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How to measure RTJ flange grooves



Ring Type Joint (RTJ) flanges are typically used in high pressure and/or high temperature services above 800°F (427°C). Together with a ring gasket, the RTJ groove helps create a tight seal between the two parts, so it is important that a groove meets the required specifications specified in API 6A.


API requires the inspection of ring groove dimensions on the flange face. To pass API requirements, these dimensions must be met. For BX Grooves, the groove’s major diameter and width are specified. For R and RX Grooves, API specifies the groove’s pitch diameter and width.

RTJ measuring kits are available from Mirage for measuring RTJ flange grooves. This may be required when you need to record the existing dimensions prior to machining with a flange facer, or to check the groove dimensions after any machining operations. Different kits are available, each designed to be used with the corresponding flange facer and supplied with extension rods to extend the working length for measuring larger diameter.




Using an RTJ measuring kit after machining with the Mirage MM1000i

In this case, the RTJM1000 kit would be used. The gauge incorporates a ‘U’ shaped Yolk, allowing it to be used whilst the MM1000i is still installed within a flange bore. The Yolk provides clearance around the hub of the flange facing machine and when it is installed,the kit can measure diameters from 187mm up to 1021mm

An important point to be aware of is that its critical that the RTJM balls (contact points) are set to the height of 0.187”. Fortunately, if you are buying your kit from Mirage, these are set to the correct dimension before despatch.

Measuring the major diameter (external faces)

  • If the seal groove ring number is known (e.g. R54) proceed to set the kit using a calibrated vernier or an outside micrometer as per the Internal/External dimensions within the table below.


  • Ensure the flange face is clean, flat and is free from debris before taking any measurements.
  • Place the RTJM1000 over the top of the flange with both ‘Contact Points’ situated within opposite sides of the RTJ groove & the ‘Wear Pads’ sitting flat on top of the flange face.
  • Apply light pressure to the ‘Lower Block Assembly’ away from the centre of the flange when taking external measurements or towards the centre of the flange when taking internal measurements. This ensures the ‘Contact Point’ in the ‘Lower Block Assembly’ is in contact with the relevant groove face of the RTJ groove.


  • Whilst maintaining pressure on the ‘Lower Block’ as described above, ‘sweep’ the ‘Upper Block Assembly’ across the opposite side of the RTJ seal groove. If taking either an internal or external measurement, the correct value for either the ‘Outside Groove Profile Diameter’ or ‘Inside Groove Profile Diameter’ is found when the dial gauge is at its highest value.
  • Record this dimension in the table of the kit’s manual. Repeat the process at several locations around the groove to get an average diameter.

Note that the unit can be easily reconfigured to measure the minor diameter (internal faces). This is done by removing the ‘Cover Plate’ by removing the 2x small, button heads socket screws, rotating it 180° & then replacing the 2x screws. Note that the small silver mark in the ‘Cover Plate’ should now be on the right hand side as shown below.

measuring-rtj-groove-minor diameter-internal-face-1.png

If you need to machine the groove faces you might be interested in our earlier blog post here

Do you need equipment for an on-site machining project? Visit our resources section to download our buyers guides or get in touch with your regional contact here.

You may also wish to visit our resources section, where you can download buyers guides, product datasheets or watch product videos.

Download the Flange Facing Machine Buyers Guide


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