HT2013: Steel 44 mm, 145 mm Long, 1-7/8" x 50 mm OD

Paragon Machine Works 44x145mm steel head tube.   Yes these are expensive, but the added wall thickness at the top and bottom as well as undersized bearing seat significantly reduces distortion from heat.   Highly recommended for beginners.   There is a weight penalty compared to normal straight gauge head tube material.  

Reference: PMW Website