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Hi ajoy!
There is a company from the UK called JME ltd. that manufactures only gamma ray crawlers for the ID dimension you're looking for...
The smallest ID's available for X-ray crawlers is 6 inches...
It is called the 3/6 gamma crawler. 3/6 means it can be used in ID's from 3" to 6". These are self propelled systems.
More info can be found at the sites:
http://www.jme.co.uk/crawler.html
Here is the data sheet for the 3/6:
http://www.jme.co.uk/JME%20Datasheet%200306%20Gamma%20Crawler.
Hope this helps.
Respectfully,
SSBN727 Run Silent... Run Deep!!!