The speedometer cable boot is sort of an Achilles' heel on the two valve twins that was never upgraded. The rubber deteriorated with time and the fit around the cable becomes loose. Water will potentially enter the transmission and if gone unchecked, transmission bearing failure is a likely outcome, all because of a cheap little rubber boot.
This bothered Siebenrock and keenly aware of the exact nature of the problem, he designed and created a better boot! The Siebenrock Speedo Cable Boot first of all is made of Nitrile Butadiene Rubber (NBR) which is a synthetic rubber that is highly resistant to oil. In addition, the to hole in the boot places a much tighter seal around the speedometer cable than does the original, so the water runs down the outside, away from the cable and it stays out of the transmission.
The highly flexible material combined with the shape and oil resistant properties will make this the last speed cable boot that you will need to buy.