well, i had problems with some issues that are undoubtedly their core business.
however, if you come into issue with proprietary information, that is another story.
that is kind of problems i was facing with when i was working in maintenance department. you assumable have problem with nozzles and want to save money by avoiding to replace the whole assembly. if manufacturer does not accept such approach, you cannot achieve what you want without having enough knowledge on the matter to allow you to engineer whole distributor yourself, eventually calculate nozzle. that is large responsibility that should be taken only if you have master knowledge on the issue. many people don't know that most of manufacturer's service engineers have large service books that are much more comprehensive than maintenance manuals they deliver to their customers. they hold such books as proprietary information, and you are actually confronting patent legislation if you try to obtain it other way.
the only "straightforward" way is saying "i am taking full responsibility for assembly from now on" you can still buy from manufacturers all parts they are willing to sell, but they will not provide technical support any more. During my time in maintenance, i took such course in few occasions, but only few ones.