Closing safety for screw caps

Closing jars with sterile screw caps

When twist-off jars are closed manually with screw caps (cam screws) it may occur that the jars aren't tight even before, during or after the sterilisation process. You should check the closing safety for screw caps to exclude technical causes and faulty products. For this purpose the manual process and the expected tightness should be analysed.

Premilinary marks:

The compound in screw cams that are suitable for sterilisation (maximum temperature up to 121°C) is harder than the compound in screw caps that are suitable for pasteurisation (up to 97°C). Sterile compounds consist of non-foamed materials to satisfy the migration regulations for the use of fatty filling goods. These caps are frequently made for (partially) automatic or rather industrial filling. The tolerance range for the optimal closing safety for screw caps (equivalent to tightness) has decreased in the last years. Manual treatment is more difficult and requires a higher physical effort!

Measuring the distance between seam and cam

Check the closing safety for screw caps during the production and without opening the jar

Fig. 1 First search the seam at the glass mouth to check the closing safety for screw caps.

Fig. 2 Mark the cam at the glass mouth.

Fig. 3 Mark the beginning of the cam at the corner of the lid and close the jar.

Fig. 4 Measure the distance between both marks. Like this you identify the distance between seam and cam after the closing process.

Recommendation for the distance between seam and cam: 
Lid TO 53-70 = 2-5 mm
Lid TO 82-110 = 2-7 mm 

With some practice you can check the distance between seam and cam on the closed jar taking a subsequent glance under the corner of the screw cap.

No negative distance between seam and cam!

The distance between seam and cam should never be zero or negative

Fig. 5 The marking on the lid has always to be right hand of the marking on the jar. 

Checking the closing safety for screw caps

Checking the closing safety for screw caps place a vertical marking on lid and jar when the jar is closed (fig. 6). Open the jar and screw it until it's finger-tight (i.e. when you perceive the first resistance). The distance between the markings on the jar and lid gives the so-called closing safety (fig. 7). The closing safety for screw caps should be 4 to 7 mm and 1 to 5 mm after cooling down (fig. 8).

Optimal closing safety for twist-off jars


Before use heat the lids in a warm water bath at a temperature of about 65°C for at least 15 minutes to assure the closing safety for screw caps. Please note that the refilling the water bath with "cold" screw caps reduces the temperature significantly. So please pay attention that the temperature is kept constantly on a level of 65°C.


Always keep glass mouth and screw cap (compound, cams) tidy.

Constant filling temperature

Ensure a constant temperature of the filling good.

No air pockets

Avoid air pockets in the filling good.

Head space

Check if there is enough head space (air volume) between the filling good and the screw cap because otherwise no vacuum can be created. 

Internal and counter pressure

Using the autoclave the difference between internal and counter pressure must be less than 0,7 bar.

Check list for the closing safety for cam closures

That's what you can check on your own

Reason for leakage => insufficient safety level

  • Correct adjustment of sealing machine
  • Increase capping force (also manually)

"Cam squeezer"

  • Head pressure is too high
  • No pre-adjustment

Vacuum is missing or too small

  • Pre-steaming: check adjustments
  • Correct filling temperature
  • Correct filling level (create sufficient head space)
  • Reduce counter pressure if necessary

Overtightened screw cap

  • Check closing safety for screw caps
  • Check distance between seam and cam
  • Correct capping force
Brochure and check list for closing safety for screw caps for download