So I recently swapped an Acera MTB drivetrain over to a road frame. I used Sheldon Browns cassette width measurements and came up with 42mm. I used the original bb as well which puts the chainrings at 50mm.

Obviously 8mm is a big discrepancy, however everything shifts smooth through all gears and the chain doesn't rub in cross chained configurations. With this being the case is it going to cause any problems if I leave it as is, or should I buy a more narrow bb and bring it in a couple mm? Thank you.

42mm measured how? 50mm measured how? and how many chainrings. Not enough info.

Mechanically defining chain line: a line parallel to the center-line of the bike ..

in a triple crank the middle chain ring is on a line matching the center of the rear cog cluster..
a cog if an odd number or the gap between the 2 most centered cogs if an even number..

do what you have to do to make it so.



...

Never Fix A Running Piece.

42 mm rear measured using Sheldon Browns chainline article. 50mm center of the center ring to the middle of the seat tube. It's a triple btw.

That's how feel, I just wasn't sure if it would cause excess wear.

Guess you'll just have to push big gears more often, to even out the wear

If you are having issues, the first quote. If you are not having issue the second quote.

IMO, chain line can't be assessed from the measurement to the seat post center alone. You still need to extend that imaginary line back to the rear cluster and see where it hits the stack. Preferably near the center of it.

Don't obsess about "precise" numbers. That number works if you spend equal time on all cogs & rings.
I tend to use the smaller/higher cogs more than the others, so I run a bit wider chain line, so my chain tends to be running "straighter" for the gears I use the most. I have no idea what the actual number is, because it simply doesn't matter.

Just put the chain on the middle chainring and the center cog if the cassette and see how square the chain is to the rear hub/bb spindle. If it looks pretty close, and since it works smoothly, you should be good.

John

Sometimes I feel like an analogue man who's been trapped in a digital world.

"If it ain't broke, don't fix it."

Another simple check is look at the clearance between the crank chain ring & the chain stay, how many mm is it ? is it too close like 1.mm or about 12.mm away, these little pointers all help to understand if you need to change out the BB.

Also if you have reduced the possible three ring crank-set onto a road bike, on the lowest chain ring possible 22. & the highest gear on the back how close to the chain stay is the chain, do don't want it rubbing across the top.