May 2000 Tech   
    Home / Features / Four Clutch tip
    Indian Four and ACE
   Checking the freedom of rotation of the clutch body -a neglected part?
   By Charles J. Qua
  Click on pictures to enlarge!
1. Picture shows the Indian-Four clutch body. Against the projecting hub, rear side of engine, a sprocket is mounted which drives the transmission. This hub (arrow) was trued up by grinding and is supported in the crankcase, by originally a babbitt lined bronze shell or nowadays a sometimes used solid aluminium/tin "clutch bearing". At the other side (engine front side) of this clutch body there is a pressed in bronze bush. The body rotates on the crankshaft "nose", when the clutch is disengaged. Although made precise by the factory including the 2 inspection holes, the outer rim can be a little eccentric, so it is advisable to balance the clutch body dynamically (arrow).
2. Picture shows the bronze bush. On an extension of the bush one part of a pressure ball bearing is fitted (arrow). In this case the indentations on the outer rim (arrow) are trued up, so when disengaging, the inner clutch plates can separate more easily. The weight of the body including the steel inner clutch plates is considerable, which results in pressure on the oil film. This film has to prevent metal to metal contact when the the bronze bush rotates on the crankshaft.
3. Originally the factory made an axial oil groove which stops 6 mm / 0.25" before the front side end of the bush (arrow), so there will be no loss of oil pressure as the oil flows in by pressure from the rear side (transmission side). The oil groove has a 6 degree inclination towards the front side end of the bush. The bronze bush can show wear, already after a few thousand miles. Besides inducing vibration, this also can cause some loss of oil pressure. An axial oil groove breaks the oil film more easily which causes metal to metal contact . The picture shows the wear.
4. Picture shows a bronze bush in an Ace clutch body (this body has a smaller outside diameter than the Indian-Four body). Three axial oil grooves can be seen extending till the front side end of the bush. If there is no pressure lubrication, the system can work, especially when the speed of rotation is limited. (When measuring the inside diameter this bush is a little oval, so there is some wear but otherwise very smooth inside. The correct type of bronze was used, 8% tin.)
5. All the Four crankshafts have an oil hole at the end of the crankshaft "nose" (arrow in picture 9). The later Fours have an additional oil hole about halfway the hub length of the "nose". In early Fours it will be an improvement to make this hole with a bevel around it to ease the oil flow. Diameter: 3 mm / 0.125" (see picture 8, arrow). When there is such an oil hole one could make instead of the axial groove, a circular oil groove (width: 4mm/0.15"), positioned exactly at the oil hole. The edges of the groove are eased off. The oil flow now comes from two sides into the bush: from the rear and from the additional made hole. Together with the circular groove, the oil film will be maintained under pressure and there will be no loss of oil pressure.
6. The clearance of the "clutch (body) bearing" is measured (0.04 mm / 0.001574"). Check if the body spins well. If not, before torquing down the bearing cap completely, give some blows with a hammer on the cap to center it. Check again. Mark in which direction the blows were given as in a later stage, when the clutch is completely assembled it is not possible to test the body separately. Watch the aluminium/tin bearings used here (min. 18% tin). Because this material is harder than babbitt and can not incorporate dirt, the crankshaft has to be surface hardened (or hard chromed etc.) to prevent scoring of the hubs. Usually the hardness of a plain bearing is 20% or less than the hardness of a hub. On the # 4 crankshaft bearing from aluminium/tin it can be seen that the shell stands a little proud (arrow) of the ground bearing flange. After torquing down the bearing cap, the shells will be "pulled up".
7. Bronze bush and crankshaft "nose" clearance: 0.02 - 0.025mm (0.001"). All bearing caps are torqued down with the same torque as during the align-boring procedure. The clutch body and crankshaft are rotated by hand in a direction opposite to each other. If at a certain point resistance is felt, the crankshaft "nose" has to be checked. (The red paint mark -arrow- on the crankshaft flange is related to the position of the fly wheel during the dynamic balancing procedure.)
8. The correct sequence would be checking the crankshaft "nose" with a clock before turning the bronze bush. The bush has to be of the correct bronze composition. Preferably honed to size and with a very light press fit and Locktite pressed into the ground hole of the clutch body. It seems sometimes difficult for a  machinist to ground the "nose" in line with the main bearing hubs, because after grinding these, he has to use a steady-support to do the job. The arrow pointing at
the crankshaft shows the crankshaft was ground during the balancing process.
9. If the eccentricity is not more than 0.03 mm/0.0015" this can be corrected with fine emery cloth till no resistance is felt when rotating (step 7). If the eccentricity is more, the "nose" has to be brought in line with the main bearing hubs and reground. After definite assembly the nuts are secured by wire so the studs too will not become loose as the prescribed torque is not that high.