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    Default How To: Install 1948--1968 Harley-Davidson Fork Lock

    Abstract:
    This guide outlines the process of installing the fork lock (also commonly called neck lock and steering head lock) for 1948--1968 overhead valve Harley-Davidsons. The fork lock operates by twisting a key, which in turn slides a plunger into a hole in the fork's neck stem. This in turn acts to lock the steering of the motorcycle. The procedure is demonstrated on a panhead wishbone frame. Existing information on the installation is scattered, hard to find, or difficult to understand. This write-up aims to provide a clear and unified presentation of the information.

    Parts Used:
    The frame used here is a 1948 wishbone replica from V-Twin (V-Twin part number 51-1949). The fork lock kit is a replica of HD part number 48150-30 (V-Twin part number 37-9021). Note that original Harley-Davidson (HD) fork locks were manufactured by Briggs and Stratton. From 1948--1955, HD used an early version of the fork lock that had stampings on the face to indicate which key fit the lock. From 1956--1968, HD used a late version of the fork lock that had a stainless steel cover and no longer had visible key stampings. The V-Twin replica kit is a replica of the late version with the stainless cover. To the best of my knowledge, nobody re-manufactures the early style lock. Therefore, using the V-Twin fork lock kit on a 1948--1955 HD frame is not correct from a restoration perspective, albeit it does fit and function properly.

    Tools Used:
    The only special tool used was a taper length (extended length) 3/32" drill bit (McMaster-Carr part number 3063A113).

    Procedure:
    The kit contains the lock body and keys, the plunger (the large black colored object in the photo below), the plunger spring, two roll pins for securing the lock body (only one is needed; the kit provides an extra), and a welch plug for plugging the hole in the front of the neck casting.

    Note that many parts books do not show the plunger spring. In particular, various forum posts state that Duo-Glides (1958 and up) did not come with the plunger spring. The spring is intended to sit behind the plunger (that is, between the plunger and the rear of the motorcycle). This is shown in the photo below.

    In the absence of the lock body, the spring pushes the plunger towards the neck stem of the fork. This may cause the plunger to actually enter the hole of the neck stem when the fork is turned, thereby locking the fork and disallowing the ability to steer the motorcycle. Such a situation is clearly dangerous. Without the spring, gravity naturally acts to slide the plunger away from the neck stem. Therefore, in a failure where the lock body loses contact with the plunger, the spring pushes the plunger towards the neck stem, whereas gravity acts to pull the plunger away from the neck stem. By omitting the spring, the steering is less likely to accidentally lock in the event that the lock body loses contact with the plunger. For this reason, I removed the spring from the assembly and decided to not use it.

    With the assembly completely back out of the frame, I greased the plunger and slid it into the plunger hole through the hole in the front of the neck casting. (Recall that the plunger is being installed without the spring behind it.) The two ends of the plunger have different diameters, and the plunger is inserted so that the end with the smaller diameter faces the neck stem of the fork.

    Below is what the key hole looks like with the plunger now installed, without the spring behind it.

    At this point, the lock body is test-fitted, as shown in the photo below. The lock body will not slide all the way into the lock hole of the casting unless the circular boss on the back of the lock is correctly oriented to match the recess on the plunger. Play with the mating between the lock and the plunger to get a feel for how the mechanism operates, and to ensure the plunger slides back and forth as the key is twisted. The plunger should be at its furthest recessed point (away from the neck stem) when the key is oriented perpendicular to the bottom of the neck casting (as shown in the photo below). Verify this by watching (through the neck cups) the plunger slide back and forth as the key is twisted.


    At this point, the goal is to fix the outer shell of the lock so that when the key is out of the lock, the plunger is at its furthest recessed point. In essence, this is accomplished by taking the key out of the lock, inserting the lock into the lock hole with the key hole oriented perpendicular to the bottom of the neck casting, and then fixing the lock in position using the roll pin. However, the V-Twin reproduction frames do not come with the roll pin hole pre-drilled. Therefore, to install the roll pin and fix the lock in position, we must first drill the hole for the roll pin.

    Looking at the outer shell of the lock, you'll notice that there is a small blind 3/32" hole, as shown in the photo below. This hole is for the roll pin to enter; the roll pin should be partially fixed in the shell of the lock and partially fixed in the neck casting, once the whole procedure is finished. Therefore, the hole in the neck casting needs to be drilled precisely to line up with this hole in the lock. To make matters more difficult, the hole in the lock is drilled at an angle, since the neck casting makes it very difficult to drill a straight hole into the area surrounding the lock hole.

    As shown in the above photo, mark where the roll pin hole is on the face of the outer shell of the lock, so that you can see the orientation of the roll pin hole when the lock is installed into the lock hole of the neck casting. Next, as shown in the photo below, mark the face of the lock and the fact of the outer shell of the lock at the top of the key hole, while the key is out of the lock.

    Test-fitting the lock, the markings should be as in the photo below.

    With the lock oriented so that the key hole is perpendicular to the bottom of the neck casting, verifying that the plunger is at its furthest recessed position (as in photo below).

    Next, draw a line on the neck casting to match the roll pin mark made on the lock. Your goal here is to have the roll pin hole radially aligned with this line on the neck casting.

    Our next goal is to determine where on the previously drawn line we should drill the roll pin hole. We start by measuring the distance from the face of the lock (to be exact, the position on the lock that matches up with the face of the lock hole in the neck casting) to the center of the roll pin hole in the shell of the lock. As seen below, this distance was about 0.475" on my lock.

    Next, mark this position on the sharpie line on the neck casting. The center of the roll pin hole in the lock should be directly beneath the mark. However, since the roll pin holes are drilled at an angle, you cannot simply drill the hole in the neck casting at your mark, as your resulting hole would be too far towards the inside of the neck casting to line up with the hole in the lock. We will compensate for this problem now.

    Measure the wall thickness of the lock hole in the neck casting at the mark you made. See the two photos below.


    The wall thickness at my mark was 0.182". Next, we will estimate the angle of the roll pin hole. Start by inserting the blunt end of a 3/32" drill bit into the roll pin hole in the lock. Start tracing a triangle, as shown in the photo below.

    Next, extend the lines of your triangle, and finish the triangle by drawing the third side perpendicular to the side of the lock. See the photo below.

    Measure the third side of the triangle and the lock side of the triangle. My measurements were 2.271" and 1.015", respectively. (Your measurements will differ, depending on how much you extend the lines when drawing the triangle. The resulting angle should not change.) Computing the inverse tangent of the third side length divided by the lock side length, we estimate the angle of the roll pin hole to be 65.9 degrees. We will now use this angle to compute the compensation (offset) needed to make the roll pin holes line up.


    CONTINUED IN NEXT POST...

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    Default

    ...CONTINUED FROM PREVIOUS POST

    Draw a diagram indicating the (uncompensated) roll pin mark made earlier. My diagram is shown below. Note that the two measurements indicate how far the center of the roll pin hole is from the face of the lock hole (0.475") and the wall thickness of the casting at that point (0.182").

    Now, we can compute the compensation (the offset of the roll pin hole on the outside of the lock hole versus on the inside). This is computed by dividing the wall thickness (0.182") by the tangent of the roll pin hole angle (tangent of 65.9 degrees). Be sure you're computing the tangent in degrees if your angle is measured in degrees, and you're computing the tangent in radians if your angle is in radians. My resulting compensation turned out to be 0.081", as shown below.

    Next, subtract the compensation (0.081") from the marked distance (0.475"). The resulting distance, 0.393", is the compensated distance on the line where you should actually drill the hole. This distance closely matched the suggested value of 13/32" found in another forum. Therefore, I chose to use 13/32" as my drilling distance instead of 0.393". I figured that perhaps this suggestion was based off of a factory specification, whereas mine was based on hand measurements and calculations, thereby subject to error. It was nice to verify the suggestion though, and important to ensure the suggested value of 13/32" will actually line up with the V-Twin part.

    Next, mark the compensated distance on the line on the neck casting. Center punch the compensated mark, as shown below.


    Now, insert the drill bit into the roll pin hole of the lock body, and rest the lock in the lock hole, as shown below. Trace a line on the neck casting to indicate the vertical orientation of the drill bit. This is to visually help you keep the drill bit aligned while drilling the hole in the neck casting.

    Insert a piece of towel or shop rag into the neck hole so that metal shavings will not go into the plunger hole. You do not want metal shavings to accidentally prevent the plunger from sliding smoothly.

    Next, drill the hole using a 3/32" taper length (extended length) drill bit. Try your best to drill at the angle of the roll pin hole (65.9 degrees). You can get a feel for this angle by inserting the drill bit into the lock body without actually drilling any material.



    After cleaning the new hole with sandpaper to remove all burrs, and removing the towel from the lock hole, you can start inserting the lock body into the lock hole. As you slide it in, watch through the new roll pin hole in the neck casting to see when the roll pin hole in the lock body lines up. In the photo below, you can see the two holes beginning to align.

    In the next photo, the lock has been pushed all the way in and the two holes are seen to align perfectly.

    Verify that the holes line up by inserting the blunt end of the drill bit through the new roll pin hole in the neck casting and into the roll pin hole in the lock. You should be able to feel that the drill bit enters all the way into the hole in the lock. To verify, put the key in the lock and twist it. Pushing and pulling on the key, you should find that the lock remains fixed and does not slide in and out of the lock hole. The drill bit is temporarily acting as the roll pin to fix the lock in place. If your holes do not align, you can try to chase the holes with the drill bit. Be careful if you do so; you do not want the fragile bit to break in the roll pin hole.

    Now it's time to install the roll pin. The pin is shown below. It is essentially a radial compression spring; it will need to be squeezed to fit into the roll pin hole.

    Hammer the roll pin into the roll pin hole. To get the roll pin started into the hole, you can try lightly squeezing it together with vise grips and inserting the end into the roll pin hole.


    Ensure the roll pin is installed all the way so that it enters the hole in the lock and fixes the lock in place. Next, the welch plug is installed into the hole in the front of the neck casting. To install the plug, place it into the hole so that it buts up against the inner lip of the hole. Then, use a piece of rod or a socket to hit the face of the plug. This will compress the plug and make it extend radially, fixing it in the hole of the neck casting.




    Remark:
    Small magnets are very helpful for removing the plunger, lock, and welch plug from their respective holes while performing the above procedure.


    Sources:
    http://www.hydra-glide.com/phpBB3/viewtopic.php?t=2947
    http://www.caimag.com/forum/showthre...ck-replacement
    https://www.vtwinmfg.com/Instructions/37/37-9021.pdf
    http://panhead51.eklablog.com/instal...at-c29671138/1
    http://www.caimag.com/forum/showthre...on-t-mean-cold
    http://www.hydra-glide.com/phpBB3/vi...php?f=340&t=66

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