Always wondered why if head and main studs provide better clamping force than bolts, why are cap screws a better way to clamp the rod cap to the rod verses a nut and bolt? The cap screw turns in the rod, same as a bolt would turn in the block to clamp a head or main cap. Am I comparing apples to coconuts? Just curious
I'm not a machinist or an engineer, but from what I understand, studs are stronger because they only stretch vertically as you torque the head. A bolt, as you torque it down, will twist, or tend to move horizontally, which can affect your torque reading. It can't hurt that there are two sets of threads rather than one, either.
Probably because the head of the bolt cuts into an area of the rod that isn't the best place to cut into at the same time that precious clearance from the cam is needed. Seems like a good enough cap screw can be made that eliminates rods failing from stress risers machined into that area. If you look outside the American V8 world and it's similar architecture, you'll see other very high performance engines using stud and nut. [You're welcome??]
The main advantage of the stud is that when you torque the nuts on there is less friction required to spin the mut I you get a more accurate torque reading. Just keep in mind that because the studs have a better clamping force it is a best practice to re size the main journals or rods when you switch fasteners. Many times better fasteners change the dimensions of rod or main journals, not always but you need to at least check.
and lastly the maximum effectiveness of any thread is diameter/length. otherwords, if it is 1/2" diameter, maximum strength is 1/2". if you have 2" of 1/2" thread, only 1/2" of it is effective.
In general studs are superior, but I assume you're referring specifically to the 73-up 350 "cap screw" rods? I always assumed there was something more to those rods than simply the use of the cap screw (I used them in my last 350 build a long time ago), but I'm sure some of the 350 experts will know for sure.
Well, cap screw rods in general vs nut and bolt. It is a curiosity of mine why Buick went to capscrew rods starting in '73 for the 350 even tho they were still cast. They are a different design rod from the earlier style, but if Buick had no intentions of offering a Hi Perf 350, why the change?Especially since the biggest HP 350 was '70 and that was the older style nut and bolt rods This is just crap I wonder about LOL
This is how I see how they work, and correct me if I'm wrong, You thread the stud into the block, hand tight, set head on, thread nut on, as the nut is torqued, the stud is motionless, BUT it is being pulled upwards as the nut is torqued downwards. So the action is the same as a bolt, EXCEPT the threads of the stud do not turn in the block, they just engage the threads of the block tighter and tighter without adding the friction of twisting. I think too much
Not sure why they changed design, but I hope mine hold. I still have the original 68-72 style rod, but with ARP hardware. Figure it should still handle 6k RPM on occasion. However, to comment on your question, I would agree that a stud and nut design rod should technically be stronger, but in the case of the 68-72 Buick 350 rod, I feel that the design sacrificed some rod integrity, therefore the cap screw rod is superior. Just my opinion
Tensile load in a bolt is "spread" over more length, a stud is much less length overall. (and in auto engine, most everything is tensile, not shear or bending). It is the tensile load that is most important, and proper torque allows maximum tensile load. Under torquing and you are not gaining the full advantage, and over torquing damages the bolt by "stretching", and leads to fatigue/failure. This is also why you see different diameter bolts and grades being used in similar applications with different torque values. Studs on mains are used in some applications, like aircraft long bolt engine assemblies, so there is a great deal to understand about all of the fasteners and the what and why. This is a good time of year for you guys (and gals) in the "cold places" to read up on such things. And you never know when you might end up on a game show and have an answer that makes you rich. If that happens, remember this thread...
They went to the capscrew rods because there were old style rods that broke even on stock engines. Dennis manner the Buick engineer told me on the phone when I was interviewing him for my book. I have a pic somewhere of the week spot in the old style rods and it’s at the location where the rod bolt goes into the rod. It is super thin there. The caps crew rod is way thicker at that location. The later rods are heavier too. The cranks don’t interchange without re balancing..
Didn't know that about the crank/rod combo not interchanging due to the weight difference, actually never thought about it It makes sense tho!
Believe it or not Buick was making improvements to the 350 over the years. Sure the compression ratios and emissions crap bogged them down but there were improvements for reliability. The later heads never crack, the early ones usually crack, the better rods, better oiling. The later stamped steel rockers are more reliable than the alum ones, even though they do get sloppy over time.
Thank you Sean for your post of explaining where the weak point is on the earlier style rod. That is more or less what I was hinting at, but I didn't want to claim something that I was unsure of
Too bad Buick couldn't keep the 350 till like 1990 like Olds with their 307. I read it couldn't meet emissions
Exactly how many stock rods, cap or nut/bolt, have been thrown to date? Anyone know? My engine guy that did the boring, align hone,etc, likes the nut/bolt setup with ARP's better than the caps.
