Rebuilding valves

The fit and finish of a valve determines the compression and ease of movement of the valve. By rebuilding a piston valve, it is possible to decrease any leaking as well as improve the "Action" or movement of the valve in the casing. This page is meant to show the basic steps involved in proper valve rebuilding. It is by no means a complete instructional guide but should give the reader a good idea of the procedure involved.

Inspection of the Valve Casing and Valve

It is important to closely check the valves and casings to assess the amount of corrosion and wear. The depth of corrosion has to taken into consideration when honing. Excessive corrosion will increase any measurements you will take when deciding on the amount of material to be removed.

Measurement of the Valve and Valve Casing

The valve diameter must be measured to determine if it is cylindrical. All measurement is done with a micrometer and ID gages. It is also helpful to see if either end of the valve is the same size. If either end is larger than the other, it must be honed down. It is a general rule of thumb that a difference of .0005" is an acceptable amount of variance between diameters at the ends of a valve before plating. Casings must be measured at the top and bottom of the inner valve sleeve as well as between ports.The file shows the use of the ID gages and measuring from the bottom of the valve liner.

Scraping Valve Ports

The valve ports inside the valve casing will produce slight burrs during honing. It is important to remove a small portion of the lip around the edge of the inner valve ports. Great care must be taken while executing this procedure. If the technician slips past the port hole and scrapes the inside of the valve casing, irreparable damage can be done. The tool used to produce a chamfer around the edge of the inner hole is seen here. The tool is made from a hardened steel ball brazed to the end of a brass "T" handle. the handle must be made of material sufficient enough to allow the technician to control the movement of the ball and apply pressure. The surface of the ball is ground to slightly less than half of its diameter then ground and polished to a mirror finish. If the tool is made correctly, it should not mar the inside of the valve casing.

Actual scraping of the valve involves inserting the tool into the casing, finding the port by visual inspection, applying the proper amount of pressure while rotating the tool in the port hole. A small sliver of material should be produced by this action and must be removed. Leaving any of these scrapings in the valve casing can severely hamper the process and potentially damage the valve and casing. Each hole must be lightly scraped and inspected. This procedure can also be performed on horns which have intermittent sticking valves. Often the valves stick due to slight prostration from heat causing them to hang on the edges of the valve ports. By scraping the ports then lightly lapping the valve, this problem can be eleviated.

Burnishing Valve Guide Slots

Often overlooked in the rebuilding of valves is the importance of the valve guide slots to the proper operation of the valve. It is important to remove any burrs or foreign debris from the slot and slot edges. The valve guide must be allowed to move freely in the slot. To insure the proper finish on the valve guide slot, burnish the slot with a smooth burnishing tool. It is also advisable to lightly scrape the edges of the valve guide slot before burnishing. this will remove any large burrs or corrosion which may be present. While burnishing pay close attention and check if the sides of the slot are parallel. If they are not, file them with fine pattern files. Do not open them greater than the largest distance between the sides of the slot.

Although difficult to determine by eye, check the alignment of the valve guide slot to the valve liners (ports in valve). Most spring cages (upper half of valve which holds the spring) are soft soldered into a grommet at the top of the valve. It is fairly easy to reposition the spring cage by applying heat and flux to the valve placed in a lathe between centers.

The honing of the valve and valve casing is done on a machine expressly made for this purpose. Grinding stones are positioned around the outside of the valve while oil is flooded over the operation. The oil provides a lubrication and helps to wash away the material being removed.The oil is necessary for both valve and casing honing procedures. The valves and casings must be precisely sized before being plated with hard nickel. The honing machine allows the technician to grind the outside diameter of the valve to very tight tolerances.

Likewise, the valve casing can be internally honed to appropriate tolerances.The difference between the valve and valve casing diameters after honing will determine the amount of plating that is necessary to achieve a good refitting of the valve. It is most efficient to plate the diameter of the valve to .001" greater than the finished ID of the valve casing. This is adequate to smooth the plated surface and remove imperfections in the plated finish before lapping.

Preparation after Plating

After the valves have been plated they must be re-honed to their new diameters. In the process of plating the ends of the valve often are larger than the center section. This is due to the adhesion of metals in a plating tank. If heavy plating is necessary, the ends of the plated valve may be much larger. Care must be taken to take the ends down without effecting the center section of the valve. Honing the plated valve uses several different stones of different coarseness. Not only is it necessary to insure the valve has the same measurements along its entire length but the surface of the valve must also be very smooth.

Lapping and Fitting

The final procedure involves lapping the valve into the casing. Fine compounds are used to gently remove small amounts of material from the inside of the valve casing and from the outside of the valve itself. A series of compounds are used ranging from relatively course to fine. It is desirable to have a mirror finish on the valve with completion of the lapping. All compound must be carefully removed before checking the fit of the valve. It is vitally important that too much material is not removed during lapping. Improper lapping can result in a tight fitting valve which hangs. Lapping is intended to be the final procedure and does not in and of itself remove all high and low spots.

Finished Valve

This page is not intended to be a tutorial but does demonstrate many of the process involved in refitting badly worn or new replacement valves.

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