Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

NPT thread sealant ....

NPT thread sealant ....

NPT thread sealant ....

You guys are going to be so happy when this project is done and I stop asking question.

Most of my connections are SAE (o-ring). From what I read this is a great seal and most of the items I ordered only gave this option as a connection.

However pump and cylinder (being 30 years old) have NPT connections (thinking about it, gauges still seem to all be NPT).

I did my usual web search and found everyone has an opinion.

I found people suggesting: nothing (since the threads are tapered and should lock in place), Teflon tape (just don't get any over the edge of the pipe, Loctite 545 (with or without primer), Permatex 54540, Rectorseal 5, .......

I even saw one person say that NPT threads are only to be used once.

There is only a few NPT connections but I want to make sure they are leak free.

What is the "best" sealant to use on them (cylinder is 3/8 ... pump 1/2 and 3/8 .... filter 3/4)?

Thanks again ..... Mike

PS ... I have a bit of a phobia when it comes to NPT thread. I have had NPT threads leak so I make sure they are "tight", like two 24" pipe wrenches tight. When building our house, the "certified and licensed" installer, finished the gas pipes, pressurized the pipes to 10 PSI, said they passed and then left for the night. He made the mistake of leaving his test gauge on the system. I noticed the next day the gauge had dropped to zero. I pumped the pipes up to 10 psi and started with the snoop .... one leak ... two leaks ... three leaks! I was getting tired of looking. I put 50 psi in the pipes to make it easier to find the leaks. WOW .... 14 leaks !!!!!!!!!!!!

I called the gas company, they said if it passed his test then there was nothing they could do. I called him. He said I should never have put 50 PSI into the pipes as it "blew out" the sealant. What a crock or s**t. He would not fix it.

So I took everything apart, cleaned the connections (many had very rough threads ... I could tell he needed new dies on his threader) and put it all back together. I bought a new gauge ( he had a 0-100 gauge ... could not even see if it lost 1 psi ... I bought a 0-25 gauge) put 20 PSI on the system ...... a week later still holding 20 PSI. I should have just bought a threader and done the job myself. He never did finish the job (and I made the mistake of paying him as he went along so he got the majority of the money). I still don't have return runs in the house. I will get to these one day.

RE: NPT thread sealant ....

I have successfully used Loctite's Teflon thread sealant. Some folks use both Teflon tape and sealant.

Be careful over-wrenching pipe fittings; you will crack the female port.

RE: NPT thread sealant ....

Where you say SAE, do you mean flanges or STOR? STOR being straight thread 'O' ring.

NPT is tapered and so will need to be sealed, but as stated in the other thread, there are plenty of adaptors available make the jump between the joints.

Lots of options...pick one


RE: NPT thread sealant ....

The folks that use tape often find bits of tape in the pump and valve.

One of the major rules I follow...Never use tape on a fluid power joint.

The PTFE gets suspended in the oil and just hangs around and occasionally get stuck in places that can cause problems.

Anaerobic sealants work better in my experience.

RE: NPT thread sealant ....

Loctite PST is a curing, albeit slow, sealant.


RE: NPT thread sealant ....

My wife keeps saying I just need to invite you guys down for a week. She is getting tired of looking at the unfinished lower floor of the house while the finished area is not being used right above us.

Thanks so very much for the advice .... again (and again, and again, ...)


RE: NPT thread sealant ....

There have been a number of threads (pun intended)on sealing NPT threads. Do a search. Over-tightening is not necessary or good. However, novices never know how tight is tight.

RE: NPT thread sealant ....

NPT threads are not a good choice for high-pressure fluid fittings. The NPT threads have a tapered pitch diameter, and they are designed to create a seal where the thread flanks are forced into contact. But there is still a leak path at the small helical gap between the external thread tips and root fillets of the internal threads. The pipe thread sealant compounds and tapes work by filling in these small gaps. But no matter how carefully you apply these tapes or sealant compounds, there will always be some residue pushed into the fluid circuit.

RE: NPT thread sealant ....

It is awesome reading these posts.

For years I have worried about this exact problem. Working for an air shop for several years, I always worried what was being left in the lines. Even on my own work here around my shop, I am always thinking about sealant being pushed by the threads along in front of the nipple being screwed in. Yet, as you can see from a previous post of mine, I have seen the horrors of not using enough sealant, rough threads and under tightening.

I have eliminated all the NPT threads I can. Only the original cylinder and the pump now have NPT threads. Everything else is SAE (o-ring) connections.

All this talk about cleanliness now has me thinking about how clean the components I am purchasing are?

When I cut tubing, I always use air to blow out the pipes but maybe this is not enough. Maybe I should be "washing" them in a cleaning solution? Now I am thinking about cutting fluids and such used to manufacture the tubing.

I have put (or watched) many hydraulic flexible lines on equipment but never thought about cleaning them before assembly. I just watched a video last night that showed the "extra care" a company takes to push a cleaning bullet through each of the flexible lines they make before sealing them in a bag and sending them to a customer. I have watched many flexible lines being made and can't even remember ever seeing the builder blow them out (I always did before installation).

Hmmmm ..... maybe I need to think about upgrading my cleaning standards.

Thanks again ..... Mike

RE: NPT thread sealant ....

I disagree that no amount of care will prevent pushing thread sealant into the system. If the leading 2 threads on the male fitting have no tape or compound on them, no tape or compound will get pushed into the system. If nothing is on the leading end of the fitting, then nothing will enter the system.

Most likely extreme cleaning measures need not be taken unless extremely sensitive components are in the hydraulic system. Certainly care should be exercised to keep tools and final piping and hose assemblies clean and free of chips and other hard debris. That would be called good shop practice.

If extreme cleanliness is required, then, too, the bulk fluid used to fill the system must be delivered through a filter cart.


RE: NPT thread sealant ....

Thanks again Ted!

I do take care to keep the first few threads clean of anything, starting the tape or sealant "back" .

Funny you mention the cleanliness of the bulk oil. That was one of the things that got me thinking. I was talking with Royal Purple (a senior technical engineer at there headquarters in Texas). He said they filter all the oil through a 3 micron filter before "bottling" and then seal the container. He said this is one of their selling point when they go to large factories and try to get them to change to their brand. They will test your present brand and show you how dirty it is when it comes in new.

Again, honestly, I am sure none of this matters in my case. I have a 30 year old cylinder and pump and a cheap Chinese back-up pump. I am sure anything I do will be better then what has been done it's life.

I always use good cleaning practices no matter what I am assembling.

Thanks again ..... Mike

RE: NPT thread sealant ....

Rolling element bearing life is sensitive to oil cleanliness down to the micron level.

je suis charlie

RE: NPT thread sealant ....

Now maybe a good time to ask ... how fine of a filter do I need on my system?

I was going to put a 10 micron filter on the return line ... is this good enough?

Should I think about a 5 micron filter instead?

It is inside the house ... a relatively clean environment.

Do you worry about things like breather caps? When the oil is drawn down, obviously air is drawn in ... again, inside a house, just dust in the air but how critical are we talking?

I am actually thinking about venting the cap outside so that when the lift comes down and the air vents from the tank I don't get an "oily smell" in the house. So now I have "outside air" going in and out of the tank.

My batteries will be on constant charge so I am planning on venting them outside also (I made the mistake of causing an spark on top of a battery fresh off charge while in college .. it was a large truck battery used to start a gas turbine we had ... blew the top off the battery, covered me with acid ... still remember that 35 years ago ... good lesson).

Thanks ..... Mike

RE: NPT thread sealant ....


The rating of the filter needs to weighted to the system. In other words, you have to assess the level of cleanliness of the system as built, then consider the environment that the system is in, then you need to consider the requirements of the system in terms of tolerance to dirt. On top of the that, consider the duty cycle and also how much you are willing to pay.

It can be tempting to go for a 5 micron filter. However, if the system is generating or taking on particles bigger than 5 microns, the filter will block. Equally, if the components in the system are not very tolerant of dirt, then a 20 micron filter might not be sufficient.

Have a look at the data sheet for the pumps and valves that you have selected. They will tell you what ISO4406 cleanliness is required to keep them working.

The ISO 4406 standard is a measure of the number of 4, 6 and 14 micron particles in 100ml of fluid. So it's reasonable to suggest that a 20 micron particle will not catch enough of these particle to get the system clean. A 5 micron filter will catch these and most of the particles that are bigger and will then become blocked.

Ultimately, the filter selection will be your choice based on the system weighting. By all means, use a 5 micron filter...if your wallet can handle it. But then, if the system is used infrequently, and you take steps to control the ingress, then you should be clean and also relatively wealthy as the filter will not block.

Good luck!!!

RE: NPT thread sealant ....

Additionally, select a return filter with flow rate capacity of at least twice the flow rate of the system so that at the system's coldest temperature the pressure drop does not open the filter bypass check valve during start up. The filter flow capacity may also depend on the return flow rate generated by the retract cycle of the cylinder. You can get reservoir air intake filters which will keep down inhaled contamination.


RE: NPT thread sealant ....


From the posts above I'm sure you can see that designing a hydraulic fluid system for high reliability applications can get quite complicated. ISO 4406 provides guidance for establishing fluid filtration levels, assuming of course the system meets several conditions. One important criteria is cleanliness of the system to start with. If you're going to filter your fluid to a 5 micron level, then all the components used to assemble the system should be cleaned of any particle contaminants 5 micron or larger. The work environment used to assemble the system must also have air filtered to the appropriate cleanliness level. For example, a class 100,000 clean room still can have up to 29,500 particles 5 microns in size per cubic meter of air. In case you're wondering, your garage or basement workshop is nowhere close to meeting class 100,000 clean room standards.

There can also be sources of particulate contamination within the fluid system itself. A common source is surface corrosion (rust) on the tank head space surfaces from condensation of humid air caused by heating/cooling cycles. Ideally your tank should be constructed of corrosion resistant materials, and should use a desiccant breather filter to purge moisture from the air volume inside the tank.

As noted above, very fine filter elements (such as 5 micron) tend to clog quickly, often creating enough pressure drop across the filter media to open the bypass valve and bypass fluid around the filter element. If you're going to use a fine filter element, as a minimum your filter housing should have an impending bypass visual indicator. Or better yet have a differential pressure sensor that alerts you of an impending bypass condition. Otherwise you will have no easy way to check the condition of the filter element.

Selecting the best filter type for your application involves other considerations than simply micron numbers. A 5 micron filter will not typically trap 100% of debris 5 micron in size. An important characteristic is the filter's beta ratio.

I'm sure this all seems like overkill for your application. But hopefully you will find the information informative nonetheless.

RE: NPT thread sealant ....

This information is all fantastic .....

I guess my biggest question now is how the heck any hydraulic equipment works without breaking down????

Being a machinist, depending on who I was working for at the time, I have been in a good number of lean rooms. My "little brother" one day explained to me how "dirty" the clean rooms were that I am use to going into. He is a professor at a college in Canada and they produce integrated circuits in their lab .... that is "clean".

Honestly, I think about the local forklift repair shop or the shop that I mentioned that rebuilds cylinders. Dirt, oil, dust, ..... it is actually somewhere I would not want to work in, never mind try to assemble something that needs to be "clean".

I am planning on 5 GPM so I can go big on a filter and still stay at a reasonable price .... I am looking at filters rated at 20 GPM.

Looking at the Parker valve cut sheets, they all say "Filtration: ISO Code 16/13 ... SAE Class 4 or better"

The cut sheet for the Brand flow valve says "10 Micron Filtration Recommended"

So this is a bit confusing to me. I will have to do some reading to see how "sae class 4" relates to a micron rating (LENZ seems to rate their filter by micron) or Baldwin seem to use the Beta ratio on their web site.

Looks like I have some homework to do again.

Thanks again so very much !!!!!!!!!!!!!!


RE: NPT thread sealant ....

For even more filter information look at Pall data.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close