TR4/4A Self locking bolts question

Interesting question. I would think the original bolts can be re-used. In my view, the early bolts were under-torqued, so you could not rely on the amount of stretch to retain the bolts, and thus the need for the lock tabs. That is one extreme. The other extreme is the method used on many modern engines, where the bolts are actually over-stretched past their yield strength. When you torque past yield, then the bolts cannot be re-used.

Maybe someone knows for sure, but I think the TR4 bolts were torqued less than yield, so are not damaged when installed like some modern bolts. I used to have a TR4 manual, but don’t at the moment. Does the manual say specifically to replace the bolts? If not then I would assume they are re-usable if in good condition.

Split lock washers are used a lot...but if you think about them, they can’t really function unless the bolt becomes loose, and by then it’s already too late. They do no better than a hardened flat washer.

I could look again but the manual talks about the early bolts that used lock tabs. Unless I missed it (possible) I don't think it addressed the later bolts.

bammons said:

I can't seem to find the answer on this question. Are the bolts that attach the crankshaft main bearing halves self locking (are they the stretch bolts like the flywheel bolts)? As far as I can tell they are used only in two places on my car - the flywheel and the connecting rod big end. The reason I ask is my car did not have any split washer on the crankshaft mains and I see in the books they are supposed to have them. I also assume that if they are self locking them they would not use split washers - is that correct? I am aware that earlier cars had tab lock washers. I want to order new ones (I should shouldn't I?) if they are stretch bolts but will not need to if all I need to replace is the lock washers. I always seem to get salesmen who don't know the answers to my questions at the place I was going to order from so I will rely on your answers.

Click to expand...

All bolts are "stretch" bolts, that is, they hold things together by stretching and creating clamp load. Lock washers are supposed to prevent bolts from backing off under vibration, but are only marginally effective, as you can see in this rather startling video: https://www.youtube.com/watch?v=IKwWu2w1gGk&feature=youtu.be.

I am not a TR guy, would someone tell me how self locking bolts work. I have never come across these except with a nylon tit embedded in the threads. I doubt that this was done in the 1950/60ies.

I think Bammons is merely referring to the fact these bolts have no lock tabs. They are not any different than a regular bolt...just torqued so that there is enough stretch on the bolt to prevent loosening.

My information comes from what I have read about the flywheel bolts that do not use the lock tabs. I understand that all bolts utilize some amount of stretching but apparently these are designed to stretch more. Enough stretch that you are not supposed to reuse them because I assume they have stretched too much. Which brings me back to the crankshaft main bearing caps. I guess I can play it safe and just order new but I would really like to know the answer and the answer affects whether or not to use lock washers. Remember from above my car did not have lock washers in this location.

I'll take a shot at this.

I have to admit, at the outset, that I'm an electrical engineer, not mechanical, but I'm accustomed to working outside the electrical area. So, some of you guys with specialized knowledge (like John, in particular) may want to comment on this. I'm fine with that.

Anyway, I recently undertook a study to try to understand why bolts loosen. It's all pretty interesting, especially for a geek like me who is easily amused. (My electronics specialties are noise and distortion; please try to contain your excitement.) Most of our understanding of bolt loosening comes from a classic 1968 paper by a guy named Junker, who showed that cyclic lateral forces were the culprit, and designed a test (now called a Junker test) to evaluate it. The video referenced above gives an example; you can find more on line.

Lateral vibration loosens the bolt, initially perhaps only microscopically, and that reduces the stress in it. Then, with less stress, the vibration moves it more easily, reducing the axial stress further, and so on until it loosens completely. This process can be prevented a number of ways, including increasing the elasticity of the joint (e.g., with a spring washer or stretch bolt). Then, as the bolt begins to loosen, there is less decrease in the axial stress, so the loosening process is impeded. Not necessarily completely prevented, though; it depends a lot on the vibration environment, the type of locking device, and probably other factors. Of course, there are other tricks, like nyloc nuts, which increase the thread friction independent of stress, and actually look pretty good in Junker tests. Belleville washers, which combine a serrated surface with a domed shape, thus a spring effect, also seem pretty good.

I found results of several Junker tests for split washers. They showed a minor improvement over nothing, at best, and in some tests, they even looked worse. Unfortunately, I found only raw tests, with no interpretation or underlying analysis, so it's hard to say what the difference was. I suspect it depends on the design of the washer and the material of the bolt and substrate that it's threaded into.

Another point: Junker's work post-dates the design of our cars, so the understanding of bolt locking, when the cars were designed, was not as good as it is today. For that reason we probably should not view the locking devices used in the original designs as the only, or even best, way to go.

Oh, and to answer the original question...while doing this study, I wrote a little utility for calculating stress in bolts. I have to caution you that these results are a little soft, as they depend on some assumptions and are quite sensitive to certain parameters (like, friction in particular) but I think they give you a decent ballpark estimate of stress. I get about 90,000 PSI stress in a 7/16 inch main-bearing cap bolt, torqued to 60 lb. ft. That's well within the yield strength of a grade 8 bolt, so I don't see any problems in reusing them, as long as they are otherwise in good condition (no bunged up threads or anything like that).

All bolts do stretch. The issue, if I correctly recall college physics, is that metals have a trait called Young's Modulus. This is the point in stretch where, when loosened, the bolt will not return to its original condition of shape and strength. If the modulus is not reached, the bolt can presumably be reused. As for the original question, I reused the bolts in my '59 production TR3 and have been running it for 8 years. I also reused the bolts in the TR3 I had in college, which I gleefully over-revved continuously (believing that balancing in the rebuild produced immunity), again with no problem. Doe the Moss catalog provide any inferential answer to the question?
Bob

For those posts above that did not know about the "stretch" bolts you may refer to the Triumph shop manual Section 1-123 where they are called "place" bolts. Also refer to Roger Williams book "How to restore Triumph TR4, TR4A" page 98, picture 7-14-17. In looking once again no reference is made to the crankshaft main bearing cap bolts as being stretch or place so I assume it is a regular bolt which can be reused. Would I be better off using a lock washer or a grade 8 flat washer? I will ask when I order but I have little luck when I have asked questions in the past. Bruce

Bob - I have looked at Moss online where they sometimes do put a note or attachment with certain items giving needed info. They do not say anything special about the bolts. Moss has been good about answering questions but they are closed now. I am surprised that someone on this forum has not answered definitely with all the "Triumph" experts on here so that is why I just decided to ask here.

bobhustead said:

All bolts do stretch. The issue, if I correctly recall college physics, is that metals have a trait called Young's Modulus. This is the point in stretch where, when loosened, the bolt will not return to its original condition of shape and strength. If the modulus is not reached, the bolt can presumably be reused. As for the original question, I reused the bolts in my '59 production TR3 and have been running it for 8 years. I also reused the bolts in the TR3 I had in college, which I gleefully over-revved continuously (believing that balancing in the rebuild produced immunity), again with no problem. Doe the Moss catalog provide any inferential answer to the question?
Bob

Click to expand...

I think you're thinking of yield stress. Young's modulus is a measure of elasticity: stress = young's modulus x strain. Strain is percent elongation; stress is in force per area, e.g. pounds per square inch.

I really don't know how the dynamic forces on, say a conn rod bolt compare to the stress of the installation torque. Now that you bring us the subject, I don't know what's likely to break first when you over-rev an engine. But I doubt that the conn rod or main bearing cap bolts (assuming they are in good condition) will be the first to go. I do know, however, that I don't trust anything technical that comes from Moss Motors. I've heard way too much techno-hogwash from them.

If you can find any info on the steel used, I could answer the question using my old material books.

Some food for thought...Properly tightened rod bolts actually see no cyclic load unless the engine is over-revved. Here’s why. When you torque the bolt, it retains a stretch, which also retains a force through the bolt. I don’t know what that is, not knowing the size and torque, but, let’s use Steve’s theoretical 90kpsi. So the bolt is always pulling the rod together with 90,000psi of force. now you set the engine in motion. As the rod pushes the piston is sees compression, and as it pulls the piston it sees tension. Until the tension exceeds the resting 90kpsi of holding force, the rod bolt never knows there is a load. Once the load exceeds 90kpsi, then the cap starts to move and the bolt sees more load. Now the bolt finally sees an additional cyclic loading, and cyclic tensile loads are what cause fatigue. Compressive loads don’t affect fatigue life.

So, in theory...a properly torqued rod bolt that is never over revved will last virtually forever.