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Diagnose and resolve 5XX errors for Cloudflare proxied sites.

Create, build and nourish the farm of your dreams in FarmVille 2. You’ll interact with a variety of interesting characters, grow and unlock special crops, take care of animals, and make your farm come alive. Search the world's information, including webpages, images, videos and more. Google has many special features to help you find exactly what you're looking for. Tap limits are designated as L-i, H-1 H-2, H-3, etc. In the chart on this page we have illustrated the difference between tap and product limit, using a 1/4-20 tapped hole as the example.

Overview

When troubleshooting most 5XX errors, the correct course of action is to first contact your hosting provider or site administrator to troubleshoot and gather data.

Required error details to provide your hosting provider

  1. Specific 5XX error code and message
  2. Time and timezone the 5XX error occurred
  3. URL that resulted in the HTTP 5XX error (for example: https://www.example.com/images/icons/image1.png)

Additional details to provide to your hosting provider or site administrator are listed within each error description below. Cloudflare Custom Error Pages change the appearance of default error pages discussed in this article.

Error analytics

Error Analytics per domain are available within the support portal for your account. Error Analytics allows insight into overall errors by HTTP error code and provides the URLs, responses, origin server IP addresses, and Cloudflare data centers needed to diagnose and resolve the issue. Error Analytics are based on a 1% traffic sample.

To view Error Analytics:

  • Navigate to the Cloudflare support portal. Refer to instructions about filing a support ticket for information on how to reach the support portal.
  • Scroll down to the Error Analytics section.
  • Click Visit Error Analytics.
  • Enter the domain to investigate.
  • A graph of Errors over time is displayed.
  • Click on a status code in the table beneath the graph to expand traffic error details.

Error 500: internal server error

Error 500 generally indicates an issue with your origin web server. Error establishing databaseconnection is a common HTTP 500 error message generated by your origin web server. Contact your hosting provider to resolve.

Resolution

Provide details to your hosting provider to assist troubleshooting the issue.

However, if the 500 error contains “cloudflare” or “cloudflare-nginx” in the HTML response body, provide Cloudflare support with the following information:

  1. Your domain name
  2. The time and timezone of the 500 error occurrence
  3. The output of www.example.com/cdn-cgi/trace from the browser where the 500 error was observed (replace www.example.com with your actual domain and host name)

Error 502 bad gateway or error 504 gateway timeout

An HTTP 502 or 504 error occurs when Cloudflare is unable to establish contact with your origin web server.

There are two possible causes:

  • (Most common cause) 502/504 from your origin web server

502/504 from your origin web server

Cloudflare returns an Cloudflare-branded HTTP 502 or 504 error when your origin web server responds with a standard HTTP 502 bad gateway or 504 gateway timeout error:

Resolution

Contact your hosting provider to troubleshoot these common causes at your origin web server:

  • Ensure the origin server responds to requests for the hostname and domain within the visitor’s URL that generated the 502 or 504 error.
  • Investigate excessive server loads, crashes, or network failures.
  • Identify applications or services that timed out or were blocked.

502/504 from Cloudflare

A 502 or 504 error originating from Cloudflare appears as follows:

If the error does not mention “cloudflare,” contact your hosting provider for assistance on 502/504 errors from your origin.

Resolution

To avoid delays processing your inquiry, provide these required details to Cloudflare Support:

  1. Time and timezone the issue occurred.
  2. URL that resulted in the HTTP 502 or 504 response (for example: https://www.example.com/images/icons/image1.png)
  3. Output from browsing to www.example.com/cdn-cgi/trace (replace www.example.com with the domain and host name that caused the HTTP 502 or 504 error)

Error 503: service temporarily unavailable

HTTP error 503 occurs when your origin web server is overloaded. There are two possible causes discernible by error message:

  • Error doesn’t contain “cloudflare” or “cloudflare-nginx” in the HTML response body.

Resolution: Contact your hosting provider to verify if they rate limit requests to your origin web server.

  • Error contains “cloudflare” or “cloudflare-nginx” in the HTML response body.

Resolution: A connectivity issue occured in a Cloudflare data center. Provide Cloudflare support with the following information:

  1. Your domain name
  2. The time and timezone of the 503 error occurrence
  3. The output of www.example.com/cdn-cgi/trace from the browser where the 503 error was observed (replace www.example.com with your actual domain and host name)

Error 520: web server returns an unknown error

Error 520 occurs when the origin server returns an empty, unknown, or unexpected response to Cloudflare.

Resolution

Contact your hosting provider or site administrator and request a review of your origin web server error logs for crashes and to check for these common causes:

  • Origin web server application crashes
  • Cloudflare IPs not allowed at your origin
  • Headers exceeding 32 KB (typically due to too many cookies)
  • An empty response from the origin web server that lacks an HTTP status code or response body
  • Missing response headers or origin web server not returning proper HTTP error responses

If 520 errors continue after contacting your hosting provider or site administrator, provide the following information to Cloudflare Support:

  • Full URL(s) of the resource requested when the error occurred
  • Cloudflare cf-request-id or cf-ray from the 520 error message
  • Output from http://www.example.com/cdn-cgi/trace (replace www.example.com with your hostname and domain where the 520 error occurred)
  • Two HAR files:
    • one with Cloudflare enabled on your website, and
    • the other with Cloudflare temporarily disabled.

Error 521: web server is down

Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain Cloudflare IP addresses.

The two most common causes of 521 errors are:

  • Offlined origin web server application
  • Blocked Cloudflare requests

Resolution

Contact your site administrator or hosting provider to eliminate these common causes:

  • Ensure your origin web server is responsive
  • Review origin web server error logs to identify web server application crashes or outages.
  • Confirm Cloudflare IP addresses are not blocked or rate limited
  • Allow all Cloudflare IP ranges in your origin web server's firewall or other security software
  • Find additional troubleshooting information on the Cloudflare Community.

Error 522: connection timed out

Error 522 occurs when Cloudflare times out contacting the origin web server. Two different timeouts cause HTTP error 522 depending on when they occur between Cloudflare and the origin web server:

  1. Before a connection is established, the origin web server does not return a SYN+ACK to Cloudflare within 15 seconds of Cloudflare sending a SYN.
  2. After a connection is established, the origin web server doesn’t acknowledge (ACK) Cloudflare’s resource request within 90 seconds.

Resolution

Contact your hosting provider to check the following common causes at your origin web server:

  • (Most common cause) Cloudflare IP addresses are rate limited or blocked in .htaccess, iptables, or firewalls. Confirm your hosting provider allows Cloudflare IP addresses.
  • An overloaded or offline origin web server drops incoming requests.
  • Keepalives are disabled at the origin web server.
  • The origin IP address in your Cloudflare DNS app does not match the IP address currently provisioned to your origin web server by your hosting provider.
  • Packets were dropped at your origin web server.

If none of the above leads to a resolution, request the following information from your hosting provider or site administrator before contacting Cloudflare support:

  • An MTR or traceroute from your origin web server to a Cloudflare IP address that most commonly connected to your origin web server before the issue occurred. Identify a connecting Cloudflare IP recorded in the origin web server logs.
  • Details from the hosting provider’s investigation such as pertinent logs or conversations with the hosting provider.

Error 523: origin is unreachable

Error 523 occurs when Cloudflare cannot contact your origin web server. This typically occurs when a network device between Cloudflare and the origin web server doesn’t have a route to the origin’s IP address.

ResolutionContact your hosting provider to exclude the following common causes at your origin web server:

  • Confirm the correct origin IP address is listed for A or AAAA records within your Cloudflare DNS app.
  • Troubleshoot Internet routing issues between your origin and Cloudflare, or with the origin itself.

If none of the above leads to a resolution, request the following information from your hosting provider or site administrator:

  • An MTR or traceroute from your origin web server to a Cloudflare IP address that most commonly connected to your origin web server before the issue occurred. Identify a connecting Cloudflare IP from the logs of the origin web server.
  • If you use Railgun via a Cloudflare Hosting Partner, contact your hosting provider to troubleshoot the 523 errors.
  • If you manage your Railgun installation, provide the following to:
    • A traceroute to your origin web server from your Railgun server.
    • The most recent syslog file from your Railgun server.

Error 524: a timeout occurred

Error 524 indicates that Cloudflare successfully connected to the origin web server, but the origin did not provide an HTTP response before the default 100 second connection timed out.

ResolutionContact your hosting provider to exclude the following common causes at your origin web server:

  • A long-running process on the origin web server.
  • An overloaded origin web server.

If you regularly run HTTP requests that take over 100 seconds to complete (for example large data exports), move those processes behind a subdomain not proxied (grey clouded) in the Cloudflare DNS app.

Error 525: SSL handshake failed

525 errors are often caused by a configuration issue on the origin web server. Error 525 occurs when these two conditions are true:

  1. The SSL handshake fails between Cloudflare and the origin web server, and
  2. Full or Full (Strict)SSL is set in the Overview tab of your Cloudflare SSL/TLS app.

Resolution

Contact your hosting provider to exclude the following common causes at your origin web server:

  • No valid SSL certificate installed
  • Port 443 (or other custom secure port) is not open
  • No SNI support
  • The cipher suites accepted by Cloudflare does not match the cipher suites supported by the origin web server

Error 526: invalid SSL certificate

Error 526 occurs when these two conditions are true:

  1. Cloudflare cannot validate the SSL certificate at your origin web server, and
  2. Full SSL (Strict)SSL is set in the Overview tab of your Cloudflare SSL/TLS app.

Resolution

Request your server administrator or hosting provider to review the origin web server’s SSL certificates and verify that:

  • Certificate is not expired
  • Certificate is not revoked
  • Certificate is signed by a Certificate Authority (not self-signed)
  • The requested or target domain name and hostname are in the certificate's Common Name or Subject Alternative Name
  • Your origin web server accepts connections over port SSL port 443
  • Temporarily pause Cloudflare and visit https://www.sslshopper.com/ssl-checker.html#hostname=www.example.com (replace www.example.com with your hostname and domain) to verify no issues exists with the origin SSL certificate:

If the origin server uses a self-signed certificate, configure the domain to use FullSSL instead of Full SSL (Strict). Refer to recommended SSL settings for your origin.

527 Error: Railgun Listener to origin error

A 527 error indicates an interrupted connection between Cloudflare and your origin's Railgun server (rg-listener). Common causes include:

  • Firewall interference
  • Network incidents or packet loss between the Railgun server and Cloudflare

Common causes of 527 errors include:

Slot

If contacting Cloudflare support, provide the following information from the Railgun Listener:

  • The full content of the railgun.conf file
  • The full content of the railgun-nat.conf file
  • Railgun log files that detail the observed errors

Connection timeouts

The following Railgun log errors indicate a connection failure between the Railgun Listener and your origin web server:

Resolution

Contact your hosting provider for assistance to test for connectivity issues between your origin web server and your Railgun Listener. For example, a netcat command tests connectivity when run from the Railgun Listener to the origin web server’s SERVERIP and PORT (80 for HTTP or 443 for HTTPS):

LAN timeout exceeded

The following Railgun Listener log error is generated if the origin web server does not send an HTTP response to the Railgun Listener within the 30 second default timeout:

The time is adjusted by the lan.timeout parameter of the railgun.conf file.

Resolution

Either increase the lan.timeout limit in railgun.conf, or review the web server configuration. Contact your hosting provider to confirm if the origin web server is overloaded.

Connection refusals

The following errors appear in the Railgun logs when requests from the Railgun Listener are refused:

Resolution

Allow the IP of your Railgun Listener at your origin web server’s firewall.

TLS/SSL related errors

The following errors appear in the Railgun logs if TLS connections fail:

Resolution

If TLS/SSL errors occur, check the following on the origin web server and ensure that:

  • Port 443 is open
  • An SSL certificate is presented by the origin web server
  • the SAN or Common Name of the origin web server’s SSL certificate contains the requested or target hostname
  • SSL is set to Full or Full (Strict) in the Overview tab of the Cloudflare SSL/TLS app

Error 530

HTTP error 530 is returned with an accompanying 1XXX error displayed. Search for the specific 1XXX error within the Cloudflare Help Center for troubleshooting information.

Related resources

Tap Sizes What is the difference between NC and NF ( National Coarse and National Fine ) thread?
Styles of tapsClass of threadGH numbers
Basic Point In Thread MeasurementConstants For Finding Pitch Diameter And Minor Diameter Of Screw ThreadsThread Constants For Various Percentages
Relation Of Tap Pitch Diameter To Basic Pitch DiameterTap Limits Product Limits And Class Of ThreadHow to order Special Taps



Tap Sizes

Tap Sizes have been standardized to conform with those of standard screws, bolts, and studs. Machine Screw tap sizes range from No. 0 through No. 14; No. 0 being .0600' outside diameter; No. 1, .0730'; No. 2, .0860, etc.--all in .0130 increments. Hand Taps, more commonly designated as Fractional Taps and used today on all production machines, are designated in fractional and integral inch sizes from 1/4' upwards.
Threads per Inch are shown for various tooth forms: the Unified series adopted by Great Britain and the United States during the war, and the corresponding American National Standard. NC and UNC mean coarse thread. NF and UNF mean fine thread. NS means special thread.
Pitch Diameter is the basic dimension of a screw, threaded hole, or a tap the diameter of an imaginary cylinder, the surface of which passes through the thread where width of thread and space between threads are identical. This cylinder, of course, would be a cone for tapered taps. It is upon Pitch Diameter that tolerance limits are based to establish Class of Thread.

What is the difference between NC and NF ( National Coarse and National Fine ) thread?


The difference between NC and NF is the pitch.

Pitch is the distance between the crests of each thread. (See Image Below)

Coarse threads have a larger pitch ( fewer threads per inch, or a bigger distance between the threads) compared to fine threads.

The pitch can be found by looking at a chart for either NC or NF and changes depending on the diameter. (See tables below)

NC (National Coarse) and UNC (Unified National Coarse) are the same. NF (National Fine) and UNF (Unified National Fine) are the same

NC ( National Coarse )
Size ( Diameter ) Threads per Inch
1/4'20
5/16'18
3/8'16
7/16'14
1/2'13
9/16'12
5/8'11
3/4'10
7/8'9
1'8
1-1/8'7
1-1/4'7
1-3/8'6
1-1/2'6
Link to the Full NC Table
NF ( National Fine )
Size ( Diameter ) Threads per Inch
1/4'28
5/16'24
3/8'24
7/16'20
1/2'20
9/16'18
5/8'18
3/4'16
7/8'14
1'12
1-1/8'12
1-1/4'12
1-3/8'12
1-1/2'12
Link to the Full NF Table

Class Of Thread

There are three established Classes of Thread, designated in the Unified series by adding “A” for screws and “B” for nuts (or other internal threads) to show definite limits and tolerances.
Class lB Thread is that in which a 1 A screw can be run in readily for quick and easy assembly. The hole is classified as 1 B. The fit is 1 B Thread, and rarely used in modern metalworking.
Class 2B Thread consists of a 2A screw in a 2B hole. This 2B Thread has wide application, accommodates plating, finishes, and coating to a limited extent and, therefore, has fair tolerance allowances.
Class 3B Thread means a 3A Screw in a 3B Nut or threaded hole for applications where tolerance limits are close.

GH Numbers

In the tables that follow, tap selections are shown for the Class of Thread desired and, under the Class of Thread heading, applicable GH Numbers are listed. 'G” designates Ground Thread. “H” means that pitch diameter is on the high side of basic. These two letters are followed by a numeral showing the tolerance of pitch diameter oversize as follows:
H1 = Basic to Plus 0.0005'
H2 = Basic Plus 0.0005' to Plus 0.0010'
H3 = Basic Plus 0.0010' to Plus 0.0015'
H4 = Basic Plus 0.0015' to Plus 0.0020'
H5 = Basic Plus 0.0020' to Plus 0.0025'
H6 = Basic Plus 0.0025' to Plus 0.0030'
H7 = Basic Plus 0.0030' to Plus 0.0035'
The diagram below, exaggerated for clarity, illustrates these several selectives in Pitch Diameter tolerance--including 'L' (undersize tolerance), although no 'L' taps are shown in this book. Pitch Diameter varies with the number of threads per inch because the number of threads of Pitch of screw determines the height of thread. Since Basic Pitch Diameter is measured from points half the height of the fully formed thread, a hole drilled to provide theoretical 50% thread engagement would be of the same diameter as the pitch diameter of the tap.


handy tip for H limits

decimal over size x 2 plus 1 = H limit

eg

.005 oversize

.005 x 2 plus 1 = H11

eg.

.003 oversize.

.003 x 2 plus 1 = H7

The Basic Point In Thread Measurement

All measurements must have a controlling point or base from which to start. In the case of a screw thread, this control point is called the BASIC or theoretically correct size, which is calculated on the basis of a full form thread. Thus, on a given screw thread, we have the Basic Major Diameter, the Basic Pitch Diameter and Basic Minor Diameter.
While it is impossible in practice to form screw threads to their precise theoretical or BASIC Sizes, it is possible and practical to establish limits which the deviation must not exceed. These are called the “Maximum” and “Minimum” Limits. If the product is no smaller than the “Minimum Limit” and no larger than the “Maximum Limit,” then it is within the size limits required. This difference between the Maximum and Minimum Limits is the TOLERANCE.
In actual practice the Basic Size is not necessarily between the Maximum and Minimum Limits. In most cases, the Basic Size is one of the Limits. In general, tolerances for internal threads will be above Basic and for external threads, below Basic. See drawing below.
For graphic representation, the Basic Pitch Diameter is commonly designated by a line with variations from it indicated by shorter lines spaced to represent a numerical scale, as shown on the left half of the drawing below.
On an actual screw thread, the Basic Dimensions would follow the contour of the theoretically perfect thread, as on the right half of the drawing below.
To find the basic pitch diameter or basic minor diameter of any screw thread, subtract the constant for the number of threads per inch from the basic major diameter.

Constants For Finding Pitch Diameter And Minor Diameter Of Screw Threads
Constants for Finding
Basic Pitch Diameter		Constants for Finding
Basic Minor Diameter
Threads Per
Inch
Pitch In
Inches
National
Thread
Whitworth
Thread
Theoretical
V
National
Thread
Whitworth
Thread
Theoretical
V
80
72
64
60
56
50
48
44
40
36
32
30
28
27
26
24
22
20
18
16
14
13
12
11-1/2
11
10
9
8
7
6
5-1/2
5
4-1/2
4
3-1/2
3-1/4
3		0.012500
0.013888
0.015625
0.016666
0.017857
0.020000
0.020833
0.022727
0.025000
0.027777
0.031250
0.033333
0.035714
0.035461
0.037037
0.041666
0.045454
0.050000
0.055555
0.062500
0.071428
0.076923
0.083333
0.086956
0.090909
0.010000
0.111111
0.125000
0.142857
0.166666
0.181818
0.200000
0.222222
0.250000
0.285711
0.307692
0.333333		0.00812
0.00902
0.01015
0.01083
0.01160
0.01299
0.01353
0.01476
0.01624
0.01804
0.02030
0.02165
0.02320
0.02406
0.02498
0.02706
0.02952
0.03248
0.03608
0.04059
0.04639
0.04996
0.05413
0.05648
0.05905
0.06495
0.07217
0.08119
0.09279
0.10825
0.11809
0.12990
0.14434
0.16238
0.18558
0.19985
0.21651		0.00800
0.00889
0.01000
0.01067
0.01144
0.01281
0.01334
0.01455
0.01601
0.01779
0.02001
0.02134
0.02287
0.02372
0.02463
0.02668
0.02911
0.03202
0.03557
0.04002
0.04574
0.04926
0.05336
0.05568
0.05821
0.06403
0.07115
0.08004
0.09148
0.10672
0.11642
0.12807
0.14230
0.16008
0.18295
0.19702
0.21344		0.01083
0.01203
0.01353
0.01443
0.01546
0.01732
0.01804
0.01968
0.02165
0.02406
0.02706
0.02887
0.03093
0.03208
0.03331
0.03608
0.03936
0.04330
0.04811
0.05413
0.06186
0.06662
0.07217
0.07531
0.07873
0.08660
0.09623
0.10825
0.12372
0.14434
0.15746
0.17321
0.19245
0.21651
0.24744
0.26647
0.28868		0.01624
0.01804
0.02030
0.02165
0.02320
0.02598
0.02706
0.02952
0.03248
0.03608
0.04059
0.04330
0.04639
0.04812
0.04996
0.05413
0.05905
0.06495
0.07217
0.08119
0.09279
0.09993
0.10825
0.11296
0.11809
0.12990
0.14434
0.16238
0.18558
0.21651
0.23619
0.25981
0.28868
0.32479
0.37115
0.39970
0.43301		0.01601
0.01786
0.02001
0.02134
0.02286
0.02562
0.02668
0.02910
0.03202
0.03558
0.04002
0.04268
0.04574
0.04742
0.04926
0.05336
0.05821
0.06403
0.07114
0.08004
0.09147
0.09851
0.10672
0.11132
0.11642
0.12806
0.14230
0.16008
0.18295
0.21344
0.23284
0.25613
0.28458
0.32017
0.36590
0.39404
0.42689		0.02165
0.02406
0.02706
0.02887
0.03093
0.03464
0.03608
0.03936
0.04330
0.04811
0.05413
0.05773
0.06186
0.06416
0.06662
0.07217
0.07873
0.08660
0.09623
0.10825
0.12372
0.13323
0.14434
0.15062
0.15746
0.17321
0.19245
0.21651
0.24744
0.28868
0.31492
0.34641
0.38490
0.43301
0.49487
0.53294
0.57733


Thread ConstantsFor Various Percentages

Formula for Obtaining Tap Drill Sizes
(Select nearest commercial stock drill)
(Outside Diameter of Thread) - (0.01299 X Amount of Percentage of Full Thread / Number of Threads per Inch) =
Drilled Hole Size
(Number of Threads per Inch) X (Outside Diameter of Thread - Selected Drill Diameter / 0.01299) =
Percentage of Full Thread
Figures in table show amount to subtract from O.D. of screw to obtain specific percentages of thread.


Nss Slot Hilesi Nas L Yap L Ru

Threads Per
Inch
Double
Depth
60%
Thread
65%
Thread
70%
Thread
75%
Thread
80%
Thread
85%
Thread
6
7
8
9
10
11
12
13
14
16
18
20
24
27
28
30
32
36
40
44
48
56
64
72
80		0.21651
0.18558
0.16238
0.14434
0.12990
0.11809
0.10825
0.09992
0.09278
0.08119
0.07217
0.06495
0.05412
0.04811
0.04639
0.04330
0.04059
0.03608
0.03247
0.02952
0.02706
0.02319
0.02029
0.01804
0.01623		0.1300
0.1114
0.0975
0.0866
0.0779
0.0708
0.0649
0.0599
0.0556
0.0486
0.0431
0.0389
0.0326
0.0288
0.0276
0.0260
0.0243
0.0216
0.0194
0.0177
0.0161
0.0138
0.0121
0.0107
0.0097		0.1408
0.1207
0.1056
0.0939
0.0844
0.0767
0.0702
0.0649
0.0602
0.0526
0.0466
0.0421
0.0354
0.0312
0.0298
0.0282
0.0263
0.0234
0.0210
0.0192
0.0174
0.0149
0.0131
0.0115
0.0105		0.1517
0.1300
0.1138
0.1011
0.0909
0.0826
0.0755
0.0699
0.0648
0.0566
0.0501
0.0453
0.0382
0.0336
0.0324
0.0304
0.0283
0.0252
0.0226
0.0207
0.0187
0.0160
0.0141
0.0123
0.0113		0.1625
0.1393
0.1219
0.1083
0.0974
0.0885
0.0808
0.0749
0.0694
0.0606
0.0536
0.0485
0.0410
0.0360
0.0347
0.0326
0.0303
0.0270
0.0242
0.0222
0.0200
0.0171
0.0151
0.0131
0.0121		0.1733
0.1486
0.1300
0.1156
0.1039
0.0944
0.0861
0.0799
0.0740
0.0646
0.0571
0.0517
0.0438
0.0384
0.0370
0.0348
0.0323
0.0288
0.0258
0.0237
0.0213
0.0182
0.0161
0.0139
0.0129		0.1842
0.1579
0.1381
0.1228
0.1105
0.1005
0.0921
0.0850
0.0789
0.0691
0.0614
0.0553
0.0460
0.0409
0.0395
0.0368
0.0345
0.0307
0.0276
0.0251
0.0230
0.0197
0.0173
0.0153
0.0138


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Relation OfTap Pitch Diameter To Basic Pitch Diameter

American tap manufacturers use a series of tap pitch diameter limits. These limits feature a .0005” tolerance in tap sizes #0 through 1 inch, and a .001 inch or greater tolerance in tap sizes above 1 inch through 1-1/2 inch diameter, inclusive. The chart shows the relationship between tap pitch diameter limits and basic (nominal) pitch diameter.



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Tap LimitsProduct Limits And Class Of Thread


Engineers frequently receive a request for a Class 3B (or other class) tap. Many times, too, the customer will ask for a tap to produce a “Class 3B Fit”. Ordering taps by these specifications is incorrect, and often impractical. The following information is presented to clarify the difference between the terms Class of Thread, Tap Limits and Product Limits to make ordering taps easier and aid the customer in obtaining the tap best suited for his requirements.
Class of Thread refers simply to the tolerances that control the closeness of fit between two threaded mating parts. This term should be used only in reference to a threaded assembly, as, for example, a screw and nut.
Product Limits refers to the limits and tolerances of the internal or female thread. The° of tolerance is expressed by the terms Class 2, 2B, 3 and 3B. Product limits refer to the various limits and tolerances applying to nuts or internal threads and are identified by the terms Class 2, 2B, 3 or 3B.
Tap Limits refers to the various sizes of taps manufactured to permit selection of a tap which will produce an internal thread within the desired product limit. Tap limits are designated as L-i, H-1 H-2, H-3, etc.
In the chart on this page we have illustrated the difference between tap and product limit, using a 1/4-20 tapped hole as the example.
Although ground thread taps are produced to precision tolerances under closely controlled manufacturing processes and are guaranteed for accuracy of individual elements, there is always the possibility of the presence of unknown factors which can be detrimental to good tap performance. The tap manufacturer, therefore, is not able to guarantee the size of the tapped hole.
To summarize, the following points should be remembered:
1. A tap cannot produce a Class of Thread. It can produce a tapped hole within specific product limits.
2. Since it is used only in tapping a hole, or producing an internal thread, a tap has no control over the fitting properties of the mating external thread.
3. To produce what is commonly referred to as a “Class of Thread,” both the external and internal threads must be within their respective product limits. Only when both members of a threaded assembly fall within the desired class limits can the proper fit be assured.
4. The acceptability of any class of threaded hole is determined only by an accurate “Go” or “Hi” thread plug gage of the corresponding class. Similarly, the acceptability of a male part with an external thread is also determined by a corresponding “Go” or “Lo” thread ring gage.
5. When ordering taps, specify either the tap limit or the class of thread that identifies the limit of the tapped hole.
6. In special applications, supply as much information as possible. The more information our engineers have to work with, the better they can recommend a tap to do the job properly with least expense.
7. Remember, it is not always the most expensive tap which will do the job best. Our files contain many examples of special jobs which were solved with inexpensive standard taps.
link to Tap Recommendations for Classes of Thread for Unified and American National Screw Threads

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link to tap and die index page