Ptgr: Engineering and Quality Control is what Defines Quality Long Term Equipment . As you have considered proper specifications and materials usage is paramount . Pro-grade units have all this and more , they have Qualified and Trained Dealer Support Networks . You have done your Home Work as can be seen by the Quality of Questions that you have asked .

Note: After your second paragraph you have asked approx eight separate questions . For sake of argument I will use Chainsaws for answer examples.

1. Forged Steel as per Crankshafts over less strong cast steel or stamped steel units. High Grade aluminium for better heat dissipation and head and cylinder strength . Also stronger Magnesium crankcase vr man-made plastics and Chrome Plating of cylinder bores vs rings only on cheaper products.

2. Internal component tolerances are normally dependant upon the form of component style . Eg: Babbitt bearing vs Smaller Needle bearing to Higher load Ball bearings . Ideally a tighter engine properly maintained (Service Periods and Specified Lubrication Conformance) will out live a less precise and balanced unit . Another example is the Box Store Clamshell Engine Designs that just do not lend themselves to rebuilding but rather a predefined hour throw away life cycle .

3. Most definately Pro-grade equipment to an extent use superior technology of design and higher quality material and design of components. EG: Open Port vs Closed Port Piston and Cylinder Design . 3-5 plate clutch vs 2-3 plate .

4. Most Top Manufacturers have a form of ISO Quality Control Conformance Certification . However not all competition does so eg: Offshore (Asian)

5. Both Zama and Walbro have high and lower grade model carburetors. Eg: Ease of maintenance and quality of material list . eg: cast stl. brass and stainless stl. vs plastic and epdm .

6. Mos quality manufacturers use Premium grade components Bosch , Champion , NGK . vs Offshore Clone Electronics .

7. Design / Construction ( 1. Balance 2. Durability & 3. ease of maintenance) With less grade units comfort , life-cycle and ease of parts and maintenance is usually somewhat compromised .

8. It's somewhat like comparing a Diesel with a Regular Comparable Displacement Gas Engine. After a few Hundred Thousand Miles most if not all Gas Units are in the Boneyard where as most Diesel are just becoming broke in . Not exactly Apples to Apples but you catch my drift. You get what you pay for more or less !

Sorry I could not be more concise but my time is somewhat limited , hope this is of some assistance within your needs.

Yes the technical info I was looking for. I yanked the carb on a freebie Sears (Poulan) trimmer - in doing so I noticed a plastic intake it was mounted too. Well, OK, Corvette LSx motors run plastic intakes too, no biggee... I pulled it off as well (to open the cover to access the muffler... and was shocked to find the plastic intake was in fact a molded cover that directly bolted to and covered the cylinder crank case. That piece of plastic gave me pause and thus this thread...

My intent in asking the question was to neutralize some of variables such as dealer networks/training as well as the types of owners/operators different lines are marketed and sold to. ...And really just get down to the technical/engineering differences in 2 cycle engines.

further assumptions
Retailer/dealer is equally competent, trained, stocked etc.
Owner/operator is a homeowner user only but is equally competent and follows recommended maintenance intervals
Same gas and air cooled 2 cycle oil is used in both cases
Cheap brand is clearly a 'homeowner' 2 cycle engine
Premium brand is from their 'homeowner' line, not their 'commercial' line

So hopefully that leaves us with the purely objective, tangible, quantifiable engineering differences between premium and cheap 2 cycle engines used on power equipment for the sake of discussion/comparison...

thanks

Sounds like you have a half crank engine in that trimmer, I'd be more concerned about the half crank than the intake.
Plastic intakes or "spacers" insulate the carb from the engine heat.
I've had some half crank stuff and never really had problems from the engines, it's all the other stuff that has broken on mine.

I've got all three, Homelite SX135, Echo SRM2200, and Olympyk 400. Homelite is typical cheapie homeowner with 1/2 crank, etc. Echo is in between, and Olympyk (EFCO) is high end commercial. That said, I bought Homelite more than 10 years ago at a yard sale for $5 and it still keeps going. It's lightest of the three so gets most usage. On the other hand Olympyk is used mostly with pruning head (chainsaw) and with brush and circular saw blade. Overkill as a weedeater.

The cheap stuff (like Homelite) uses a 1/2 crank, starter on the output shaft, one piston ring, sleeve bearings, and no plating or tratment on either piston or cylinder bore. I have no problem with plastic parts, but do object to self threading fasteners going into plastic parts. After you've been apart five or six times, threads are gone. The premium stuff uses a full crank (better support), starter on opposite end of crank (much easier to rewind rope or spring when inevitable maintenance required). Also flywheel on opposite end means ignition is more accessible than on cheapy half-crank machines. The Olympyk has chromed cylinder and needle bearings on the rod big and small ends, and ball bearings on both ends of crank.

Although all three manufacturers recommend 50:1 gas/oil, I'm more comfortable doing so with the Echo and Olympyk. I deliberately keep the Homelite a little rich (40:1).

Homelite uses Zama carbs, while Olympyk uses Tillotson, Zama and Walbro carbs. Interestingly both Homelite and Olympyk use Phelon magnetos. They are generally conceded to be almost bullet proof and have 75% of the market.

Although not specific to the engine, cheap vs. professional weedeaters differences include curved shaft with cable drive vs. geared, angle head with solid streel shaft; bushings at string head vs. ball bearings (and zerk nipples for greasing), higher quality throttle controls and operator handles.

Please emlighten me on this "Full Crank" vs "Half Crank" thing.
Most of the engines have had full crankshafts, not a half of one, unless it was broken in half. ;0)

I think a 1/2 crank has only 1 main bearing. If I'm wrong , let me know.

Things are probably a little different here in Australia. I had an old 20" McCulloch and it was a great saw did everything I wanted and more. But it met with an accident and I had to get a new saw there and then. So I went to the local hardware chain store and the best saw they had was a Poulin rebadged McCulloch so having no other choice I bought it, and that's when my problems began.
After two tanks of fuel the saw started to bed or run in and went out of tune but the carby required a special tool to adjust the mixture which they won't sell you so I made one. That overcame the initial problem. I then took the saw up to my brothers place 3,500 Ft up in the mountains and the saw ran lean due to the altitude but I could retune it. I don't know what your fuel is like in America but the blend of fuel over here can change from brand to brand and tank full to tank full and while youc car has a computerised injection system it can adjust to suit these variations but highly tuned motors such as you have on a chain saw can not
As a consequence if you are just a handyman user you may not be aware of these sorts of problems with saws and so what is not a problem for a professional user can be a major problem for a not so expert user. I do not know Brian as I was not there, but this may have been the problem you had with the first saw.
Now having worked for a company importing and distributing saws and being in the engine design department of the same company as I said this was a nuisance to me but to a less experienced person it is a major problem. Especially if you have to keep taking the saw back to the shop for a quick tune up instead of being able to do it yourself. I also spent 30 years as a volunteer Bush Fire Fighter and as there were always variations in the blend of fuel and we could just about go anywhere from the oceans edge to the mountains so each saw had a screw driver attached to it to adjust the carby mixture for the blend of fuel or altitude. In fact in such circumstances it could be a matter of life or death. Again because we needed reliable saws we always used Sthil. This standardisation also meant parts were interchangeable.
But now I have had my McCullock/Poulan saw for a while you start to notice other things like the chain sprocket is wearing out fast. But for me the biggest bug is the fact that on this saw the exhaust comes out towards the top of the muffler and blows forward where it tends to strike the log and blow back in your face. This results in you running out of breath quickly and also at least in my case suffering an angina attack. The result is I will be replacing this saw fairly soon.
So my advice to even a home handyman is, when you buy a saw check it out well. Have a look at what the Pro's use and be guided by that. Also for peace of mind be prepared to spend a little bit extra as it will in time pay for itself. Finally remember that in the first few tanks of fuel the saw will run or bed in and the tune may change so if you can not perform a tune up yourself take it to the shop and have them do it for you. The problem could be as simple as that.