One end is a 3/8" square hex drive, OAL 4-3/4".  Odd combination of coarse and extremely fine threads- the coarse are RH while the fine are LH.  Almost like a micrometer adjustment thread.  It was in a bag hand marked R175-P2.  Some kind of replacement part? 

My first WAG is a mandrel. For what?, i don't know yet, but will look for it.

Is the fine thread left handed?

Doesn't look like it is threaded, but if it is- perhaps it is part of a puller of some sort.

Is it all one piece, or does the square part turn a shaft inside the fine-threaded part?  If it does, could it be the core of a Greenlee-type punch?

machine shop project?

Possibly the guts of an Adjustable Blade Reamer???  Regards, Lou

No size reference so I'm guessing a bit but it looks like the center shaft for a "ridge reamer" for engine cylinder bores. I've used several different types but I think they had a hex rather than square drive. The course thread part expands the 3 support legs to anchor tool in bore. The cutter assembly mounts and turns (with large wrench or socket) on the fine thread portion. The fine pitch works like the lead screw on a lathe to feed cutter a certain amount with every turn. Will try to attach a pic from eBay.

Those course threads on the one end are throwing me, a handle? or threaded in to something like a fixture for a ???? mill, lathe, grinder??

Quote from: JohnD on April 05, 2018, 11:57:34 AM
No size reference so I'm guessing a bit but it looks like the center shaft for a "ridge reamer" for engine cylinder bores. I've used several different types but I think they had a hex rather than square drive. The course thread part expands the 3 support legs to anchor tool in bore. The cutter assembly mounts and turns (with large wrench or socket) on the fine thread portion. The fine pitch works like the lead screw on a lathe to feed cutter a certain amount with every turn. Will try to attach a pic from eBay.

Ding-Ding-Ding, I think we have a winner!  Nice job and thanks John.  The super fine thread pitch makes perfect sense now- its like the feed rate for a hand driven boring bar.