Joining tapered and straight cut threads

[ifoxwell]

I recently asked the question of how to seal a joint for a seacock, thankyou all that replied... but it does raise another question.

All the standard bronze fittings I see around appear to be a tapered thread and yet my saildrive leg and possibly the dzr ball valve I've bought could be straight cut.

Now I'm sure this mix up happens all the time so how much of a problem is it. I'm sure in pure technical terms it has to be bad but in practice how much difference does it make?

Ian

 

[lpdsn]

I've always just taken care to always buy the right type of thread. Takes a bit of double checking and I'm sure I'll get it wrong one day. I've never risked mixing and matching. I guess it could be done with enought PTFE tape, but I doubt my insurance company would be happy if it resulted in a claim.

ASAP are a very good reliable source of fittings and they have a wide range, so you should be able to find parallel fittings.

 

[prv]

Tapered male into straight female is considered acceptable, but not vice versa. Teflon tape on the threads as lubricant will help them engage far enough to jam and make a seal. The teflon isn't meant to be a sealant, so you don't wind loads on.

Pete

 

[Jaguar 25]

Tapered male into straight female is considered acceptable

Rather than acceptable it is a requirement for tapered male fittings wrapped with PTFE tape to be used with parallel female threads.

 

[sailorman]

Do not over tighten tapered male threads, it may split the thing its being screwed into

 

[ghostlymoron]

What about tightening against a backnuts or applying thread sealant? No-one has mentioned these yet - they usually do.
I'm firmly into the male taper into parallel female with three rounds of PTFE tape camp.

 

[Lon nan Gruagach]

What about tightening against a backnuts or applying thread sealant? No-one has mentioned these yet - they usually do.
I'm firmly into the male taper into parallel female with three rounds of PTFE tape camp.

I'm prepared to be roasted here, or at least put right...
Back nuts are only used where orientation or axial position of the mating parts is required, otherwise just wind it in till it hits something (like the end of the thread)
An example would be bent/formed conduit into box. Both box and conduit can only have one orientation, so to make tight a back nut is used.

 

[ghostlymoron]

I'm prepared to be roasted here, or at least put right...
Back nuts are only used where orientation or axial position of the mating parts is required, otherwise just wind it in till it hits something (like the end of the thread)
An example would be bent/formed conduit into box. Both box and conduit can only have one orientation, so to make tight a back nut is used.

I agree. I think some people are a bit anal (can I say that?) about it especially when it comes to sea cock orientation. I have found that using the tapered/parallel/PTFE method you can always get the valve handle in the correct orientation. I've had no leaks or accidental 'unscrewings' either.

 

[ifoxwell]

Thanks for all the advice guys