This demonstration guide answers one field question about rails that cannot keep a chain in plane. Measurements come before parts, and the notes are not a generic equipment FAQ.
Diagnose before replacing parts
Start with rail height and groove width. That reading separates rails that cannot keep a chain in plane from adjacent faults that share the same complaint.
If the measurement is out of spec, treat the bar as the primary path. If it is in spec, do not replace the bar just to complete a parts list.
Repair the failed system
The usual correction is the bar, installed to the documented spec for this complaint.
A demonstration shop records the finding so the next technician is not repeating a guess about rails that cannot keep a chain in plane.
Stop before you make it worse
Do not welding a rail tip to "get through the pile". That shortcut hides the original measurement and creates a second failure.
If the machine still fails after the bar is corrected, convert the visit to a broader diagnosis rather than stacking parts.
Written and published by Ridgeline Small Engine.