To protect your engine from destruction you need the jump-guard together with the new-style, larger cam-chain tensioner, identifiable by being approx. 2.5" or 63mm tall.
The new cam-chain tensioner is the FIRST LINE of defence against the dreaded cam-chain jumping the teeth on the crankshaft sprocket.
The cam-chain is held in tension by a hydraulic system which is fed from the engine oil pump system. As the engine oil pressure builds up, this forces a piston down, which in turn pushes a sled against the chain to keep it tight and stop chain slap, noise and tooth-jumping. This problem is caused by the old-style tensioner leaking oil back into the sump after standing for a while with the engine off, particularly on the side stand.
If this happens, the cam-chain is slack on engine re-start and this slackness causes a VERY, or just a SOMEWHAT noticeable rattle for a few seconds when the engine is started. In extreme cases this slack can allow the chain to jump the teeth with potentially disasterous results.
If the chain does jump the teeth, this loses the synchronisation between cranckshaft (and therefore the pistons) and the cam-shafts (and therefore the valves) which can result in the valves and pistons colliding. Ouch! And very £€$¥ as this can result in bent valves and broken pistons which means a full engine strip and lots of new, expensive parts.
The new-style tensioner has an oil reservoir built in which in theory holds enough oil to ensure that the tensioner is fully up to pressure when the engine re-restarts.
The rattle on engine start is not, in and of itself, disastrous; it is merely a symptom of the problem existing and can mean one of two things, either you still have the old-style tensioner OR you have the new-style tensioner (which was fitted in production in 2011(?) ) AND it is getting worn. There is a small 'O'ring seal which costs pennies and takes minutes to fit and which will restore the ability of the tensioner to hold oil and in theory stops the chain becoming slack and noisy.
This is where the 'jump-guard' comes in. Despite having the new-style tensioner (and for all bikes with the old-style), the chain can still get slack if the 'O' ring seal goes and therefore the chain can still jump the teeth etc. etc.
The plastic 'jump-guard' that Tom so clearly describes, was fitted as a 'back-stop' and is the second line of defence in the sequence. If the chain gets slack and the chain attempts to 'climb up the teeth', the chain will make contact with the plastic sheild and NOT be able to jump the teeth!
So kiddies, we ALL need BOTH the new-style tensioner AND the jump-guard fitted to our K12 or early K13 bikes, noisy or not. There is nothing else between you and disaster; regular servicing and maintenance makes no difference. Make sure you have em or get em fitted now!
Brian (who never thinks sticking plasters are a satisfactory solution to a problem caused by bad design