Centrifugal Clutch and Sump Helicoil Installation on a Honda NBC110 Supercub. – A Space for My Projects, Thoughts, and More…

Earlier this year I made the decision to replace the centrifugal clutch on my NBC110. This was probably the first clutch replacement in the eight years and 60,000km life of the bike, and it was certainly long overdue.

While still ridable, the slippage was quite noticeable particularly on hill starts, and my previous inspections with the clutch cover open revealed the presence of a rough carbonous layer on the clutch bell. If this coating was as a result of burnt oil as I suspect, in must have been getting very hot due to slippage in the past!

*Above: Original clutch with burnt layer.*

Removing the Engine.

The first order of business was to remove the engine from the frame. While this is not strictly necessary, it does make working on the engine sigificantly easier.

This process is conducted simply following the steps described in the service manual (linked at end). This requires the removal of most of the body panels, in order to gain access to the engine mounting bolts.

Note that the engine is fairly heavy (~25kg) and so having wooden blocks available to sit it down on is not a bad idea.

Removing the Clutch

With the engine off the bike, the bolts holding the clutch cover can be removed. Take care when seperating it from the crankcase, as the gasket can be easily torn. A new one will run you ~$15 from Honda.

To remove the clutch, the three hex head bolts need to be removed from the centrifugal filter cover to reveal the lock nut underneath (left image).

You should use the special tool for these lock washers (right image); I have seen videos where they use a flat-head screwdriver and a hammer to turn it, however my attempts with this method only ended up damaging the nut.

Note that the tool has dog teeth on both ends. The larger end is for the centrifugal clutch, and the smaller for the seperate plate clutch (actuated by the foot shifter). To access the 1/2″ square through the center you will need an extension bar on your torque wrench.

Note also that due to quite frankly poor design, to remove the centrifugal clutch bell the shifter clutch must also be removed.

The clutch tool can be purchased for $40 from Lambda Motorcycles in Australia, or from Aliexpress a quarter of the price.

Lots of crud on the oil strainer from the clutch, but nothing metallic.

Installing a Sump Helicoil Insert

At some point in the bike’s lifetime, the sump plug was either cross-threaded or overtorqued, resulting in damage to the threads.

To correct this, a stainless steel Helicoil insert was installed into the stripped hole in the crankcase, providing a much more robust threaded connection in future. The sump plug uses an M12x1.5 thread, for which generic helicoil kits can be readily purchased for ~$30 NZD from Aliexpress or elsewhere.

The hole is first drilled out (not shown) using the provided 12.4mm drill bit, allowing the new threads to be subsequently tapped in. The provided tool can then be used to screw in the helicoil insert. The small metal tang must then be broken off and removed.

Tapping new threads for the helicoil insert. Note that the tap should be lubricated to avoid galling.

Installing a New Clutch

I purchased my new clutch from Aliexpress for about $80 excluding shipping. They are also available from Auckland City Honda for around $200 if you want a genuine product, as well as from Lambda Motorcycles.

From the box my clutch appeared to be missing a small washer behind the freewheel, causing the clutch bell to be clamped against the end of the crankshaft when the lock nut was tightened. This I only discovered when pulling the bike apart again after several days of having to shift into neutral at every stop to avoid stalling. In the end however I was able to reuse the washer from the old clutch, albeit after an afternoon of hair pulling.

The lesson here is check that all works as it should before you reassemble everything!