There's no two ways about it. If you install the thing and it fails ..you're eating the bill if it results in issues. This would be the same for anything. The false assumption is that if the engine fails ..and you installed the spin-on conversion ..that it, in itself, constitutes a reason for denial of warranty. There's nothing, inherently, that establishes that the cause of the failure is a result of the alteration. Even if you don't change your oil and the engine fails, there's nothing legally "assumed" that it was the cause. Suppose they had a run of bad bearings that spun in service? Suppose they didn't let this get around as common knowledge and, instead, wanted to see if they could get away with blaming customers/consumers.
Many try to do this. It doesn't mean that you need to let them.
It's really sorta laughable. If you and I had a deal ..and then ..when something went wrong, I just say "Oh, well since it went wrong I SAY you need to empty your wallet in my hands." JUST because I said so, I think you would see it a little differently.
It's the old fall back position that most bosses take when something goes wrong, "Did you do anything different?" "..well, I did blow my nose within 2 miles of the event this time. It was a first for me."
"THAT'S IT!!! You blew up the machine!!"