TM 5-4240-501-14&PTHEORIES OF OPERATIONCompressionFigure 10.The margin on a new Briggs & Stratton valve is 1/82 ofan inch, so that when it becomes less than 1/64 of aninch the valve should be discarded. Remember, this isafter all pit marks and burn marks have been removedfrom the valve face. If the valve is bent, the face will beground unevenly, and if the margin becomes too thin onone side the valve should also be discarded. A valvewith too thin a margin will not be able to withstand theheat and will quickly crack and burn. After facing thevalves and the valve seats to a 45° angle, place a littlefine grinding compound on the valve face, and verylightly lap the valve to the seat. Use of fine grindingcompound removes any grinding marks and gives aclear picture of the valve seat width. Be sure to removeall grinding compound from seat and valve.Figure 11.The valve seat width is usable up to 5/64 of an inch, buta new seat should be between 3/64 and 1/16 of an inch,and it should be in the center of the valve face. After thevalve seat and faces are ground, the valve should beinstalled in the guide, the cam gear turned to the properposition, and the tappet clearance checked. Refer toRepair Instructions for tappet clearance. Usually theclearance will be too small, and the end of the valvestem will have to be ground off to obtain the properclearance. Care should be taken not to overheat the endof the valve stem while this grinding is taking place; besure the end is square with the stem. It is recommendedthat the valve springs and retainers be assembledimmediately after setting the tappet clearance to preventchances of dirt getting under the valve seat.614
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