Prussian Blue Marking Compound

Part: Prussian Blue 

Why You Need This: Prussian Blue is a non drying marking compound used check the contact surface of  a valve face to valve seat when working on a cylinder head.  If you are replacing your valves with new ones or simply lapping and reinstalling the original valves on your Honda CB350 / CB360 / CB450 / CB550 engine, then Prussian Blue is needed to determine the proper fit and seal between the valve face and seat. Since it is impossible to see how well a valve and seat fit together after years of wear, Prussian Blue is applied evenly and thin around the valve face and pressed it into the matched seat to check the consistency of the face contact the two parts make. This is done pre-lapping to determine the initial state of the parts and post-lapping to see if the lapping process was successful.  

Sold as a .75 FL oz tube, one tube will do many valve jobs. 

• new CB350/ CB360 engine valves
• new CB450 / CB500T engine valves

Installation Tip:  If you are replacing your engine valves then they must be lapped to the old seats in the head. Apply a thin even coat of Prussian Blue to the valve face. Press the vale to the seat (do not rotate the valve during the checking process), remove and check the seat for contact (where the blue has transferred). Wipe clean and rotate the valve 90 degrees and test again, Repeat two more times to ensure the valve and seat are making good contact. Thin, spotty or incomplete contact means the valve seats will need additional lapping or cutting to bring back them back to true. Do not squeeze on the gel like toothpaste along the surface. 

NOTE: Valve guides must be within service limits before attempting a valve lap. If not replace valve guides first.