Since I’ve had my Cub, the engine has always had a pretty noticeable rattle when cold, which would only go away once the engine was up to full operating temperature. This is a clear sign that the camchain and idler sprockets are in need of replacement, which with over 60,000km of Postie use isn’t surprising.
The new camchain, tensioner sprocket, idler roller, and cover gasket were purchased from Lambda Motorcycles in Queensland, for $74AUD, plus shipping ($18AUD including other parts to replace my piston).
The engine was also experiencing a loss of power and was burning oil when hot, which I guessed was due to worn piston rings from prolonged riding at 90km/hr. A complete new piston and rings was ordered on sale from Lambda for $55 AUD, as well as new head and cylinder base gaskets for $17.
Camchain Replacement
The fairings, footpegs, and engine guard were first removed and the oil drained, allowing the alternator cover to be unbolted to expose the flywheel. A neodymium magnet inside a ziplock bag was used to check for any ferrous debris in the oil, to which happily there was none.
To access the tensioning mechanism, a custom tool was created to help remove the flywheel, using a printed paper pattern as a guide to cut the tool from 2mm aluminum plate. (A4 PDF at end of post). The tool fits around the 112mm OD of the flywheel, with cutouts for the 9 protruding tabs spaced 30° apart, preventing it from turning as the locknut is loosened.
To remove the camchain, the head cover was first taken off to expose the camshaft sprocket, and the locking bolt removed to allow it to be slid off of the camshaft. A simple tool was created to hold the sprocket steady, using offcut lengths of aluminum plate, and two short lengths of 6mm steel rod.
The tensioner arm and cylinder idler roller bolts were then removed, allowing the chain and roller to be extracted into the crankcase side.
The new camchain, roller, and tensioner sprocket were installed, and the engine timed using the marks on the flywheel and alternator cover. With the camshaft sprocket, tensioner, and flywheel torqued to spec, the alternator cover was finally reinstalled with a new gasket, and the engine refilled with oil.
Piston Replacement
Replacing the piston was a straightforward process, requiring only the removal of the cylinder head and cylinder to access the piston.
The head cover was first removed, giving access to the four stud nuts and two bolts adjacent to the camshaft sprocket. After removing the sprocket and camchain idler roller, these can be loosened and removed in a progressive mannor, allowing the head and cylinder to be slid off of the studs.
Note: Be careful to not let the camchain go loose and come off of the crankshaft sprocket, otherwise you’ll have to take off the flywheel to put it back! (ask me how I know). If you make this mistake, a useful tip is to tilt the bike at a ~30-degree angle so that the oil pools to the right, saving you from having to drain it.
The circlips for the gudgeon pin were then pried out using a fine flat-head screwdriver, and the gudgeon pin pushed out. On inspection the piston did infact appear to be in good condition, with some skirt wear, but nothing out of the service limits. The rings also appeared to be in fairly good condition, though they were visibly less stiff compared to the new set. The gudgeon pin showed no wear, apart from some slight surface marking.
Due to the lack of a micrometer the cylinder wear and distortion could not be properly measured, but at the head the cylinder was measured with calipers to be ~49.6mm and ~49.9mm respectively before and after honing.
With the old piston off, the new one with new rings, circlips, and gudgeon pin was installed, taking care to mount it in the correct orientation (the “IN” marking on the side with the larger valve cutout must be facing upwards towards the throttle body). The cylinder and head were then remounted and torqued on, and the idler roller and camshaft sprocket reinstalled. The head gasket was replaced during this process, but the base gasket still appeared to be in acceptable condition and was left original.
The engine was then retimed in the same manner as when the camchain was replaced, and the head cover reattached.
As always make sure to turn the engine over by hand a few times before using the starter to make sure you’ve got it right.
Compression was measured at ~115 PSI (120 PSI spec) with the throttle open, and ~75 PSI closed. The old piston also measured ~75 PSI closed, but the open throttle measurement was accidently not measured before the engine was apart.
The top speed however has returned to its original value of ~90km/hr on the flat, up from arount 80km/hr, and the engine feels less sluggish at low rpm.
With the new camchain parts the rattling is completely gone, and even from cold the engine runs smooth as butter, with no noise except the exhaust thump and slight whine of the chain and straight-cut gears!
