|The MGA With An Attitude
The Nature of THREADED STUDS -- UT-109
At 01:03 AM 2/11/03 -0500, Bruce Vay wrote:
>"I began the reinstall of the head and am using ANR new head studs. I was torquing along and was two studs from finished and one of the short studs for the rocker arm assembly simply broke off. I had done 20#, 30#, and was on 40# when the failure occurred. Did I use the wrong torque. ANR says 40# and the Haynes is over that? Could it have been a bad stud?
Probably too much torque for the small studs. Also if you run out of threads on the stud it binds up and twists the stud rather than turning the nut on the threads.
By the book, the original 3/8" head studs call for 50 lb-ft torque. I don't have a torque spec specifically for the smaller 5/16" studs for the rocker pedestals, but by all accounts a 5/16" stud or bolt will tolerate barely more than half the torque of a 3/8" stud of similar material. So my recommendation would be for 25 lb-ft torque for the 5/16" studs. Also a host of other 5/16" bolts and studs on the engine are spec'd for 25 lb-ft torque, such as clutch cover to flywheel, gudgeon pin clamp bolt, manifold studs, and water pump bolts.
If you are building a competition engine, and both the block and the head are known to be very flat (freshly machined), and you are using special high strength studs, then you might follow the recommendation of the stud manufacturer for the larger (3/8") head bolts if they call for more then 50 lb-ft torque. I personally consider this a bit risky, as the increased tension on the studs may deform the cylinder head. At any rate, the 5/16" studs should still call for about half the torque of the 3/8" studs.
You should always be able to spin the nuts down the studs with one finger with no binding in the threads. If it binds, then run a thread cutting die down the stud and a tap through the nut to be sure the threads are clean. If the die removes any significant amount of material from the threads on the stud, that is indication that it was previously over tightened and the stud is stretched, in which case it should be replaced. Always put a drop of oil on the threads with tensioning.
The 5/16" studs securing the rocker pedestals only are NOT a structurally critical application for securing the pedestals (being low stressed). Unless you have special high strength studs for this point I would never torque these higher than 25 lb-ft. I am pretty sure that 40 lb ft would be very close to the break point for standard strength 5/16" studs. There are some additional issues with over torquing these studs (see below).
The B-series engine I have been running in my MGA for the past few years (same size studs as the A-series engines) has a high lift cam and high performance valve springs, and I regularly run it to 7000 rpm (and occasionally higher) when autocrossing. I have never used any special studs, only stock parts, and I never replace a stud unless it is somehow damaged (stretched or stripped). Through many years of use, about 200,000 miles of very serious driving, spanning a few different engines and many, MANY iterations of reassembly, I have replaced perhaps 3 head studs. I have never broken one during assembly, and have never had any problem in operation, always using stock parts (and mostly old used parts).
For me this is a non-issue, and I do not recommend the expense of replacing all studs with new parts, and especially the high cost of the special high strength studs. They just aren't necessary. The stock engine works fine and is quite reliable using stock studs and 50 lb-ft torque. From what I hear from some old timers, the engine assemblers at the factory didn't even use a torque wrench. These engines are conservatively and moderately over-engineered for strength of parts, so the torque settings are not particularly critical as long as you get in the ballpark.
If you intend to R&R the head quite often, as you might do with a regularly used competition engine, then you may find that the high strength studs will hold up better to abuse an will be less likely to suffer wear on the threads or stretching or other such damage from mishandling or from repeated reassembly. Otherwise the high strength studs may be of some interest for a very high compression engine for competition use, but generally a waste of money for a street engine.
I do recommend that you use the original type tall hex nuts for the head studs, as they will put less stress on the threads with more length of thread engagement, and will be kinder to the threads on the stud, and will give you more consistent results with torque settings. Also use the original type heavy (thick) flat washers under the nuts to avoid deforming the washers and causing a divot in the head around the top of the hole that could cause the head to bind on the stud during later removal of the head. This is especially important for the aluminum rocker pedestals used for the MGA. Thin flat washers deform easily, even hardened steel thin washers, and will deform the top of the aluminum pedestals causing fracture failures. Thin washers will even cause similar divots in the iron head.
The four smaller 5/16" studs used in the rocker pedestals may give a few problems, but always because of abuse or misuse, usually from over torquing. The longer two front and rear 5/16" rocker studs also provide the anchor point for the valve cover mounting nuts. People fairly often over torque these in a variety of ways and end up with a broken stud, more commonly in operation somewhere down the road than during initial assembly. You torque the hex nuts up to the specified torque setting. Then you install the valve cover and torque up the two nuts on top, which adds more tension to the #1 and #4 studs. If the valve cover gasket leaks, it is tempting to tighten the valve cover nuts, adding more tension to these studs. Since the cap nuts have a 5/8" hex size on top, it is very easy to forget that the underlying stud is only 5/16" diameter, and you have two different nuts pulling on it. You don't need a lot of torque on top. If the valve cover is straight, and the bottom gasket surface it flat, and you start with a fresh cork gasket (the first time), then wrist torque only is sufficient to secure the top nuts and seal the gasket and properly retain the valve cover. I sometimes use large knurled thumb nuts here, especially for a competition engine where the cover may be removed more often. See picture here.
If the valve cover gasket gets crushed to less thickness over time, the valve cover sets lower, and the nuts get lower on the studs. If you over torque the cover nuts the top of the cover may be deformed downward, and the nuts get lower on the studs. As the rubber grommets on top of the valve cover deteriorate and get crushed thinner, the nuts get lower on the studs. If the edge of the cup washer touches the top of the valve cover, the buffer effect of the rubber grommet disappears, and then the torque and tension increases rapidly with more tightening.
These cap nuts have a blind hole that may bottom out on the top end of the stud, after which additional turning twists the stud directly rather than tightening the nut downwards on the threads. This is much more likely to break a stud with a given torque. If the nut gets far enough down on the stud, the bottom end of the cap nut can come into direct contact with the hex nut on top of the pedestal. After that, further tightening is only turning one nut against the other nut in a jam nut configuration, which could break the stud between the nuts. It will also twist the stud with further turning, which combined with stress of tension more commonly will break the stud below the hex nut at the top of the pedestal. If you break the stud you no longer have tension for the valve cover nut, and the cover is then held with only one nut, which can result in breaking of the other stud in turn.
The MGA with aluminum rocker pedestals, can suffer a cumulative combination of such problems. Using a thin or soft flat washer will allow deformation of the aluminum pedestal around the top of the hole. This can promote stress cracks and can deform the material enough to bind on the stud during next removal. If you subsequently break the stud in operation at this point, there is a high probability of fracture of the top of the aluminum pedestal over the rocker shaft. Once the rocker shaft breaks loose at either #1 or #4 pedestal, there is a very high cantilevered upward load on the shaft at the next pedestal, which commonly results in fracture of the next pedestal as well, even if it is in otherwise perfect condition. The aluminum parts are much weaker in this respect than the iron pedestals.
All things here considered, you must be very careful with the studs securing the valve cover. Start by using studs of the correct length. Extra length causes bottoming of the cap nut on the end of the stud. Lesser length gives too few threads for engagement with the cap nut. Use heavy flat washers. Do not over torque the hex nuts. Use a good valve cover gasket (not too thin). Replace the rubber grommets on top regularly. Be sure to use the correct combination of hardware on top, starting with the rubber grommet, cup washer, thick high collar flat washer, and then the cap nut. If you leave out a part you can bottom out the cap nut either on the end of the stud or on the hex nut underneath.
>Then how do I remove it? I know how to do the removal, but I hesitate as I have broken one and not been able to remove it.
To begin with, studs should never be torqued tightly into the castings. Be sure the base threads run freely with finger force only. Install the studs with wrist torque only, never to exceed 5 lb ft of torque for the stud installation. When you do finally tighten the nut on top, that stud is not going anywhere. The studs are commonly coarse threaded on the bottom end and fine threaded on the top end. With tension and torque, the fine thread has a shallower ramp angle and will turn easier than the coarse thread, so the nut will turn on the stud during tensioning, and the stud will not turn further in the casting. For later removal, if the nut binds on the stud, the stud may unscrew before the nut, but this is not a problem. Just take it to a vice, remove the nut, clean up the threads, and generally reinstall the stud if it is not damaged.
If your stud is broken flush at the top of the pedestal, it is likely that you will have to remove the rocker assembly to unscrew the stud with pliers or vice grips. If the stud was installed initially with finger torque only, then you might have a chance of unscrewing it with a center punch and light hammer. Place the punch tip near the outer edge of the end of the stud, lean the punch over at about a 45 degree angle, and tap gently with a light hammer to rotate the stud counter-clockwise. If you're lucky and it's not too tight, it might come out that way. Otherwise remove the pedestal for access to the stud with pliers.
At 05:22 PM 7/1/03 -0700, Randy Trautman wrote:
>What is the correct method to install and torque new cylinder head studs?
Clean out the threads first, both female and male, so that you can screw the studs in with your fingers. Believe it or not, finger tight is tight enough. Never apply more than 5 lb-ft torque to seat a stud, as you could mushroom out the top thread in the block and raise a burr that could interfere with the head gasket. The damaged thread could also give a problem with removal of the stud later. Over torquing a stud can also crack the block when you run out of thread and start wedging the hole open.