Base circle

Discussion in 'Softail Models' started by GSM2, Apr 1, 2011.

  1. GSM2

    GSM2 Member

    i just changed my cam shoe tensinors. i have my plate back in and i am about to put in my push rods and rocker support plate.
    -i dont fully understand how to find base circle for a cylinder. i have the bike on a lift and in 5 gears. i am having a hard time knowing when it's at the bottom of the correct stroke.
    is there an easy way to know for sure?
  2. D.Bradfield

    D.Bradfield Active Member Contributor

    How to tell if the Tappet is at its lowest point.

    There is not one precise location that is the lowest point so it is easier to find as it is many degrees of the cam heel. Of course, I didn't know this simple little fact when I started out so I methodically used to roll the bike back and forth looking for the precise spot wasting time and causing much anxiety. After all, the penalty can be a bent valve if a lifter/tappet is adjusted on a high spot of the cam lobe. If you look at the lobe of a cam, which you can ask to see in the shop you patronize, you will notice quite a distance where the lifter will remain at its lowest position. Why? The corresponding intake valve will remain closed while the exhaust valve is open to allow the spent gases to exit on the exhaust stroke. Or, the exhaust valve will be closed on the intake stroke while virgin air/gas is sucked into the cylinder. Or both valves will be closed on the compression and the power strokes. So, what are some techniques for arriving at the cam lobes low point? One way is to put the piston at TDC, (top dead center), on its compression stroke. Huh? I'll explain three ways to figure this out. With spark plugs removed put your thumb firmly over the spark plug hole of the cylinder for the pushrod that needs adjusting. Have a friend rotate the rear tire or gently kick the bike over if it has a kicker. The piston will build up pressure on your thumb as it rises on the compression stroke. When feeling this pressure, look into the spark plug hole with a flashlight and the TDC of the piston can easily be determined. A second way. Remove the timing plug on the left center side of the engine at the base of the cylinders, (barrels). With the piston at TDC, the timing mark will be aligned in the center of the timing plug hole. The front cylinder is indicated on most engines by a straight vertical line. A round dot near the top of the hole on some year engines also indicates TDC. The rear has a rectangular horizontal mark or a single dot near the bottom of the hole. Harley has changed these marks over the years so it is wise, as usual, to check a manual whether it be Harley or a variety of others like the user friendly Clymer. Or you can physically watch the lifter go up and down while rotating the engine. The first two ways allow adjustment of both lifters in the same cylinder side once TDC on the compression stroke has been determined. In these two cases the lifters on the opposite cylinder will be at their highest point.Every Harley wrench has a favorite way of doing this. Remember that there can be more than one right way
  3. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

    Check these:

    Pushrods keep dropping - Harley Davidson Community

    Adjusting Pushrods - Harley Davidson Community

  4. GSM2

    GSM2 Member

    so after i find TDC; the piston goes back down.... and up.... and back down. that is my BASE CIRCLE for that cylinder?.?.
  5. gs34

    gs34 Junior Member

    The base circle is the portion of the cam lobe where the lifters are at their lowest point...this would mean the valve for this lobe is closed.
    If you will draw a circle on a piece of paper, this would represent the "base circle" of the, pick a point approximately halfway around the circle and draw a lump on it.....This would represent the lobe of the cam, or the part of the cam which allows the valve to open and close.
    As dbradfield 203 said, when the piston is at TDC of the compression stroke, the valves for that cylinder will be closed....indicating that the lifters are at the lowest point, which is the "base circle" of the cam.
  6. GSM2

    GSM2 Member

    i was over thinking everything. it clicked yesterday when i was putting it back together. thanks for all the help