Thread sealant for diesel fuel lines?

[wingcommander]

I don't think so there must be a mating surface or the pipe will pull out.
It is that mating surface that needs to be sealed not the thread


This was the type of fitting I was referring to .
My apologies, replying to wrong forum here . Remind me again men my age can't multitask.

 

[Gustywinds]

You can buy BSP tapered TAPs and BSP Parallel taps. BSP Parallel Holes are spot faced to create the seal at the face and not in the thread and flanged BSP fittings with appropriate washers should be used. BSP taper thread is used on pipework where the thread angle is the same on both male and female. The seal is thus formed along the full length of the thread and not the first turns if it was parallel.

Precisely, the BSPP seal on a face with a rubber, fibre, copper or in the case if really high pressure systems, sometime metal to metal seal, A BSPT going into a BSPP parallel as the OP describes is only sealing on the one thread where the thread diameter clashes. It is not a good idea and no reputable engineer would design a system where this is the case. The same thing is true of America fittings such as NPT into SAE parallel.
The OP (and apparently others according to DaveRw) get away with it as the pressures are very low (maybe 4-5bar max in a diesel system, I'm not sure?), I am used to dealing with systems running everything from 200 to 800 bar. I just wonder what would happen if his boat was destroyed by fire due to a fuel leak and an inquisitive insurance assessor spotted the bodge?

 

[Daydream believer]

Hawk White then Hemp wound round the thread
Purchased from a plumbing & heating supplies

 

[Freebee]

I use ptfe tape intended for gas fitting which is slightly thicker, 6 turns at least followed by a generous smear of slic tite ~(screwfix) will seal most fuel system pipe joints