Manual - Chapter 1 Engine

17. Examination and renovation: camshafts and camshaft drive mechanism

  1. Examine the camshaft lobes for signs of wear or scoring. Wear is normally evident in the form of visual flats worn on the peak of the lobes, and this may be checked by measuring each lobe at its widest point. If any lobe is worn by a significant amount the camshaft must be renewed. Scoring or similar damage can usually be attributed to a partial failure of the lubrication system, possibly due to the oil filter element not having been renewed at the specified mileage, causing unfiltered oil to be circulated by way of the bypass valve. Before fitting new camshafts, examine the bearing surfaces of the camshafts, and cylinder head, and rectify the cause of the failure.
  2. If the camshaft bearing surfaces are scored or excessively worn, it is likely that renewal of both the cylinder head and the camshafts will be the only solution. This is because the camshaft runs directly in the cylinder head casting, using the alloy as a bearing surface. Assemble the bearing caps and measure the internal bore using a bore micrometer. If any internal diameter exceeds that specified, it will be necessary to renew the cylinder head and bearing caps. Note that it is not possible to renew the caps alone, because they are machined together with the cylinder head and are thus matched to it. It may be possible, however, for an expert to effect a repair; many machines suffer from top-end lubrication problems and several experts offer a repair service involving (usually) the fitting of shell or needle roller bearings to rectify this. It is possible that one of these may be prepared to undertake the repair of such a basically similar design.
  3. Measure the camshaft bearing journals, using a micrometer. If any journals have worn to less than the specified outside diameter, the camshafts should be renewed. The clearance between the camshafts and their bearing surfaces can be checked using Plastigage or by direct measurement. The clearance must not exceed the specified limit.
  4. Camshaft runout can be checked by supporting each end of the shaft on V-blocks, and measuring any runout using a dial test indicator running on the bearing journals. Since no service limits are specified, expert advice must be sought if runout appears excessive; the camshaft should be renewed, however, if warpage can be seen with the naked eye.
  5. Excessive camshaft endfloat can produce a loud, regular, ticking noise noticeable mainly at idle speed. Each camshaft is located by a thrust flange which bears against a thrust face on the rear of the front bearing cap. Endfloat is measured by mounting a dial gauge on the cylinder head so that it is parallel to the camshaft, with its tip touching one end. Push the camshaft as far as possible away from the gauge, zero the gauge, then push the camshaft towards the gauge as far as possible and note the reading. Note that this will be very difficult if the valve gear is still in place; it should be removed if possible. If the reading taken appears excessive, expert advice should be sought. If excessive wear is found, it will be necessary to renew the camshaft or the cylinder head or both; repairs to reclaim such wear are extremely difficult and may be undertaken only by an engineering expert, if at all.
  6. The camshaft drive chain should be checked for wear, particularly if the tensioner plunger was found to be fully extended, this latter condition being indicative that the chain is probably due for renewal. Check the chain at points all along its length, looking for cracked, broken or missing rollers or side plates, or for any links which appear stiff or unduly sloppy; if any of these or other signs of wear or damage are found, the chain must be renewed. To check the chain for wear, lay it out on a flat surface, mark the pins at each end of a one-foot length and compress the chain endwise against a straightedge. Measure exactly the distance between the two pins, then anchor one end, draw the chain out taut and measure the stretched length between the two marked pins, repeat the test at points all along the chain's length. If the stretched measurement exceeds the compressed measurement by more than 1-2% (1/8 - 1/4 in per foot) at any point, the chain must be renewed.
  7. The tensioner and guide blades and the tensioner assembly should be examined for wear or damage, which will normally be fairly obvious. Renew any parts which appear worn or are damaged, especially if a new chain is to be fitted. The same can be applied to the crankshaft sprocket and the two camshaft sprockets. The tensioner plunger must be smooth, unworn, free from dirt or corrosion and able to slide smoothly in the tensioner body. If the spring pressure is doubtful or if any other damage is found, the tensioner assembly must be renewed complete.
  8. Note that the tensioner blade plastic face can be detached and renewed separately. The blade metal backing should be checked to ensure that the rubber bush at its bottom pivot is in good condition, also the rubber buffer which engages the tensioner plunger; otherwise it is not likely to suffer any wear. On refitting, slide the backing top fork over the locating lug on the rear of the plastic face, then insert the backing bottom end into that of the plastic face. Lastly, clip the plastic face centre to the metal backing at the tensioner plunger locating lugs.


17.3 Measuring camshaft bearing journals


17.8a Fitting a new cam chain tensioner blade plastic face - slide metal fork over plastic location plug ....


.... and insert bottom mounting eye into recess in plastic face ....


.... then clip plastic face to metal blade at centre lugs