I have a Mauser 98 bolt that was originally modified for 375 H&H. A 375 H&H cartridge fits fairly loose, is a tight fit desirable or is loose ok?

There are 100 different ways to approach that. Were it not for the claw extractor pushing the cartridge to the side, loose would be better. The fact that its an old military Mauser and the bolt and barrel are probably no where in alignment to one another, being tight would be a handicap anyway as the entire bolt would be holding the cartridge off center.

Considering what you have, it really doesn't matter. If you want to make the best of what you have. Start with virgin brass and remove the extractor from the bolt. Fire form them all at full power at the range using a cleaning rod to evacuate the cases and then just neck size after that. After that, the force of the extractor pushing the case to one side won't come into play.

Thanks for your reply. I'm trying to figure out the original chambering of an old Mauser action. The bolt face is a hair too small to feed a 404 Jeffery, but might fit a 333 or 280. I have a feeling though that it might have just been a 375 H&H. I know for a fact that the action was originally modified specifically for use as a 375 H&H chambered rifle.

Right; as Speer said, they vary a lot. I had a 700 here last week that had a bolt face of .495, for a .473 head. Very sloppy, but it worked.
Too tight is not a good thing; worse actually.
Bottom line is, does it function? If so, ignore it.

The boltface diameter only comes into play during extraction. If the boltface is too large, the extractor may drop the case before it contacts the ejector. As far as cartridge alignment during firing is concerned, it is meaningless.
My Lee Enfield has no lip around the bolt the face is flat and open. My Ruger Number One, like all single shots, has a flat breech. The boltface recess is meaningless except to control the case during extraction and ejection. Regards, Bill