Teflon tape for thread seal?

Messages
23
Location
Houston, TX
Hi everybody, I'm doing a coolant flush & the old water pipe drain plug (Not the rad. drain plug--the cyl. drain plug) wasn't threaded in all the way by the previous owner or shop, so the exposed threads are rusted. I got it backed out & its not leaking as-is. Metal on metal threads To NOT screw up the threads on the water pipe should I- Reuse the bolt as-is since it doesn't leak, or get the rust off the exposed threads w/ penetrant first? (Any chance the water pipe threads have formed around the rust? - The reason I would reuse the bolt as is) Also I need some kind of thread sealant so this won't happen again - is it cool to use Teflon tape? Any chance of the teflon disintegrating and getting into the cooling system over time? I'm trying to avoid buying a huge tube of silicone since I only need a little dab for this, but if its a better option than teflon I'll do it. Thank you
 
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Messages
25,045
Location
ON, Canada eh?
I would use a wire brush to clean off the rust, a grinders wheel with a spinning wire disc is perfect for this, but a hand held wire brush will do it with some time. Put it back in without Teflon tape and tighten down. Then cover the exposed part in wheel bearing grease to keep the moisture away from it. ;\)
 
Messages
25,176
Location
Upstate NY
The threads are NPT and one cannot screw them in all the way as they are tapered. Why are the threads rusted, in most cases the plug is brass. If you have a steel plug, you might consider replacing it with a brass plug. Its OK to seal NPT threads with teflon tape but should not be needed.
 
Messages
2,431
Location
Toronto, Canada
 Originally Posted By: Donald
The threads are NPT and one cannot screw them in all the way as they are tapered. Why are the threads rusted, in most cases the plug is brass. If you have a steel plug, you might consider replacing it with a brass plug. Its OK to seal NPT threads with teflon tape but should not be needed.
NPT threads require a thread sealer (either tape or compound) to properly seal. In a low pressure application like a cooling system, you MIGHT SOMETIMES get away without a thread sealant. NPTF (National Pipe Thread Fuel) will seal without a sealant. To the OP, silicone is not normally used as a thread sealer.
 
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12,385
Location
Northern CA
 Originally Posted By: George7941
NPTF (National Pipe Thread Fuel) will seal without a sealant.
It's National Pipe Thread Tapered Fine (NPTF)
 
Messages
2,431
Location
Toronto, Canada
 Originally Posted By: XS650
 Originally Posted By: George7941
NPTF (National Pipe Thread Fuel) will seal without a sealant.
It's National Pipe Thread Tapered Fine (NPTF)
No ,not necessarily. A number of variations of the NPT thread have been introduced to overcome the problem of spiral leakage and are known as Dryseal threads (See SAE standard J476). The best known is the NPTF (F for Fuel). With this thread design, there are controls on the crests and roots of both the male and the female threads to ensure the crest crushes or displaces material into the root of the mating thread. The interference fit between the crest of one thread and the root of the other, along with the thread flanks matching, seals against spiral leakage. Figure 3 shows an NPTF male tightened into an NPTF female hand tight. You can see the crests of both the male and female thread come into contact with the root before the thread flanks meet. Figure 4 shows the NPTF male and female threads tightened approximately 1 turn past hand tight, and you can see the flanks meet and the crests are displaced into the roots. Although these threads are considered Dryseal, a Teflon tape or liquid is still recommended to aid in the assembly process. The Teflon works as a lubricant to avoid galling of the material when tightening the two threads together and also fills any voids that may cause leakage. A variation of the Dryseal thread is the NPSF (National Pipe Straight Fuel). It is used for internal threads and a NPTF external thread can be screwed into it to provide a satisfactory mechanical connection and a hydraulic seal. The combination of a parallel and tapered thread is not regarded as ideal but is widely used. Highquality plastic quick disconnect couplings typically use NPT threads. 4 Copyright © 2007 by Colder Products Company. Colder Products Company, Colder Products and Colder are registered trademarks with the US Patent Here is a link to the source http://www.colder.com/Portals/0/pdfs/NPT.pdf
 
Messages
8,711
Location
Nothern USA
Water is quite easy to seal. Or at least get close enough that any leaks quickly corrode shut themselves. Fuel and oil are much tougher. This is why the auto manufacturers have moved away from metal to metal fittings and toward O-Ring types.
 
Messages
25,176
Location
Upstate NY
 Originally Posted By: George7941
 Originally Posted By: Donald
The threads are NPT and one cannot screw them in all the way as they are tapered. Why are the threads rusted, in most cases the plug is brass. If you have a steel plug, you might consider replacing it with a brass plug. Its OK to seal NPT threads with teflon tape but should not be needed.
NPT threads require a thread sealer (either tape or compound) to properly seal. In a low pressure application like a cooling system, you MIGHT SOMETIMES get away without a thread sealant. NPTF (National Pipe Thread Fuel) will seal without a sealant. To the OP, silicone is not normally used as a thread sealer.
I have a pair of brass drain plugs in my Merc 5.7L marine engine that are pulled every fall to winterize. I never use any sealant and they do not leak. Maybe brass seals better going into cast iron than a steel plug would. One can talk about NPTF, but there is already threading in the block and one is stuck with whatever it is and I would assume its plain old NPT.
 
Messages
2,431
Location
Toronto, Canada
You must torque on the brass plugs pretty tightly for them not to leak at all. Brass is soft and enough torque will deform the threads enough to seal. You will find that as the years go by the plugs will have to be torqued harder to make them leak-free. Two reasons why you should be using a threadsealer are 1)It takes less torque for a leak-free joint when a threadsealer is used. This leads to less deformation of the threads of the plugs. 2) If, for some reason, there is corrosion at the plugs, the plugs will come out easier if threadsealer is present. My favourite threadsealer is Loctite 567. Been using it for years and it has served me very well.
 
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