Universal 1/8" NPT to 3/8" Barb Fitting Use w/Radiator Cap This billet weld on fill neck is ideal for use wherever a heavy duty, custom application filler neck is required. 6061-T6 billet aluminum construction, for durability in racing or street applications. Includes 1/8" NPT to 3/8" barbed fitting for plumbing of an expansion tank. Accepts standard radiator caps or our 074-81-016 and 074-81-030 or Billet Cap 074-81-116.