Starting location: The Steel Bridge (see location). This is a classic steel truss bridge located 5 miles northwest of Fort Hoskins. There is room for several cars to park on the north side of the bridge.
Distance & drive time (from downtown Corvallis) (see route): 24 miles; 33 minutes
RWGPS route map: 49 miles, +3350'; 100% unpaved
Limitations on public access: This route uses private timber company roads that are subject to limits on public access and fire season restrictions and closures. This area is most likely to be open between Mid-September and early July. Non-motorized travel and passenger vehicles are allowed 1 hour before sunrise to 1 hour after sunrise. The following activities or vehicles are prohibited: Camping; fires; electric bikes; motorcycles; ATVs; target shooting; trapping.
Before visiting the Valley of the Giants, I recommend that you call the Salem office of the BLM at 503-375-5646. Hancock Forests controls the gate at the south end of the Old Railroad Grade and I believe they may control the Road 500 Gate (near the confluence of the North and South Forks of the Siletz River). The number of their office in Independence is 503-838-1610.
More information: I published this route on the MTB Project: Ft. Hoskins to Valsetz (mile 5 to mile 18.6); Siletz River Road (mile 22 to mile 16); and Valley of the Giants Road.