It would be helpful to know why this is a concern. In addition, it's difficult to know what you mean by " taper/chamfer." If you're referring to an intentional countersink, its diameter, depth and angle should be specified. If you're referring to the natural small chamfer that results sometimes from drill entry, that's a different story.
The length of full functional thread engagement depends on the needs of the application. It's generally understood that a "no go" thread plug will enter the first few threads. I've attached a paragraph on the subject from an old Machinery's Handbook that I photocopied a few years ago when I was having a discussion on the subject with a customer.
Tell us more about your application and concerns.
I'm refering to the natural small chamfer that results from drill entry.
The tapping hole is on the casing of the DC motor used for installation on the customer's unit. There are 2 thread holes.
The issue here is we receive one incoming lot with bigger chamfer (also bigger top diameter) from previous lots. Checking with 'Go' & 'No Go' gage result is still Good. Question here is are we good to release this lot to production? Is it enough just by checking thread size by go no-go gage. Immediately what comes to me is the concern on the thread length or the engagement length, as bigger chamfer naturally will reduce the thread length.
As we have the thread length control in the Unified thread (inno. of thread per inch), so shouldn't we have a standard as well for the min. thread length for Metric thread?
Since the product drawing print does not specify control dimension/tolerance on the chamfer size, nor the top diameter size, just M2.5 & pitch 0.45.
Maybe we can judge by tightening torque & loosening torque, but I'm not sure it matters because I think loosening torque will be depending on the amount of the input of tightening torque, regardless to the thread length.