Are caps included to fit on the shaft?

No, I connect the shaft with the same diameter copper tube. then I crimp the tube so it fits snugly to the flat side if the shaft. I use a copper tube instead of brass, its easier to solder and is available through amazon. The copper tube makes it easy to connect things to the shaft. Hope this helps.

what batteries can I use with these motors?

Any battery above 3volts will work.

What is the shaft speed (RPM) at 6 volts?

most motors are differant, Put a plastic disc on the shaft and hook the motor up to a variable pwr supply, you will get all the answers you need includeing current draw

ASIN	B09DG3GCGK
Best Sellers Rank	#37,142 in Toys & Games ( See Top 100 in Toys & Games ) #12 in RC Electric Motors
Brand	Acxico
Brand Name	Acxico
Customer Reviews	4.6 out of 5 stars 193 Reviews
Horsepower	1 Horsepower
Included Components	Motor
Item Dimensions W x H	0.47"W x 0.39"H
Item Weight	30 Grams
Manufacturer	Acxico
Material Type	Metal
Model	YY640
Model Name	gear
Product Dimensions	0.47"W x 0.39"H
Speed	104 RPM
UPC	735209935333
Voltage	3 Volts, 6 Volts

Acxico 3Pcs Mini Micro N20 Gear Motor DC 3V-6V Slow Speed Full Metal Gearbox DIY Robot Car

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