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ANVER Vacuum Fitting Threads Explained: NPT vs G (BSPP)

NPT seals with sealant on the threads; G seals with a washer under the fitting head. Many G male fittings thread into NPT female ports but will leak.

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Vacuum Fitting Conversions Explanation

Standard Sizes for Pipe Threads used in the Vacuum Components Industry

Are you Confused by Pipe Threads used in the Vacuum Industry?

The explanation is actually straightforward. There are two main pipe threads used for vacuum components worldwide.

NPT and G (BSPP) threads
“NPT”
Tapered
Thread

NPT thread information

The first is “NPT”, which is a tapered pipe thread used in the USA and regulated per the American National Standard Institute (ANSI B2.1-1968).

The common thread standard in North America; the same as used by plumbers.
Seals tight by means of sealed threads. The technical references say it was not designed for vacuum but it does seal tight with sealant.
Does not require rubber O-Rings or washer seals but requires sealant or teflon tape to seal the threads. Just apply sealant and tighten fully. Cons:
Multiple cup suspensions or any multiple fitting applications will result in uneven heights. Also, when you reuse the fitting, the height shortens.
NPT fittings for vacuum should be nickel plated brass or steel as the fittings take a lot of abuse when reused. We do not recommend aluminum NPT fittings unless absolutely necessary as they tend to have their threads ruined after only one use. Also NPT fittings tend to get snapped off, or have their threads ruined more than straight G threads. NPT fittings vary by batch and one needs to be more sensitive to the force required to seal them.

NPT and G (BSPP) threads “G or BSPP” Straight
Thread

The second is “G”, which is a straight pipe thread also known in the trade as BSPP, NPS or “G”, commonly used in Europe, Asia and most of the rest of the world. It is technically a Whitworth thread which is actually a British Standard Pipe Parallel (BSPP) thread known as “G”. Per the ISO standard, the proper designation is “G” in front of the fraction.

The common metric vacuum thread standard in the industrialized world (but not North America). The way to go if you export.
Seals tight thru Rubber O-Rings or Plastic Washer Seals around the head of the fitting and not the threads. Does not require sealant or teflon tape making less of a mess when assembling equipment.
It ensures that all items have the exact same height when fully tightened initially, and when reused. Fittings screw on and off easily, and threads show no wear after reuse.
Cons:
Requires that the seat area around the fitting be flat, clean and even to get an air-tight seal.

This all sounds more complicated than it is; there are basically two major pipe thread types used for these pipe fittings and many combinations work together. ANVER often builds both the tapered NPT or straight “G” thread versions or uses adapter fittings to go between them. Confusion exists because many sizes of G male fittings thread into NPT female fittings without adapters even though they will not seal correctly. Here are some metric screw and pipe combinations that demonstrate this:

1. M5 and 10/32″ Male fittings thread into a M5 Female equally well. However, an M5 Male will not thread into a 10/32″ Female.

2. G 1/8″ and 1/8″ NPT Male fittings thread into a 1/8″ NPT Female though only the NPT to NPT combination actually seals tight. Even though G males can thread into a NPT female, they will almost certainly leak. A 1/8″ NPT Male will not thread into a G 1/8″ Female at all.

The 1/4″ and 3/8″ combinations do not work like this but 1/2″ and 3/4″ work marginally sometimes as they have the same threads per inch. In general BSPT (Tapered) / BSPP (Parallel) threads have different angles (55 vs 60), shapes (rounded vs flattened valleys and peaks) and in most cases threads per inch than NPT.

If you would like to know if the fittings you have selected are compatible, feel free to contact us. For a more technical explanation, see the 2700+ page “Machinery’s Handbook” from www.industrialpress.com, which is the bible for these specifications. The above information is a layman’s explanation.

Pipe Size	Pipe OD	Threads Per Inch (Pitch)		Some BSPP Male thread into Female NPT ; However, they Leak and are Not Recommended
Pipe Size	Pipe OD	NPT	BSPP/BSPT
1/8″	0.405″	27	28	Male BSPP “G” screws into Female NPT but leaks
1/4″	0.540″	18	19	Does not screw in at all
3/8″	0.675″	18	19	Does not screw in at all
1/2″	0.840″	14	14	Male BSPP “G” screws into Female NPT but leaks
3/4″	1.050″	14	14	Male BSPP “G” screws into Female NPT but leaks
1 “	1.315″	11 1/2	11	Does not screw in at all

NPT and G (BSPP) threads
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ANVER Vacuum Fitting Threads Explained: NPT vs G (BSPP)

Vacuum Fitting Conversions Explanation

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