Why do aftermarket/performance rods come with bolts rather than studs. We use main studs and head studs instead of main bolts and head bolts, why do we use rod bolts instead of rod studs ?? Is this only a clearance issue? is because we prefer using the bolt strech mesurement instead of the bolt torque?
What do you guys think ??

Charles

 

I'm glad you asked this question, because I forgot to!

Although I'm not sure, I would guess because a high dollar rod with bolts would probably be built by a professional engine builder, who would use the bolt stretch method to determine proper clamping force. I think it is also a weight issue. Although I am just guessing, I think a thicker rod bolt, like those used in H-beam and other high perf rods, still weigh less than a skinny rod bolt and nut. I'm guessing this thicker bolt is stronger as well.

This is just an educated guess. Lets hear what the experts have to say!

 

I'd suspect that the bolt is preferred to the stud for a few reasons. The one that really comes to my mind is the necessity of machining flats for the nut and head seats. That makes a nice place for a crack to start.
The other thing.... threading the bolt into the body of the connecting rod would yield a stronger, more rigid assembly. At least this is what I've seen from my experience designing and working with finite element analysis software some years back.

-Mindgame

 

Yes, you can thread the rod bolt into the body of the rod. My thought is that you don't get quite as much out of the rod material because you are loading the threads rather than clamping two pieces together. The high end bolts have quite a bit more tensile strength (or yield strength) than the rod, so perhaps you can get more strength and less cost with thru bolts. If you radius the corner of the spot-face on the rods you go a long way toward preventing stress risers.

Howerer, Arrow for one, produces high-end rods with internal threads not for studs, but for bolts. My guess is that you should used rolled threads for strength, like APR bolts, but forming internal threads in a heat-treated rod is a significant challenge. If you form the thread first, then HT, you probably have a less strong thread, and risk HT distortion of the threads. If you've priced Arrow rods recently, you'll know why most people don't internally thread really strong rods.

[Usually formed threads on a bolt or stud are called "rolled" because it is done with 2 or 3 rollers. Internal threads are "formed" with a tap that has no flutes and is often triangular. It pushes or "forms" the material into a thread. No chips are produced. The hole is drilled oversize and the "form tap" displaces material so that the finished inside diameter (minor diameter) is correct and smaller than the original drilled hole.]

If rod weight is very critical and clearance is very critical and cost is NOT very critical, you might choose a threaded rod. FWIW, measuring bolt stretch in this applicatiion might also present a challenge.

Both methods have their use.

My $.02

 

I have high dollar 4340 rods in my current engine. They don't use the traditional rod bolt and nut. The rods are drilled and tapped to accept bolts that hold the cap on. There's more strength with a bolt holding onto the rod that a nut holding the end of a bolt. The bolt into the rod grabs more threads than a nut will.

I have some Howard aluminum rods for my next engine and they have the same setup.

 

I think you'd be hard pressed to find an aftermarket rod that isn't threaded on the small end. Even the inexpensive Scat I-beam rods are doweled and threaded and use a bolt.

Interesting on the threads OldSS.

Had an interesting discussion with a buddy about this (threads in general) one time. He's a tool and die maker. As I remember, he said that forming works real well on non-ferrous alloys along with low carbon steel and some stainless. No good for higher carbon steels, tool steels and the tougher stainless grades. At least that's the way I interpreted it. He also mentioned that the hole size was critical to forming, especially in apps where you were locked in to a precision "thread classification".

So I'm wondering if the rod makers are cuting the thread after heat treating (seems that'd still be tough) or are they forming prior to heat treatment and keeping tight control over the HT process. Maybe that works well if you hold your mil stock to very high standards?

-Mindgame

 

 

Interesting concept. I always assumed the bolt-stretch was so critical that studed or nutless-bolt designs were out of the question. I guess it makes sense that Arrow (and probably other exotic-tech-rods) would go the extra mile to lighten a rod with a bolt design like that... but I don't even wanta guess was they'd cost for something like that. Might be one of those "lighter than titanium" steel rods we hear about in shadows from time to time.

As for J19... I keep getting conflicting reports on wheither it's street-duty capable. I like the idea of larger bolts (all the J19's I've seen advertised were 7/16" although I assume you could get some in 3/8" if you called ARP), and really wouldn't mind the small weight penalty from doing so... but is it really as tempermental as everyone claims?

I guess this is one of those "find the threshold" things a pro engine builder can nail down while the rest of use are stuck paying for more strength than we need (better too much than too little). I'm hoping Pro Mods will be enough for a 500hp, 6500 rmp red-line.

 

A quick question here somewhat related. Are there any cars that race using the obique type rods? Would there be any shear load(force), or would the rods be TQed to where there's none?

 

Bolts are commonly replaced by studs in the mains/heads, but I think that's mostly due to the block having coarse threads.

If connecting rods only had coarse threads, it would also recieve a stud similar to one you would see in a block (coarse threads on one side/fine on the other).

Another thing to consider, is a threaded rod with a bolt takes up less space than a stud/nut which is important in a stroker application.

 

I have a set of Summit Pro Line 4340 rods in my current 355 that have studs instead of bolts in them.Summit no longer sells these exact rods.I built the motor 6 years ago now.Don't remember what size the stud even is,but they are a definite stud threaded into the big end of the rod along with a nut and washer to hold the cap on.