3.9.2 Straightening on a press
3.9.3 Opening the groove
/ STIHL guide bars are spring
hard. When bent slightly within their resilient range, as can
I occur during normal operation,
the bars spring back to their original shape. However, if force is applied a bar will remain permanently bent. If the defor¬mation is only slight, the bar can be straightened.
It is essential to first check the whole bar for any signs of cracks, especially at the point of deformation. The bar is un¬serviceable if cracks are found.
There are several ways of straightening the bar. If this is done in a press or a vice, the bar must be supported at two points equidistant from the cen¬ter of the bend. Pressure is then applied to the center of the bend until the bar is straight and re¬mains straight after pressure is released.
Finish straightening is per¬formed on a levelling plate with a hammer until the bar is per¬fectly flat in the rail area. The groove width is then checked by placing a piece of chain with the correct drive link gauge in the groove and pulling it around the bar. If the drive link tangs catch or stick at any point, the groove must be opened as necessary.
Once straightening has been finished the bar must again be checked for cracks which may have been caused by straightening. If any cracks are found the guide bar must not be used again even though so much time and effort has been invested.
A pinched groove can be opened by means of a groove drift which matches the speci¬fied groove width. This is done by clamping the guide bar in a vice and inserting the drift in the groove at an angle of about 45° at a point where the width is still correct.
A hammer is then used to knock the groove drift through the pinched portion of the groove. If the groove is badly distorted, it may be necessary to repeat this process several times until the chain can be pulled along the bar without resistance.
There is a risk of cracking the guide bar during this process, particularly at the nose of hard tipped bars. For this reason the guide bar must be examined for cracks after the groove has been opened. If any are found, the bar must not be used again!