Timing Belt

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Later style cam belts are reinforced with Kevlar instead of fibreglass. The Kevlar reinforced belts can be identified by the red lettering on them. Other color lettering indicates and older belt.

Caution – if you are not comfortable performing maintenance on your machine, see a dealer for your service. This is a guide and may not have all of the steps needed for you to do your own maintenance successfully – use at your own risk.

Contents

Timing Belt Adjustment

By dharhay
Original document:
Timing belt adjustment & replacement for the 02-03 ST4s

Ducati ’03 ST4s Timing Belt Adjustment This instruction on cam belt adjustment starts after you remove the fairing pieces. Plan on an hour to remove these. Since you are performing your own service, buying Ducati belts may not be that expensive. There may be another belt source, but I have not confirmed this for the 996 engine of the 02-03 ST4s.

Fig 1 - ST4s ready for belt adj./repl.
Fig 2 - Crank tool installed. See dwg.
  1. Ensure maximum of ½ tank of gas. Fuel will leak into the charcoal canister, then onto the floor if the tank is over half full and the tank is tilted up on its hinge. So put the tank up on its hinge.
  2. Remove spark plugs. Cover sparkplug wells with boots or rags. This is a great time to toss out those Champions for a set of NGK DCPR8E’s.
  3. Remove the front shield from the horizontal cylinder head.
  4. Remove crankshaft cover on Riders LH side and insert crank tool. A crank tool can be made by following the drawing at the end of this document. See Fig 2 above. The flat head screws used are kind of soft so beware on their removal.
  5. Remove oil pressure sending unit cable and remove oil sending unit. No oil should pour out.
  6. Remove battery and remove bolts fastening battery box to engine/frame. You can let the battery box dangle or partially support it via a bungee cord. Be careful of the cable on the ground of the ECU. By dangling the battery box excess stress could be placed on this cables connector and you don’t want to troubleshoot an intermittent electrical issue.
  7. Remove cam belt covers. The two covers over the cams are real easy. The center piece is removed by pulling it down. You may have to move some hoses and cables out of the way because it is a tight fit. Removing the oil pressure sending unit gave clearance for the removal. Fig 1 shows bike ready to be worked on.

On the Rider’s LH is a window on the case cover. Using a flashlight to illuminate the area, marks can be rotated into view using the engine turning tool. There are two marks denoting the TDC of each of the cylinders. When a mark in the window aligns with the pointer, look at the crank driving pulley on the Rider’s RH of the engine. If the driving pulley’s mark is aligned with the pointer on the engine case, you are at TDC horizontal cylinder. If the pulley mark and the pointer on the engine do not align, rotate the engine until the mark in the window on the Rider’s LH side case cover aligns as well as the pulley’s mark and the engine’s pointer. Fig 3 shows the engine pulley and pointer.

Fig 3 - Engine pulley and marks

You can check that the horizontal piston is at TDC by inserting a welding rod into the spark plug well and touch the top of the piston with it to sense its position.

Timing marks on cam pulleys are clearly stamped on all four pulleys. When the crank driving pulley aligns with its corresponding pointer on the engine case and the marks on the pulleys align with their corresponding marks on both cylinders, you are in position for belt adjustment and replacement.

Note - TDC for the vertical cylinder is 270 deg CCW from TDC of the horizontal cylinder. When you rotate the crank via the turning tool, you will see the other mark in the site glass. You can also verify this by inserting the welding rod to detect the piston location on the vertical cylinder. This is not needed for the belt tensioning and replacement.

If you are re-using the timing belts and will be removing them, then mark them H or horiz and V or vert., mark the direction of rotation, and mark the side of the belt to the number of pitches or teeth between the alignment marks on the pulleys. For belt replacement mark the side of the old belts to the number of pitches or teeth between the alignment marks on the pulleys. Transfer the side markings to the new belts. Horizontal cylinder is 20 teeth and vertical cylinder has 21 teeth between the timing marks. The marks are another visual aid in addition to the timing pulley marks that you are not changing the timing of the engine.

Fig 4 - Vertical cyl adj. nut obscured
Fig 5 - Horizontal cyl. adj. nut

You do not need to mark the belts if you are not removing them or replacing them with new ones. Double check that the cam pulley marks are still in alignment with the marks on the engine as well as the crank driving pulley and its marks.

Belt Tensioning – no belt removal

The horizontal cylinder is adjusted first and removed first. The vertical is then either adjusted or removed second. Notice that the nuts on the movable tensioner are special in that they resist movement and vibration with their design. These nuts can wear out and you must either replace them each time you remove them or use locktite on them. Fig 4 and 5 show the movable pulleys. Note that the frame obstructs the vertical cylinder’s movable pulley such that a torque wrench cannot be used. Should the bearing need be replaced, the engine will need to be rotated to gain access for that bearing replacement.

Fig 6 – straightedge, 3 & 5mm Allen wrenches
Fig 7 – Measuring the belt

The engine is still at TDC on the horizontal cylinder and all five timing marks are in alignment. There are many ways to get around using the Mathesis tester that you all have heard about. Below are the methods commonly used:

Method Comments
Mathesis The latest and greatest, $2500
Ducati tension gauge Still one of the best methods, $275
Gates sonic belt tension meter 507C From the belt mfgr. – not tested, $500
Gates spring tension gauge From the belt mfgr with tables and instructions, not tested, $20
Fish scale 0-25lb Commonly available, adds a repeatable number to the 3mm method, $20
3mm method Down and dirty, subjective, use fish scale to reduce subjectivity
45 deg method Still down and dirty, use in addition to 3mm method

For simple home use, the Gates Spring Tension Gauge, and the twist method work very well. Secondly, the 3mm method along with the twist method is good also. In addition, the 3mm method in conjunction with a fish scale then checking via the 45 deg method will afford the home mechanic a reasonable way to tension the valves.

The 3mm method is: simply a 3mm deflection of the belt midway between the two cam pulleys measured to the bottom of a straight edge that spans the two pulleys. This is subjective just like using feeler gauges because you are still sliding a piece of steel between the belt and the bottom of the straight edge. The 3mm method can be made more repeatable by scribing a line on the engine behind the belt. Using a spring scale to load the belt with 10lbs of upward loading, the deflection of the belt above that scribed line is limited to 3mm.

The twist method is even simpler and more subjective than the 3mm method. You hold the belt between your thumb and forefinger midway between the movable pulley and the engine pulley. If you can twist the belt more than 45 degrees from its original position with slight pressure on your fingers and thumb the belt is too loose. You twist using moderate force of your thumb and forefinger and decide when you reach the 45 deg twist if the force was moderate or not.

Fig 6 shows a 3mm and a 5mm allen wrench used for the deflection and a piece of steel for the straight edge. Fig 7 shows the sliding of the allen wrench under the straight edge. When the fish scale is used there is no straight edge needed just the scribed lines and the 10lb pull on the scale to reach the 3mm deflection at the midpoint of the belt between the two pulleys.

When the belt is tensioned by your chosen method, ensure the moveable pulley is fastened well and proceed to the vertical cylinder.

Belt Replacement

On belt replacement the vertical cylinders belt is replaced first, then the horizontal cylinder belt. Transfer marks from the old belts to the new ones and place the belts onto the proper pulleys. There is no need to remove the tensioner to remove or replace the belts. The vertical cylinder intake cam may resist staying in the proper position but its spring pressure can be overcome by holding the cam and then placing the belt over the cam. Ensure that the marks on all four pulleys and the engine pulley are still in position. Also are the proper number of teeth between pulleys verified.

Rotate the engine through several cycles to see if there are any obvious problems such as the valves hitting the piston due to the timing marks being slightly off. Once this is checked out and is ok go on to tensioning the belts.

Crank Tool Drawing

Crank Tool
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