Part: Prussian Blue Marking Compound
Why You Need This: Prussian Blue is a non-drying marking compound used to check the contact surface engagement between an engine valve in relation to a valve seat on a cylinder head. If you are replacing your engine valves with new ones or reinstalling the original valves on your Honda CT90 / CB175 / CB350 / CB360 / CB450 / CB550 / CB750 cylinder head, then Prussian Blue is needed to determine the proper surface engagement and seal between the valve face and valve seat. It is impossible to see how a valve and seat fit together after years of wear; to inspect the wear surfaces, Prussian Blue is applied evenly and thin around the contact face of a valve and the valve is then pressed into the matched valve seat. Prussian Blue compound is transferred to the valve seat and the shape and pattern that is visible represents the engagement between the two surfaces. Depending on results lapping or a full cylinder head rebuild may be in order.
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.
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 line, spotty or incomplete contact means the valve seats will need additional lapping or machine work 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.
