BBK and reliability

[coachhejl]

Why do I read that it causes an instability?

I am referring to going from 49cc to 72cc. I feel like this upgrade is simple as long as you jet the carb accordingly and maybe variator weight change to keep in rpm range.

I don't see after that why it wouldn't last as long as stock.

[aeroxbud]

Going from a 40mm bore to a 47mm bore requires a much bigger and heavier piston. This piston greatly increases the rotational mass. Placing more stress on the crank and bearings.
More performance equals more stress. Followed by shorter engine life.

[GrumpyUnk]

The change in reciprocating mass is a definite addition to the stress on the connecting rod, big end bearing, and the crankshaft itself. How long it will last depends on original equipment parts installed at the factory when built, or replacement rod & crankshaft if installed.
Many have had reasonable service without changing the crankshaft, but anything can be turned to scrap given the operator. "I there is a way to screw it up, a sailor will find it." was common knowledge back when.
The same holds true on these little midgets. If you push it too hard, it definitely will likely fail a lot quicker. I do think they were designed to accept more power given the thickness of the cylinder walls, as built and sold. That indicates the ability to overbore when worn, and substitute piston & rings of a proper larger diameter. In the countries where they are prime transportation, .05mm overbores are done, and parts are available to repair using the original cylinder, along with new piston & rings. A common practice to give more life at relatively low cost. Over there. Here, in US, not so low cost as there are likely no shops set up to overbore a 39mm cylinder. It might be a .5mm overbore, decimals escape me.
Any way, the longevity depends on quality, and also on the operator. If/when you do the work, be sure to chedk the fit of the piston in the cylinder, and also check ring end gap. There's a gap calculator on site which I have used. Works well. Also check the piston for cylinder wall clearance. I have had one come in that just seemed to be too tight. It did not 'fall' through the cylinder freely with dry walls. I 'adjusted' the fit using a file until it felt 'right'. Worked fine after. Indication is that out of about a half-dozen, only one had what I thought was a too-tight fit. I'd have to think to actually count the number done without problems...
That said, inspect the whole thing for defects, be sure to check ring gap, and space the gaps about 120 from each other. I slather the thing with lots of oil for assembly,; whether needed or not. Be sure the ferrules are in place and the cylinder and head fit over them properly. If you re-use the original head, using a composition gasket will relieve a bit of the compression increase, and may seal better on a used head.
tom

[br4inl3ss]

one thing i know for sure is that no matter what people say, stock crank is total garbage if you put any amount of additional stress to it. even a aliexpress 15$ crank will be better in quality than STOCK ( that exclude kymco ofcourse ).
if anyone think cheap aftermarket is the same as stock, i suggest them to learn about how its all done and how business works. the crank they install at factory isnt even worth 10$. keep that in mind.

now about compression... what they say is 100% right. but one thing they forget.
some piston are domed and NEEDS a matching head to lower the compression.
a domed 50mm piston with a stock 39mm head will not work together even with 3 base gasket. been there done that.
way too much compression. actually more than 13:1 according to my measurement and maths.
heavy pinging and extreme lack of power when hot. when i say extreme, i eman imagine the engine had only 10cc. thats what i mean by extreme.
the glixal kit i have ( and most KIT are like that but not all ) is made to match together with acceptable compression. dish piston with dish head ( if its called like that... my english is justin trudeau IQ level ). i think it end up near stock compression ratio.
that said, the mass is still a problem.... but not with a repalcement/aftermarket crank. anything but stock isnt a problem.

must be noted tho that some people was lucky enought to get more than 4000 km on STOCK factory crank with a decent 47mm BBK. RARE.