Are the sheave bolts shoulder bolts? (With a polished cylindrical section through the cheeks and sheave followed by a smaller threaded section for the nut. Sometimes called stripper bolts.)
Or are they plain bolts? (Partially threaded, but essentially all the same diameter.)
I recommend using shoulder bolts, which will allow you to tighten the nut fully, and still allow the sheave to rotate freely. The cylindrical portion of shoulder bolts is typically polished and of an accurate diameter, which would allow your sheave to turn more freely.
If you have to use plain, partially threaded bolts, let me suggest using elastic stopnuts (sometimes called Nylock™ - which is a tradename of Forest Fasteners) like the one pictured at the top of the vertical shaft. This will allow you to leave the sheave loose without risking having the device come apart at an inopportune time.
Upper bolt: 5/16"-18 partially threaded bolt w/ elastic stop nut.
Lower bolt: Shoulder bolt 5/16 x 5/8" shoulder, 1/4-20 thread w/ plain hex nut.
Here is a US source for Shoulder Bolts: http://www.mcmaster....-screws/=9bd6t7
I was planning to use all the nylock bolts and shoulder bolts to complete the dogstake.
The picture shows just a temp. fixed dogstake have to polish everything first.
But i am glad that you informed me about it in case that i did not know.