles on getting this going. I slathered cam and lifters with cam lube and ran a bottle of break in fluid as well...

Mike

[Joe L.]

Re: Rocker arm nut backing off

OK, got the '55 running! Adjusted valves while running and a few minutes later started to get small backfiring through carb. Removed valve cover and found that #2 exhaust rocker arm nut had backed off allowing rocker arm to jump off pushrod. These are supposed to be self locking right? Hopefully a new nut will cure the problem. I hope break in is OK with all the fumbles on getting this going. I slathered cam and lifters with cam lube and ran a bottle of break in fluid as well...

Mike

Mike-----


Yes, the nuts are supposed to be self-locking. However, if they are original nuts or well-used, they have likely lost a lot of their self-locking ability. I always recommend using NEW self locking nuts when an engine is rebuilt. They are not very expensive. Another possibility is a stud so worn that the nuts will not lock.

Another less likely possibility is that the nuts are not backing out but the stud(s) ARE "backing out". This can be a problem with original-style, pressed-in studs. However, I consider it unlikely in your case. If the new nuts don't solve the problem, though, this is the next place I'd look.

[Clem Z.]

Re: Rocker arm nut backing off

back in the day of using selflocking? adjustment nuts i always used "pal" locking nut as a back up lock nut on top of the regular adjustment nuts

[Jim S.]

Re: Rocker arm nut backing off

I ordered a new 1965 Impala with the 396 engine. It arrived at the dealer here in Saginaw, Mi in May 1965. After about 6 months one of the rocker arm nuts started to back off. When I took the valve covers off, I found that several of the torque prevailing nuts were cracked! As I remember the original nuts had a thin neck on the top with a distorted thread. The crack occurred right through the thin section and the nut lost its torque prevailing characteristics.

I gave one of the broken nuts to a metalurgist at Saginaw Steering Gear. He indicated that the crack was caused by hydrogen embrittlement. After the nuts were plated there was a draw operation that was missed. The nuts cracked in the highly stressed thread area over time. The replacement nuts from Chevrolet were of a somewhat different design.

Jim

[Stuart F.]

Re: Rocker arm nut backing off

All good advice. I recall some 55's used a different type of nut, possibly with a lock nut, but not a "pal" nut. They were infamous for backing off. The lock nuts looked almost like they had a triangular shaped pinch on the top. Even with them, specially with solids, we always added "pal" nuts to hold the adjustments.

Stu Fox

[Joe L.]

Re: Rocker arm nut backing off

I ordered a new 1965 Impala with the 396 engine. It arrived at the dealer here in Saginaw, Mi in May 1965. After about 6 months one of the rocker arm nuts started to back off. When I took the valve covers off, I found that several of the torque prevailing nuts were cracked! As I remember the original nuts had a thin neck on the top with a distorted thread. The crack occurred right through the thin section and the nut lost its torque prevailing characteristics.

I gave one of the broken nuts to a metalurgist at Saginaw Steering Gear. He indicated that the crack was caused by hydrogen embrittlement. After the nuts were plated there was a draw operation that was missed. The nuts cracked in the highly stressed thread area over time. The replacement nuts from Chevrolet were of a somewhat different design.

Jim

Jim-----


Over the years, 2 different types of self-locking nuts have been used for both small blocks and big blocks. One style was the "Stover" type, distorted thread nut pictured below on the left side. The other type was the "pinched", distorted thread nut depicted on the right below.

In GENERAL, small blocks have used the "pinched", distorted thread type of GM #3744341. However, there have been exceptions. During certain periods, the "Stover" type was used.

Big blocks have GENERALLY used the "Stover" type. However, exceptions exist here also. The 65-E66 nut was GM #3860307 and I believe this was of the "pinched" thread style. L66-74 used GM #3896648 which is a "Stover" type nut.

Many of the "Stover" style nuts have been of natural finish. This eliminates problems with hydrogen embrittlement. In a fully lubricated environment like rocker arm nuts, I really don't see the need for zinc or cadmium plating.

The "pinched" style nuts are usually zinc plated. I have no idea why.

By the way, the "Stover" nut pictured on the left is plated but it's NOT a rocker arm nut. It's just a "typical" Stover nut I had handy.

[Stuart F.]

Re: Rocker arm nut backing off

Joe;

Great pictures. Neither look exactly like I remember, but it has been a while since I had my valve covers off (soon though). The top end (distorted end) looked more triangular - perceptably so.

Also, about the early 55 nuts, didn't they try a tall nut with a lock (set) screw in the center? I have this picture in my mind, I think from an Uncle of mine's car. He had one of the first V8 Chevy's I had seen, and he had trouble with rocker nuts. My brother's was a later 55, and I had the covers off it to chrome them (of course). His was different again. as I recall, all the early stick shift models had solid lifters, but the later ones (like my brother's) had hydraulics.

Stu Fox

[Joe L.]

Re: Rocker arm nut backing off

Joe;

Great pictures. Neither look exactly like I remember, but it has been a while since I had my valve covers off (soon though). The top end (distorted end) looked more triangular - perceptably so.

Also, about the early 55 nuts, didn't they try a tall nut with a lock (set) screw in the center? I have this picture in my mind, I think from an Uncle of mine's car. He had one of the first V8 Chevy's I had seen, and he had trouble with rocker nuts. My brother's was a later 55, and I had the covers off it to chrome them (of course). His was different again. as I recall, all the early stick shift models had solid lifters, but the later ones (like my brother's) had hydraulics.

Stu Fox

Stu-----


The "Stover" nuts used for the rocker arms did have a "triangular depression" on the top. As I mentioned, the one pictured is not a rocker arm nut. I just used it to depict the general configuration of a "Stover" style nut.

[Jim S.]

Re: Rocker arm nut backing off

Joe,
As I recall, the "pinched" nut was the type that originally came on the early 396 engine. The "stover" type nut was the service replacement.

Jim