66 461 Press In Stud Question

[Craig S.]

Hi guys - I am helping a fellow NCRS friend Dave Barclay today with his 66 L79 327/350. He was driving it earlier today and right near his home he heard something let loose in the top end after a blast to 4000 rpm. The engine had been recently rebuilt before he purchased it. I went over and it was backfiring in the carburetor, and I heard clatter in the LH side in the valve rocker area. I suspected a broken spring, but we pulled the covers and checked all the rockers and sure enough found problems with cylinder 5 intake and exhaust. 5I stud was up about 3/16" and of course had huge slop, but 5E stud was fine, pressed in all the way but the stamped rocker had let go and fractured out the bottom in the rocker ball pivot fulcrum area.

I am used to dealing with threaded in studs not pressed, we checked all the others and they all seem good, even thread count above the studs, and correct tight valve setting. It is very clear this engine has indeed been recently rebuilt, there is no varnish and all the valve retainers and springs are new. It appears like a mix of new and old stamped rockers are in use, evident by varnish on the older ones and darker in color. The 5E that fractured was actually one of the newer ones. While we don't know the details of the cam, it sounds like an L79 cam or blueprint, idle is about right for that cam, this is not a high lift hot cam. I see no evidence of coil bind at full lift on the other cylinders, so I think we can rule that out as a reason why 5I stud lifted out of the head.

I looked carefully at the boss on the stud mount area, and see no evidence of any cracking that would cause a looser press fit. There is no evidence of any bending of the studs, they look straight and true. I also took a hard wooden dowel and tapped the valves lightly with a rubber mallet and they move freely and there is no evidence of any bent valves or stem binding in the guides. Given the blast he gave the car was only to 4000 rpm, I wouldn't expect any float with apparent new springs.

What does confuse me is if the stud working up on 5I was the initial problem, resulting in failure of the 5E rocker, or vice-versa.

I am not a fan of pressed studs, but Dave really doesn't want to pull the heads on a recently rebuilt engine just to install theaded studs and spot face the stud bosses.

We are planning to get a set of roller time comp cams investment cast rockers on Monday, and install. I made an adapter on my metal lathe to use with a slde hammer to pull the 5I stud completely, and plan to clean the boss and pocket with brake clean, and re-install the stud with Loctite red bearing mount.

I thought I would mike the stud to see if there is any apparent diameter reduction in the press fit area, and maybe install a new stud if I can find one readily.

Does anyone have experience with using red loctite to anchor in press fit rocker studs? I am out of my element with the press fit heads, I am used to building BB's with theaded studs or SB's with threaded studs.

We are trying to get the car back up quickly without major tear down.

Anyone have an idea why both valves would fail like this on the same cylinder, this may be a case where the failed rocker was faulty and broke, and somehow that caused the 5I stud to move up......any ideas welcome....thx!......Craig

[Joe L.]

Re: 66 461 Press In Stud Question

Craig-----

I don't know why the problem occurred on the #5 cylinder. However, I wouldn't even attempt to re-install the existing stud with Locktite. I don't think that there's a chance that it will hold.

Oversize studs are available from GM. The .003" oversize is GM #3814692 and the .013" oversize is GM #3815892. I would use the Locktite with one of these, although I don't think that it's really necessary and may "complicate" things if he ever does want to remove the studs and install the screw-in type.

Installation instaructions for the studs will be found in the factory service manual.

[Craig S.]

Re: 66 461 Press In Stud Question

Hi Joe - thx for the info. I like the OS idea to gain a better press fit. I was worried about recurring loosening. Is there any danger of the 0.003 OS causing the cast boss to crack with installing? I am guessing not since this is a factory approved procedure/process. I will pull out the 66 service manual and read up, and have Dave order the stud.

Any thoughts on using the comp cams roller tip investment cast rochers with the standard rocker ball instead of the usual stamped rockers? Since these appear to be a mixed bag, and one broke, we want to install new rockers in the process. I have used the Comp Cams full needle bearing rockers on my BB LS6 crate, and they are very nice rockers, but I am not as familiar with the roller tip only rockers vs stamped. I would think these would be a bit better and not that much more costly than the stamped rockers. I think Doc Hulst used a set of them on his 71 if I recall and was happy......thx for your input and quick response!.......Craig

[Dick W.]

Re: 66 461 Press In Stud Question

Use green Loctite #640 item #37424. It is a high temp and designed for press fits. From prior experience it does work in high temp, high stress applications.

[Michael H.]

Re: 66 461 Press In Stud Question

Craig,

The "pulled rocker arm stud" is very common. It is definitely caused by valve float. It only takes a few RPM over valve float for a split second to pull a stud and from that point, the rocker arm failure is sure to follow because there is now a lash that causes loading far beyond what the rocker arm can endure. The cylinder with the weakest springs will be the first to float and if the engine is still making power from the other 7 cyl's, it goes up to an even higher RPM.

The problem with installing the .003 OS size studs is the fact that this is definitely not recommended without reaming the hole for the new size. I've seen this done many times over the years and in most cases, it cracks the stud boss. The reamer was part of GM essential tools for every Chevrolet dealer in the 50's and 60's so it's qiote possible that you can find one at a dealer. If not, any good machine shop should have one.

I've seen a few guys replace studs with .003 OS in an emergency using freon to chill/shring the stud before installing but the results are generally the same.

I think the factory shop manual gives information and the Kent Moore # for the reamer.

Michael

[Craig S.]

Re: 66 461 Press In Stud Question

Michael and Joe - thx for the info, I did read the overhaul manual and saw the OS reamer....makes sense, we will ream and not try any OS stud without reaming. I am thinking maybe Goodson machine shop supplies will also have the reamer since the Kent Moore tool is probably long gone. The precedure is a simple hand reaming opeeration followed by a press with a tool that appears to act as a depth control hammer in driver. I am sure this prevents bottoming the stud in the boss and avoids risk of cracking the boss. I suspect I have all I need to go on, and I am sure the 0.003 OS is sufficent.

The one curiousity thing I still don't understand is the stud on 5E that has teh fractured rocker is fully seated and not pulled. The stud on 5I is the one that is pulled and that rocker was intact, but sure would not have lasted long. Maybe for safety we should replace both the 5E and 5I studs, since both were associated with a cylinder with excessive lash.

I also seriously doubt that Daves 4000 rpm blast would cause valve float on the new valve springs that are evident, with aftermarket retainers (black oxide like Comp Cams etc, not the GM issue). However, the prior owner could clearly have twisted the engine harder in break in and started the stud pulling process. Maybe there was excessive clearance on 5E too and this is why that rocker fractured. If that stud is pulled up at all, it is not evident from a crude scale, but a few thousandths of clearance can cause havoc on valvetrain so it may well be.

Thx again guys, I will check Goodson.com for reamers and tools so Dave can get what we need to do the job right.....Craig

[Joe L.]

Re: 66 461 Press In Stud Question

Craig-----

Yes, the rocker stud holes definitely have to be reamed prior to installing an oversize stud. Depending upon how much "looseness" currently exists will determine whether you need the .003" or .013" OS stud. The reamers are still available from Kent-Moore/SPX. The .003" reamer is J-5715 and the .013" reamer is J-6036. Both are currently still in use for SERVICE of Gen I small blocks (which are only recently out of PRODUCTION use), the 4.3L V-6 which is right at the end of its PRODUCTION use, and the 3800 V-6 which is still in widespread PRODUCTION use in GM vehicles.

I think that the Comp Cams Magnum rockers are ok. They've been used successfully for years now.

[Craig S.]

Re: 66 461 Press In Stud Question

Joe - a big thx for the reamer info......I was on the Goodson website and couldn't find any reamers for studs in their 2004 catalog. I will dig up the Kent Moore website and find the reamers there.....thx again!......Craig

[Craig S.]

Re: 66 461 Press In Stud Question

Joe - digging more in the overhaul manual, the installer and puller are listed as J5802-1 and J6880. Basically, I think I could make these if the are priced exhorbitantly, one is just a steel rod with a hole through it to use an nut to pull the stud and could be bored on my lathe from .675" steel stock. Sounds pretty simple. The installer looks to be similar with a depth pocket to control the depth of stud installation, I could measure with a calipers on the studs and get an average installed height measurement and bore in with a drill on the lathe. The only question wouuld be if a 59 degree drill point would cause any top thread issues on the stud during driving,I am sure the Kent-Moore tool has a square face bore in the bottom of the pocket.

Do you know if those tools are still available? I guess we will be calling Monday when they open.....thx, Craig

[Joe R.]

Re: 66 461 Press In Stud Question

Craig,

Where have you been lately?

I don't think putting another press stud in that head is a good idea. Turning 4000RPM ain't much for a L79. Don't you think next time Dave runs it up the RPM scale it will pull another stud?

With what you have said, and the used/new parts, it sounds like a half ass rebuild on the engine to begin with.

Here's a fools (good) advice! Pull the heads and take them to a machine shop and have screw in studs put in. Then you will have a good basis for building the 461 heads back to specs. Use a good machine shop and the head guy will guide you or Dave and you'll end up with a dependable upper engine rebuild.

Yes, it's going to cost more but you (Dave) will end up with a great finished product that will go beyond 4000RPM and be dependable every time.

Regards,

JR

PS. Are you going to NCRS at Old Town in a couple of weeks?

[Joe L.]

Re: 66 461 Press In Stud Question

Craig------

Yes, the J-5802-1 and the J-6880 are still available. One thing, though: most of the Kent-Moore tools have gone up enormously in price. Be prepared for "sticker shock" when you inquire as to price. Maybe you'll get lucky on these, though.

Also, I tend to agree with JR. I really think that the best thing is to remove the heads and have screw-in studs installed. One thing for him to think about is reliability when on a road trip. It's bad enough to have a stud pull when you're around home. But, if this happens out on the road, one is in a real fix.

When I rebuilt my original 350 I had the heads converted to screw-in studs. Originality be damned for an item like this; RELIABILITY is FAR more important to me.

The way the engine factory built up these heads the press in studs were much cheaper and quicker to install. Unfortunately, the opposite is true in SERVICE. The press-in studs create a SERVICE nightmare. Yes, it's true that millions of these things have performed satisfactorily for years and billions of miles. I don't like them, though, and I never did.

[David D.]

Re: 66 461 Press In Stud Question

Mike, What's wrong with just pinning the studs? Most any speed shop used to carry the kit for small block Chevy's. Dave

[Rob A.]

Re: 66 461 Press In Stud Question

Hi Craig,

I had the same thing happen in a '66, 300HP I used to own. I pulled the heads and had the one that pulled loose replaced with an oversize one. I had all the others checked, and they were fine. Some time after that, I happen to find an article in the restorer describing a kit that is available for replacing pressed in studs with screw ins. The article is very detailed, and the kit very complete and does not require removing the heads. The article included the part# of the kit, which included everything needed for the stud change. They are done one at a time using the kit. If you're interested, I'll find the article and email you the information.

[Michael H.]

Duke and cam dynamics

Dave,

I agree. Either pinning the studs or replacing with screw in studs would be the ultimate fix if he wants to remove the heads. However, neither are absolutely necessary, as long as an engine is operated within the limits of the valve train. It's valve float that causes all the problems and if screw in studs eliminate that part of the problem, then something else will give instead. It's a matter of physics.

If you watch a valve/rocker arm in motion on a running engine, it looks like the valve is kicked open and then slamed shut but that just isn't the case. The long acceleration ramps built into a cam lobe profile start the valve motion gradually as the valve opens, then set it back down gently on the seat when it closes. This is a critical part of the design of a cam lobe and it keeps acceleration forces within a range that all of the components of the valve train can live with, no matter what the frequency of this actio is. (within the limits of the valve spring) If you look at a graph of the acceleration of the valve/rocker arm, you see that there is a smooth transformation from zero to the maximun velocity and back to zero and all of the components are designed to work within this range. (see if you can find Dukes post a few months ago about valve velocity/acceleration in the archives)

When valve float begins to occur, the acceleration goes completely off the chart and well beyond the working range of the components. No valve train components made today will stand up to this kind of enertia loading. If the pinned studs don't fail, the next weak link in line surely will, and that's usually the rocker arm or push rod.

If stronger valve springs are installed, this will raise, slightly, the point in the RPM range where this valve float/bounce occurs but that comes at a price. Cam lobe wear is now an issue, along with rocker arm wear and increased oil temp from the rocker arm which results in hotter oil being dumped on valve springs etc etc.

The best fix would be, as JR suggested, replace with screw in studs but just don't expect that to be the end of the problems if the engine is operated at or near valve float often. A new set of correct GM valve springs would be the best, set to exact specs, and keep it under the valve float RPM range.

Michael

[Clem Z.]

if a exhaust rocker fails

the exhaust gas can not get out of the cylinder and when the intake tries to open there is too much pressure in the cylinder for the intake to open and will bend a push rod or pull the stud like you found. i have seen this happen when a exhaust lobe goes bad.