I’m building a frame up, according to the original spec it came with a Truvativ Howitzer BB. When I look at these on crc they are specced for either a 51 or a 56mm chainline. How would I know what chainline this frame is designed for?

Would rear axle length be the factor?

standard 135mm axle? what size BB shell is it?

i had a look on CRC, i think thats a typo on their behalf
51mm chainline for 68-73mm MTB BB
or 56mm for 83mm Downhill BB
the 83mm shell is 10mm wider( 5mm each side) so 51mm plus 5mm equals 56mm you see
so measure your BB shell and thats your answer, if that helps 🙂

rbrstr, thank you for that it kinda confirms the way I was thinking about it.