Truthfully, engineering courses are taught on the questions you ask. Think of design as an orchestra of consideration.

Let's quickly with not a lot of detail examine what you wrote here:

I guess I'm suggesting or asking that if torque specs are for bolts (ie. max rating before concern of stripping the threads?), does this imply anything at all for how much torque is needed for different designs of clamps and the surface areas that the clamps themselves exert on what is being clamped.

You can start with the interface of parts. What are you trying to accomplish? Let's take a handlebar to stem clamp interface. What is the goal? Forces applied at the extremity of the bar where the leverage is highest aka moment arm....Torque at the clamp equals Force X Distance....clamping force should be sufficient to keep the handlebar in place. Ever not have the stem tightened satisfactorily and have the handlebar rotate in your hands if putting too much pressure on it?
What kind of clamp do you need? How much surface area? Why are modern clamps 31.8mm and not say older school 1 inch diameter? For simple reason for the same relative resistance to bar rotation with a larger surface area clamp, clamp pressure can be reduced. Why aren't stem clamps 50mm in diameter? More weight and poorer aerodynamics. This stuff can be calculated. How much surface area do you need? But there is so much more. As it turns out, a broader perimeter of clamping for the same amount of screw torque actually lowers force/area. Is this bad? Yes, the bars can slip with lower clamp pressure. What do manufactures do? They create a clamp with a broader perimeter of clamping but interior of the clamp is hollowed out. This increases PSI and creates a broader clamp that stabilizes the bar. Higher PSI mitigates slippage for the same clamping pressure. Zipp for example obviously hires smart engineers based upon their stem designs. The 4 bolt torx clamp plate they engineered uses a degree of elasticity and bite based upon its clever design. I therefore normally run lower stem bolt clamp pressure with Zipp stems say compared to Ritchey which uses stiffer stem clamp face plates. Zipp clamp design is conducive to holding bars in place with lowest possible clamping pressure. Very clever and very light. I can feel just how good the design is in my hands as I tighten the stem bolts as the stem face plate elastically bends around the handlebar.

OK, how about materials? This comes down to modulus of elasticity, yield strength and even surface finish which creates coef. of clamping pressure. Aluminum for example can be alloyed to be stronger. What does this do? Stronger Al alloys can be used in stems to lower weight by reducing amount of material. Typical lightweight stems today are in the 120 gram range or so depending on length. They can be made lighter but they will lose stiffness and are more subject to manufacturing variation because wall thickness is reduced. By contrast a stem can be made heavier and stiffer and strong but will weigh more and take more energy to drag up the mountain. How is the correct amount of material determined? Today, its done on the computer with FEA. Handlebar forces are measured and stresses at the stem clamp can be computer modeled say for the widest possible handlebar with a 250 lb weight lifter in the saddle trying to bend the bar when sprinting.
Stress can assessed which translates to stem clamp design including volume of material.

About the stem bolts. It is a matter of minimizing weight and maximizing stem clamping strength. What are the considerations there? Qty of bolts, bolt dia., bolt material and thread pitch. Of course you could put Qty=10 4mm bolts on a stem but stresses are such that Qty=4 is sufficient. You could make the bolts out of magnesium for low weight but their tensile strength would be too low. They would elongate and fail. Thread pitch can be increased but the minor diameter of the bolt is reduced and the bolt is weakened and torque onset is much higher when torqueing the bolts and retention i.e. propensity to loosen is increase due to the involute ramp of the thread...why fine thread pitch is preferred if pullout forces aren't too high. If thread pitch is chosen to be too fine, tensile strength of the thread itself is compromised which can result in greater bolt failure, i.e. pulled out bolts ergo the bolt doesn't fail but the thread does.

How in the world do all these complexities culminate into a singular design? Evolution. A long and arduous trail of challenging the status quo and painstaking trial and error.

When I look at a design, I think about everything including how I would design it and why I may do it differently. This is because this is the world I come from and what I am interested in. Bicycles are beautifully engineered machines with a long lineage of trial and error to get to this point in time.

I hope that answers some of it. Design could be called 'tradeoff' i.e. many times robbing Peter to pay Paul. In design parlance this is called no free lunch. Change one thing and there is a ripple down effect. How much strength do you need and how light can I make it?

My initial post / questions were less than stellar / competent. Thanks everybody for replies.
Let me try to re-state / ask again:

1. I am looking at something that can last a few years
2. The only carbon I have on my bike is the fork hence I'd need a torque wrench decent enough to give me peace of mind that the stem . handlebar, etc are correctly torqued but not cracked / damaged
3. In terms of price max would be 300$ - I am an amateur not a pro. What are the most precise torque wrenches for amateurs (300$ or so)?
4. The reason I asked for "bike shop level torque wrench" is because I'd hoped it is within the 300$ budget
5. For example - Park Tools 5.2 - is it any good ('cause it's well within my price)? Is https://www.effettomariposa.eu/en/pr...a-ii-2-16-pro/ worth the money?

Thank you - and pardon my initially vague / incomplete post.

This, $40 on ebay:

Purchase with suitable 1/4" drive socket hex allens or...3/8" drive metric socket allens with adapter:
https://www.ebay.com/itm/NEW-Craftsman-1-4-3-8-1-2-3-4-in-Drive-Socket-Ratchet-Adapter-CHOOSE-SIZE/152096438432?hash=item2369a778a0%3Am%3AmWnCdnz9wS7v9R_3E65qUpw&var=451299120775

That looks a bit pricey... How do they compare with Park Tools 5.2 or Effetto Mariposa?

An alternative to consider. For ease of use in the torque range you need for Stems, Seatpost, etc there are a number of decent T-handle drivers out there as well. Both fixed and adjustable. That and a few hex & torx bits and you are good to go.

Yes, a couple easy tools that could be in the saddlebag or the kitchen junk drawer
Ritchey Torquekey
and, this is kinda elegant:
Topeak Nano DX

Thanks for the thoughtful and detailed response. I was however a bit confused on the Zipp reference, who specify a 10nm/6nm torque rating for Upper/Lower bolts respectively, which I think is generally higher than most traditional stems would specify on the handlebar mount.

I have a number of torque wrenches for different applications. But for working on my carbon bike I use a Bontrager 5nm T handle and have a S/K 1/4" drive for the other torques.

I figure since the T handle came with the bike, it's what should be used.

Thanks...a light gloss really. Mechanics of solving the riddle for optimizing design is painstakingly detailed. Truthfully engineers anguish over 'what is best'.
Compare Zipp stems with Ritchey for example. Have ridden them both a lot. Very different design philosophy and of course every once in a while even a big manufacturer really screws the pooch. Ritchey's 260 degree clamps for example have been an unmitigated disaster for those close to the industry. They were trying to invent a better mousetrap. Their 220 degree clamp design stems are really excellent and absolutely grip the handlebar with tight retention at very low screw torque.

Look at the size of Zipp Course stems...how large their body is. This is their design philosophy. Very low body material thickness for highest moment of inertia. Very stiff and very light weight. Ritchey goes with a more sleek stem body which requires more wall thickness because of lower moment of inertia. A larger body is inherently stronger with the same material and wall thickness.
Kind of analogous to 30mm crank spindles with thin wall section for overall greater stiffness most of us can't even feel compared to Shimano 24mm spindles...or Campy 25mm spindle cranks.

Design tradeoffs. A choice of philosophy. Guys that designed both all have the same education. Ritchey uses hex or allen screws, Zipp uses torx.

If you get a chance, please post any link you have to Zipp's stem plate torque. I generally only tighten Zipp stem bolts to about 4 Nm or so...if that. The flexible stem clamp plate just grabs the handlebar. It just can't move even with low stem bolt torque. Beautiful design.

Wonderful article and thanks for posting. No need for snarky comments directed to me. First the very veracity was challenged about the use of torque wrenches which had to be addressed. Then questions about design came out requesting explanation about torque. Why does it come into being. Many for example believe it is to prevent breakage. Equal reason is for assembly function. I was asked and I answered with good intention.

Good stuff oldnslow. Well done. Me too. You have it covered.

My bad/confusion.. I saw the reference to Zipp, while I had originally posted an Enve stem photo, so had looked up the torque spec on that Enve aero stem.

I agree. Torque wrenches are to ensure that fasteners are neither undertightened nor overtightened. In the first case, they can back out with vibration, which is unsafe, or just not perform correctly. In the latter case, the fastener can be damaged or the excessive pressure cause other problems.

Actually, I'll disagree here a bit. There's another good reason to use a torque wrench in some applications: even torquing across multiple bolts. Sure, you may be able to get "close enough" on a single bolt, and there may be a relatively wide range that'll work fine, however, for many things (like connecting rod bolts, or engine head bolts) you need to torque the bolts evenly to prevent warping over time. The absolute accuracy of the torque wrench here isn't all that important, but the precision is, and any half-decent torque wrench will be far more precise than your "feel". (Note to non-engineers: look up the definitions of accuracy and precision; they're two entirely different things.)

Agreed again, and this BTW is coming from someone who's an engineer, with an actual engineering degree.

Degreed engineer here as well and why we share the same opinion. Of course, we develop torque spec's...lol.
No argument from me...your slight counterpoint. Undeniable and a big benefit of a torque wrench as you point out.

The BMW big six is one of the largest single cylinder heads in the car industry. No more important application to use a torque wrench. Bolts are Torque to Yield and torqued in 3 sequences. I restored a 7 series a few years ago and remember it well.

Cylinder Head in place in second pic. Qty=14 TTY head bolts. Massive head.

VW air cooled have 2 different head torque specs and sequence. Also 2 different torque specs depending if you have 8mm or 10mm head studs. The heads are aluminum, the cylinders are cast iron and he case is magnesium so the engine actually "seals" as the different metals expand.

Quite a feat of engineering isn't it oldnslow? How the engine parts are toleranced with different materials you mention and each with a different coef. of thermal expansion.
I had a 914 2 liter with that engine config but not nearly as hotrodded as you show.
Can you tell us what displacement that engine is?...compression ratio? Est. HP? Are those 4 webers?
Very cool. I played around with VW's a little but not nearly what you have done there.

2180cc, 9.5:1, 170rwhp, dual Weber 48 IDAs.
Not to clog the bike forum, but i have tons of info on my website.
Complete specs Glenn's 1974 Beetle Sedan (Type 1)
Resto pics https://www.glenn-ring.com/resto/
Engine build 2180cc Type 1 Engine build
Transission build https://www.glenn-ring.com/berg5/

Awesome Glen. Bet its a hoot to drive. My 914 below had less horsepower of course but still loved the car...its balance. All I did was strip the paint and put a custom paint job on it...never had the motor apart...adjusted the mechanical lifters under the car. Car had optional 2 liter and appearance package. I found original forged factory wheels. Drove beautifully. Ultimately sold it to a collector from CA after I had my fun with it.

Since you are Beetle nut, have to ask, have you seen the Outlaw street race TV show on Discovery where the VW in the show driven by Asian...his name...defeats countless muscle cars when trolling for street races? They don't talk much about the engine in that car...blown of course...not a V-8. Any idea the spec's on that motor....~3 liters? 500 hp?...any idea?

His name is AZN. Engine is turbo with fuel injection. Not sure of the power. Personally if i wanted HUGE power, i'd put a Subaru STI engine in with 350hp and daily driver reliability. They put them in 914 also.

Nosed around the web as they are careful about posting hp for the Dung beetle. Apparently they picked up 50hp when going to injection versus carb.
Best I can surmise its a 9 sec quarter mile car and close to 500 hp and why it wins so many street races. One thing you and I know is how hard a rear engine car will launch. My 914 even with skinny tires would give a lot a much stronger cars a good run to 30mph. I am sure your beetle is a rocket off the line.
They likely could punch it out even more and make it stronger with bigger turbos but then it would need wheelie bars and the car would lose some of its sleeper quality.

Not much of a sleeper with it on TV.

This is more of a sleeper.... body is bone stock, even the Porsche alloys stock factory paint color. Only give away is the stance and exhaust.

Yep.

Sure, you don't need to use a torque wrench but if you screw up, it gets expensive really fast. Seems like cheap insurance to me.

Even the cheapest one is wastly better than guessing. Like having a cheap carpenters ruler vs none.