Limits
Do not take to following as "the gospel", but rather food for thought;
An engine manufacturer will state "limits" for an engine. They could be the maximum allowable diameter of a bore for something, etc. That limit, in and of itself, may not truly be an "airworthiness limitation".
Airworthiness limitations must be followed absolutely for the certified product to remain "airworthy". These limitations will always be stated right on the Type Certificate for the product. A pretty common example would be the "life limts" common to some helicopter parts. There is no option to go beyond the stated limitation (hours or cycles), and still declare the pruduct as "airworthy". To be "airworthy", it must conform to the limitations and conditions stated on the Type Certificate. So, if an engine TBO is stated on the Type Certificate as being limited in hours - that's that, no going over, without specific approval.
From here on, it gets a little more grey...
The Type Certificate will refer (sometimes rather indirectly) to maintenance and parts manuals. Those manuals may contain "limits". Unless those limits are also stated on the Type Certificate, they don't have quite the same absolutness (if that's a word). In theory, the mechanic should certifiy that everything is "within limits" for it to continue to be airworthy. However, some of the things you could measure, can only be accessed with an inspection much more in depth than the required interval inspections. The interval inspections are designed to "red flag" things going wrong. No red flag, don't inspect any further.
Those manuals have authority akin to a Service Bulletin. My experience has been that SB's are optional for private aircraft, and mandatory for commercial aircraft.
When I STC'd some replacement piston engine parts many years ago, I had a "life limit" in mind. To assure it was followed without option, I made it an airworthiness limitation of the STC. Adherance was mandatory. I am aware that one U.S. A&P was "cited" for not following it. It was there for a good reason, based upon test data, and vast experience. Otherwise, were it not an airworthiness limitation, and just a limit, it might not have been followed.
I hope that helps a bit. To get a valid, local interpretation, you'll have to seek local authority advice...