This is my 4" (101.6 mm) diameter, 4-jaw independent chuck with the 2 modified jaws, and also note the excellent welding and alignment pins.
I have only made one pipe, so far (I just got it) and it was made from a very irregular shaped briar block. The chuck performed perfectly, first time, very easy to set up, center, rotate, etc., and I'm sure I will get better using it with practice.
Jaws Specs:
material: 1/2" thick (12.7 mm) 4140 alloy steel (annealed)
min. block thick: 1-3/8" thick (35 mm) (could be 1-1/8" (28.6 mm), if I grind off the points of the orig jaws)
easy max. block thick: 2-1/2" (63.5 mm)
absolute max. block thick: 3-1/4+" (82.6 mm)
Jaw Dimensions: 1-1/2" x 1-7/8" long (2-5/8" above chuck face)
(38.1 mm x 47.6 mm long) (66.7 mm above chuck face)
Alignment Pins: 1/4" dia. (6.4 mm) pointed hex socket (allen) machine screws with locking nuts
Overall Length: Base Plate + Chuck + Jaws = 5-1/2" (139.7 mm)
A Picture is worth a 1000 words...

2 pictures = 2000 words...
