POST 7: Blue bike; Rebuilding the motor and transmission
I hadn't planned on doing this rebuild although the possibility must be always on the cards when you buy a bike of unknown history like this. Gasket sets were easy enough to locate on ebay, although bearings and oil seals are not available as sets. I needed:
For the cover:
Kickstart oil seal 16x28x7
For the transmission case:
Final drive bearing 6203uu double shielded bearing
Final drive oil seal 24x4x7
Crankshaft oil seal 15x25.5x7
Crank bearing 6004 unshielded 20x42x12
Oil pump O ring 18.5x2.2
I also bought new 12x17 shim washers in 1mm and 0.5mm thicknesses, and new 12x22 stainless steel wave washers but these will need machining to 17mm OD.
For the crankcase:
Crankshaft oil seal 15x25.5x7
Crank bearing 6004 unshielded 20x42x12
I also bought a new small end bearing
I started with the small final drive in the transmission case and levered out the oil seal towards the inside of the case. This exosed the final drive bearing.
I pressed the bearing out, also towards the inside of the case from the outside using a bench press. The bearing was quite rusty and ots location socket needed cleaning.
I next levered out the crankshaft oil seal, again towards the inside of the case.
Before pressing the bearing out towards the crankcase/crankshaft side. I had to use a socket to press on the bearing and a receiver from my press-and-pull kit.
I then moved to the small crankcase cover and tapped the oil seal out towards the alternator side
The bearing itself could then be pressed out from the alternator side towards the crankcase/crankshaft using a socket that could penetrate right though and an extenson bar to transmit the force.
Once more the location socket was quite dirty with rusty deposits so I cleaned them all up before starting to refit the bearings. I chilled the new bearings in the domestic freezer before fitting, however this was still a tightfit and needed the press.
I could then tap in all the oil seals.
Its now time to address the shimming/washers on the kickstart slave shaft. This shaft runs a helical gear pinion onto the main drive gear but being helical its rotation will develop a thrust along the shaft towards the transmission case cover; for this reason the shaft has a flat roller bearing sandwiched between 2x 1mm thrust washers to dissipate the load. The roller bearing is preloaded by a wavy spring washer at the other end of the shaft and another thrust washer. This all sounds very sensible if overly elaborate but there are problems. The main one being that some of the parts were missing in my bike. The thrust washers sandwiching the roller are available and I can replace the missing one. At the opposite end of the shaft the wave washer is missing and sadly spares are unavailable. To make matters worse both the wave and spring washer sit in a 2.5mm deep recess in the case. Presumably these washers can do nothing at all unless their combined thickness exceeds 2.5 mm. Since the thrust washer is 0.5mm thick then the wave washer must have an uncompressed thickness of more than 2mm or it wouldnt bear on the slave shaft sprocket at all As I can't find one to measure I dont know how thick it should be or what sort of preload it should exert. Im hoping this isnt critical.
I couldnt source an original Honda washer and 12x17 is an unusual size. 12x18 however are available so I bought a selection of shim washers and a wavy washer in this size. I tried to reduce these by 1mm in OD but this proved impossible to do with accuracy and concentricity. Searching the data shows that the rear washer is 18x12x0.5mm and the two front ones are 20x12x1.0. There is no data on the wave washer although I've been told its 17mm in diameter and so sits in the recess. However, I can't find data on its convoluted thickness or whether, or by how much it should protrude from the shouldered recess in the bush. I therefore decided to use a flat-end mill 18mm diameter to enlarge the recess concentric to the 12mm bore but no deeper. I set the drill up using a straight 12mm rod, clamped everything securely and then swapped the rod for the mill.
18mm flat end mill to enlarge recess
18mm flat washers now fit snugly.
Wave washer protrudes app 0.5mm when underlain with 1mm flat shim washer
I decided to split the 1mm shim into 2 0.5mm shims and sandwich the wave washer between them. This preserves the squish but prevents the wave washer riding directly on the spocket.
Also I forgot to work assembly lube into the main bearing so I'll add some clean oil directly on the bearing prior to closing the case. I did smear thecassemby lube on the other side bearing though.
Assembly lube worked in. I'm also showing the gasket in this pic but assembly is the work of the devil. I found it worked best if the location dowels were fitted to this smaller case and the gasket then stuck to the case using aviation Permatex. This held the gasket tight enough to allow assembly without it dropping off. I stuck it to only one surface in case I need to split the case again. I used my tusk puller to draw the crank into the larger case thus closing the two together. Taking care to align the location dowels as the gap closed. You need 3 hands which is why I couldn't take photos during the process.
I started to assemble the alternator side by refitting the stator. I suspect this won't work but here's trying it! I found it is essential that the crank oil seal on this side is pushed in at least 3mm to allow the stator boss to locate into the case before its bolted down.
Refit the key and then the rotor collar
The rotor locates over the collar and is secured with the end nut.
Turning to the clutch side I refitted the oil seal cover.
Have to put my hand up here... I'm blaming the poor manual but I did press on with fitting the clutch and drive chain into the case only to find that the chain guides and tensioner have to be fitted first! I had to take it all out again and refit the clutch sprocket chain and final drive intermediate shaft along with the tensioner. I put assembly grease into all the shaft location bushes.
Tensioner and guides were then fitted followed by the clutch (ratchet) base. This has projections on the underside that key into the bolt holes, refit the bolts and knock the tabs over to lock. Strangely this is't in the parts book I have.
Next I fitted the clutch centre ratchet
... and applied assembly grease to the ratchet...
and to the clutch assembly.
I dropped the clutch cup in next, rotating it to make sure it meshed properly with gears underneath. Finally I refitted the clutch Woodruff key. Note THIS KEY IS NOT SHOWN IN THE PARTS BOOK.
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| Woodruff key refitted |
I followed this with the automatic clutch assembly and secured it with the nut.
Should add here that the Woodruff key is unusual.this is a 3mm thick key, 2.5mm high and 12.8 mm long. Honda use these in a number of bikes and theyre quite expensive. However they are also used in many garden tools so you may get one of those more cheaply- often described as 3x2.5x13mm
Next I tried to install the kickstart shaft. This fell out during the strip and sadly the manual fails to show how it should be fitted so it was a matter of trial and error. It is further complicated as many illustrations show the case upside down compared to its final fitted position.
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| Picture from web- the case here is upside down so the starter quadrant is actually located in the bottom of the case when fitted. |
Here I have to confess to a misunderstanding! I had expected this bike's kickstart to operate in the same was as every other bike's kickstart I've worked on. These would usually kick down in a clockwise direction towards the rear wheel. Unbeknownst to me the NC50 works in the opposite way. This is probably a hang over from the original wind up spring starter mechanism that was converted to a kick start in later models. This means the lever has to be kicked down in an anticlockwise direction towards the front wheel! Its actually a step-on start not a kick-start.
The spring seems to fit naturally in this position but its not clear which side of the lug the straight projection should locate. I chose this position where the spring would be tensioned by the a kick in the conventional direction.
8Pressing on I Greased the kickstart shaft.
This shows the cases in juxtaposition. The centre join will form the bottom of the case when theyre folded together and bolted. This would put the starter quadrant in the lower half of he case when assembled.
I checked the assembly dowels were present
Fitting the final drive slave shaft roller bearing including both thrust washers at the top (wavy washer fitted at the rear).
Assembly grease in all bushes
And gasket in position prior to joining the cases. Note that I have added 0.5mm shim washers to the kickstart and clutch slave shafts which otherwise would run directly on bosses in the aluminium case. This doesnt seem to me to be a good idea but when I added shims they prevented the starter from operating.
I then assembled the case, however once assembled, although the kickstart could be rotated towards the rear this did not turn the motor over. The motor turned only when the kickstart was returned anticlockwise to its starting position. I couldnt understand this until I realised that the kickstart operates in the reverse direction compared with most other bikes. Consequently the starter quadrant and kickstart spring are not positioned correctly and will have to be changed.
I reopened the cases and repositioned the kickstart quadrant. I got the kickstart working in the "step on" manner quire easily but couldnt get the tension on the return spring. This fails to return the lever to the starting position but this might be due to weak spring. I can readdress rhis issue later, so I have left it as is for the time being and will reconsider if the bike runs. - note added later- the diagram in he Haynes manual is correct, but you need to wind on one turn to get enough tension to return the pedal. As you turn the quadrant the spring will ride up and foul the shaft. Its necessary to lever the wire turns back over the shaft with a screwdriver before it can slip down to let the shaft slip home. It helps to fit the kickstart lever so you can control the movement more easily.
The tension I arrived at was to lock the spring tang on the case molding and tension it as far as the hole in the quadrant. Further tensioning involves some serious forces to rotate the quadrant and the spring itself tended to climb up the shaft preventing me f⁸rom dropping it back down to engage the pinion with the quadrant. There might be a better way of doing this but for the time being Ive assembled it as shown (pinion engaged obviously).
I could then rejoin the case... new gasket but no sealant and tighten down. Next jobs are to fit piston and prepare and fit cylinder and head. Then its carb plus manifold/valve assembly and finally the centre-stand before refitting the motor in the bike and trying to connect the hoses and wires correctly.
Sadly my new piston turned out to be oversize so whilst I wait for a std one I refitted the inlet manifold and reed valves
I used a smear of RTV because the plastic on the valve plate seemed worn.
Fitted the centre stand... copper grease on the bolts and pivot pin.
Piston and barrel
The barrel I have is used. I know the lettercodes are supposed to match and mine is a "B".... however I have no idea what was fitted originally so a "B" it will be. I doubt it makes any real diffence and anyway I have a 50% chance of being right!
The barrel came with a nw piston but this was a +0.5 and the barrel std. This might indicate that the-PO had intended to rebore so maybe the barrel is worn? I found it 40.1 with calipers but these arent a good way to measure it. Ill try later. In any event rebore will be appx £60 and a new piston only £15 so Im going to try it as it is.
My barrel also is missing an exhaust stud. I was going to replace with 6mm studding but found I had one in stock so a simple replacement.
The barrel does have surface rust from storage so I honed it carefully with a cylinder hone. When doing 2 stroke cylinders you need a hone with stones longer than the port openings it they will get snagged in these. I used a brake hone... too small so honed from opp ends with care down to the port level. Its improved but not perfect.
I wasnt too happy with that so re-honed using a flexihone
This gave a much better finish. It doesnt photo very well but its a nice satin effect.
I worked assembly lube into the small-end roller bearing and fitted that into the con-rod eye.
I fitted the piston pin clip to one side of the piston before I chilled the gudgeon pin in the kitchen freezer for an hour and then warmed the piston with a hot air gun. I could then fit the piston quite easily from the unclipped side and push the pin through until it contacted the clip on the other side. Make sure the piston side marked "EX" faces towards the exhaust.
The piston was received without instructions. Although not marked (pattern parts!) the rings are not the same... the lower ring being thicker than the upper, and fits in the groove with the expander ring. Fit them with the wider side of the stepped gap uppermost where it will accommodate the piston location peg when the ring is compressed in position. Test fit in the bore.
Finally then in a position to reassemble. Pretty much straightforward although I hate gudgeon pin clips wthout plier loops... blooming fiddly to fit.
Reinstalling to the frame is awkward but manageable and it took a while to work out how to plumb in the carb and oil pump. Note that the oil pump drive shaft does detach and should be coated in general purpose grease before its refitted.
Finally, having cleaned and refilled the oil tank- see separate post, undo the bleed screw (crosshead) on the oil pump to allow fresh oil to fill the pump and hoses... this may take a while.
Of course, I do not know if the oil pump is working. Therefore, as a precaution I attempted to start the motor using premix 50:1.
Got it sorted... and nothing!!!.. I found no fuel was entering the carb as nothing drained when drain screw was undone. This turned out to be down to the needle sticking in carb float valve... cleaned it... carb overflows. Cleaned it again with attention to the needle valve socket (not the tip) as the needle was jamming on the side. Much better!!! I was able to start the motor (no exhaust at this stage) but the engine simply raced, Air and throttle screws had no effect. This indicates to me that the motor is flooding and control is being exerted by some other system. As there was no exhaust I did think that excess air might be being drawn in through the ex port. I refitted the exhaust and found things much improved. The motor no longer raced away with me and throttle control was restored, however, the motor needed a lot of throttle to run and even then would run only for a short time before dying. Looking inside the carb everything looks very wet and "petrolly". This again still sounds like the carb is flooding and once again suspicion falls on the needle valve in the float chamber. Ill need to do something about this because I cant set the motor up when it has excess fuel. Ive ordered a new needle but my suspicion is that the carb body is damaged.
To my joy and delight I have a spark and (eventually) a running motor! Im going to drain and clean the oil tank though... Id assumed that was the correct oil but actually I've no way of knowing whats in there, and as this motor had obviously siezed once before, Id better make sure its the right 2-stroke oil. The previous blue oil has left a gelatinous precipitate.
Testing compression I have 82psi in a warm motor. Seems the compression ratio on this bike was raised from 6.5:1 to 7.5:1. This gives an expected max compn of 105 psi. 82 isnt great but it should be usable- esp as earlier bikes worked at 91. I may try a rebore anyway but at the moment Ive bigger problems.
*Note added- rebuilt in this way the motor ran but lacked power. I swapped the piston and cylinder for a plus 0.5 rebore and compression rose to 120psi!
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