Here is the long and short of the issue, IMHO.
Disclosure:
I sell both OEM and several brands of aftermarket cylinders including Big Bore kits.
I make more profit on an aftermarket kit than on an OEM kit, even though the OEM has a price tag much higher in most cases.
I have worked with / provide cylinders to several dozen different builders and try like hell to pay attention to all of their feedback.
Most aftermarket cylinders are an attempt to "clone" an OEM cylinder.
Cost is also a major concern for the aftermarket makers, if they spend as much as the OEM to make a perfect cylinder they have to charge as much as OEM and there is no benefit.
In making the clone the aftermarket gets as close as they can and then calls it good enough.
As any of the builders here can tell you a change of 1 degree or a thousandth of an inch in certain areas of the cylinder creates changes in performance.
There are also a ton of variations in OEM cylinders so if the one they use as a sample isn't the best and then they do a decent job in cloning they may end up with a cylinder that visually looks fine and has good fitment to the crankcase and crank but produces less power due to inefficiencies and inaccuracies in the copying.
So when this cylinder goes to market the average homeowner dude is reasonably happy that his recently dead saw now makes 89% of the power it formerly made, but that isn't enough for the pros and pro builders/mechanics.
Since "there is no replacement for displacement", the aftermarket companies then overbore the same castings that produce a mediocre standard size cylinder to produce the same cylinder with the same port timing and measurement errors, just a little bigger.
The company has no additional molding or R&D to do and now has two offerings for the marketplace including the "Big Bore" which of course sounds impressive. this helps them offset the considerable cost of making a new casting.
Most builders/porters/modders who build saws for a living or for pay are unwilling to chance working with a subpar cylinder when the cost difference isn't really that great in relation to the overall cost of a pro saw and porting work.
Plating is an issue for builders including overplating, uneven plating, plating flash in ports and intake/exhaust. Overall molding and finishing may also leave a lot more for the builder to do himself and the customer doesn't see or pay for the extra work.
Since quality control and overall uniformity is better on the OEM cylinder and it has years of research and science behind it most guys feel more comfortable starting the journey to a ported saw from a known origin.
An aftermarket big bore cylinder can be made to run like a beast, but builders are less likely to undertake this as it isn't a repeatable process. If Builder Bob ports an MS660 OEM and takes notes his next OEM 660 build goes faster and maybe better. With an aftermarket he is starting from scratch each time and can't just use the same numbers and process every time a saw arrives with a check.
Big Bore cylinders sell well, even when I tell a potential customer they are better off with OEM or a strong AM with rigid QC like Meteor, more than half the time they buy the cheaper big bore kit.
If you go with any aftermarket kit make sure you use Caber or OEM rings in the right size for your piston, cheap rings kill power, PERIOD.
A big bore kit does reportedly do better than even an OEM cylinder in a big bar, big wood, stumping situation and reportedly in some milling use. A couple of builders say they like the big bore for those uses in pro tree service / milling applications with strong oil mix as the displacement and additional size of the piston give more uniform power and cooling.
Locally the 660BB I built for a tree service that uses it for smaller stumping got a "meh, about the same as the oem 660" reaction.
Dave
also worth noting (as shown by weimedogs excellent vids) that some castings aren't off by merely thousandths of an inch but by more than a 1/4 inch and are freeporting calamities
If a builder took the time to correct all the problems and built a strong saw, most customers wouldn't want to pay the same price for that saw as all OEM. So your customer base turns into bargain shoppers, who statistically are a bigger pain in the butt to deal with.
Definitely lose/lose for the builder I would think.
