These extension rods help you with deep-hole measurement, and offer you the following benefits: 

• Using a single, longer extension rod rather than several short ones reduces error. When several extension rods are joined together there is the possibility of small errors arising from the extra joints involved. Therefore it is good practice not to connect more than 2 rods to a bore gauge at any one time. 
• You can use extension rods up to 1,000mm. 
• If you are using an extension rod longer than 500mm, use the bore gauge in the vertical orientation. 
• You can confirm accurate and satisfactory operation after connecting an extension rod.