fixing fuel line leak

[innellan House]

It is not uncommon for petrol to weep from the threaded inlet of the AC fuel pump. Why is this? The inlet adaptor is 1/4 BSPP coned x 1/8 NPT i.e. the female thread in the pump casting is American. If the original adaptor goes missing, its likely replacement would be 1/4 BSPP x 1/8 BSPT i.e. all British. Unfortunately NPT and BSPT threads are not strictly compatible, risking cross threading. Another reason for leakage is over-tightening (causing permanent distortion) to create a seal, without understanding these threads are not dry seal i.e. this type of thread is meant to be used with a sealant, either PFTE or anaerobic resin. When torqued, the male/female thread flanks are wedged creating a very secure joint, but a spiral leakage path (by design) remains along the crest of the male thread - see picture. Not using a sealant/lubricant also increases the risk of galling damage/leaking. Reusing a taper thread is generally not recommended. The 1/4BSPP x 1/8NPT male/male adaptor does not exist in Australia. I show how one can be made from a barb connector.

[innellan House]

(Image 1) shows another solution. The fibre (or copper) washer seals the joint. A parallel thread into the pump casting will not expand the female part, hence reuse is a better tolerated. The .405" (10mm) x 27 tpi thread does not exist commercially, but a nice job for the home shop machinist. By the way, the pump exit adaptor (image 2) has a 1/2 UNEF thread on the end that engages the olive nut. This is an extra fine UNF thread, also found on vintage motorcycles.