Main bearing thrust washer ?

Long story = short. MG Midget, 1976 built in 11-75, 1500, original engine, 34,760 miles, stored for 18 years, re-build from ground up, Engine rebuild... The thrust washer was in the oil pan when I took the engine apart. Putting all new gaskets, washers, seals, etc in rebuild... the thrust washer goes on top of the crank to the rear of the engine block.
What keeps that little sucker in place? I put it together and turned it over( with starter hook up and battery while still on engine stand) and found that it would barely turn/ if at all. I took the bearing caps off and went to re-install them and found the washer lying on the floor next to my engine stand. Engine is very tight and will crank slowly with starter and new battery. Also replaced the piston rings.

The 1500 is a Triumph-designed engine, and like all TR engines, it has a tendency to kill thrust washers. I change the thrust washers in my 1500 race-Spridget every two years or sooner.
They should stay in place because they are "trapped" between the block and the crank cheek. If they become worn, they fall out. Once they fall out, the crank rubs on the block, creating all sorts of havoc. If you've put new ones in and they fell out straight away, you may have a damaged or worn crank cheek or adjacent block area....a bad situation.
New thrust washers are available in .005" and .015" oversize, if the damaged area can be remachined.

I'm concerned about your comment that the engine is "tight". Are you installing the washers correctly? (and I'll assume that you are not using oversize thrust washers in a "standard" setup).
Also, be sure that the ring gaps are about right (roughly .005" to .010") and that they were installed in clean piston grooves.
If you have installed new bearings, be sure they are the correct size (have the crank journals been turned down?).

Here's a story about TR6 thust washers (similar to the 1500 design):
https://users.arczip.com/zntech/tr6.html

More info:
https://www.teglerizer.com/triumphstuff/thrustwashers.htm

Here's a way that one fellow make's the thrust washers "stay put" (not sure I'd do this):
https://www.tr6web.com/Documents/tr6/mjbtwasherfix.html

A similar "stay put" ploy:
https://www.geocities.com/MotorCity/Speedway/1080/thrustwasherinstall.html


Here's a picture of the thrust washers...shown as "28" in this Moss picture (you probably already have a similar picture in your manual):

I didn't have anything machined due to the true mileage of the engine (under 40,000 miles) but possibly should have. I am going to try again this afternoon and see if I can tell if there is undue wear anound the crank. My concern was that I couldn't see anything that kept it in place and I guess I should just pull it apart and start over instead of trying to fix it in place. Just hated to undo all the work to this point. I had clean (very clean) engine parts throughout, ring grooves were fine, rings were atandard stock replacement, gaskets seals and misc were also stock since there was so little wear. It ran great but smoked, so I tore it down mainly to replace rings and since it was apart... why not everything...??? I've done a bout a dozen engine rebuilds but not enough to feel near an expert. My local MG shop is 45 miles away so I do more on my own than having it done.

PS.... Thanks... I think you answered exactly what I needed and the links were great. I'll be out in the garage in a few minutes and check back tonight to see if all was successful.
Cheers, Steve