Nominal thickness is thickness of material before machining.  Both sidewalls  are machined to insure    
                            proper mating surfaces for gaskets and flanges.        
                            * = CROWN TOP
                           

OPTI-Klear sight glasses are machined on CNC machining centers and CNC lathes to impart the best optical sight glasses available. They are machined flat on both side walls (with the exception of the OK4S1)  to keep the sidewall tolerances in line with standard API flanges and meet TTMA specifications. All of our sight glasses are annealed to help reduce cracking and crazing caused by certain fuel additives. All of our sight glasses are date coded by heat stamp and each sight glass has a heat branded part number. E-mail us or fax us at 402-445-6112   with any special custom sizes you need quoted. For special sizes include: Inside diameter, outside diameter, bolt hole circle (BHC), bolt hole diameter, number of bolt holes, thickness and quantity.