Rebuilding our 1980 Honda CR125 Engine DoctorDanger.com

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>Rebuilding the engine of our Project CR125

Our CR engine was so full of sand we could make sandcastles with the cases. Thankfully we have managed to clean it up and get into the position where we can rebuild it.

Having gathered together the parts we require all we need to do now is bring out the lump hammer and set to rebuilding the engine.

Ohhhh I do love a new bearing...

	Here is the starting point, a couple of empty crankcases. Now we have cleaned these and painted them. We have removed the knackered main bearings and so the first thing to do is fit some new bearings.
	Start by putting the new bearings in the freezer for a few hours and then heat the cases. The bearing can then be gently tapped into the cases - DO NOT HIT THE INSIDE BEARING RACE - this will knacker the bearing, drive the outer race only (we use the old bearing that was removed or a socket if we have a big enough one).
	Make sure that the bearing is seated properly (in this case the bearing must sit against the case). Having installed the bearing you then need to fit the oil seal.
	Make sure that you fit it the right way round (i.e. to stop oil getting in or out) - CHECK THE MANUAL - in this picture the oil seal is facing outwards to prevent oil entering the combustion chamber. Fitting the oil seal is simply a case of applying even pressure around the seal to drive it squarely into the case.
	Having fitted the bearings and oil seals the next thing to go in are the gears. Start by fitting the two main gearshafts into the case...The two gearshafts must mesh properly - this is critical - no bodging allowed. The two shafts should spin reasonably freely in the neutral position, once fitted the next item to go in is the shifter drum and forks. Note the position of the shifter drum in the next picture, as this is roughly the neutral position and is the position in which all the forks and the external shifting mechanism should be assembled around - it also allows the drum to pass through the crankcase half.
	First fit the shifter drum, and then fit the selector forks, these run in the tracks of the shifter drum and you will of course have noted which fork goes in which groove when you dismantled the gearbox. To fit them simply remove the bar and fit the fork into the appropriate gear and the pin into the appropriate track then insert the bar into the holes to hold the forks in place. You may need to rotate the shift drum to get everything to line up but this is easy - there are three shifter forks in total (see completed gearbox later).
	When fitting the shifter forks make sure you put the correct ones in the right and left position, they are marked and you should see the L on the side as shown in the picture...
	Having installed the majority of the gearbox you need to slot in the kickstart shaft. The shaft fits easily but be careful that the lug on the kickstart gear is located behind the retaining plate (we didn't and assembled the engine only to find that the kickstart didn't work - dohhh).
	We had our crank rebuilt and fitting it is eased by chilling it in the freezer for a few hours. The crank is a tight fit and you will not be able to simply push it into the bearings - don't try hammering it into the bearing - this is precision aligned and wacking the end will throw it out of alignment.
	And, if all is going well the gearbox should look like this, now you can offer up the other crankcase half and begin to draw the two together
	Special tools are available to draw the crankcase halves together, but you can apply even pressure to the crankcases by using extra long bolts to bring the halves together. You can't use the standard bolts as they won't reach until the cases are a little closer together, use a spacer under the bolt to vary the length as often you can only go so far before you run out of thread depth in the opposite case.
	Tighten the bolts evenly around the crank as it needs to be true to go in easily - and don't forget to fit the gasket between the two halves (yes we have done it before)! Make sure that the gear shafts run true into there respective bearings on the opposite case...all pretty straightforward so far, except for the gearchange mechanism...

The costs so far...

Rebore and piston £160, parts £200, used carb and reed £70...just think that when you pay £100 for a bike you are really buying some useful spares not the basis for a restoration...

and here's the plan...

1	Get the Bike...
2	Strip It
3	Clean it
4	Paint it
5	Engine
6	Suspension/wheels
7	Test it
8	Race it
9	Costs and Parts

A new piston, crank rebuild we're over £500 already and we haven't bought a carb or those shocks...hmm, cheap fun is starting to look like a remote possibility.

The gear change mechanism

	To begin rebuilding the gear change mechanism put the spring and shift arm in the crankcase and then place the shift gear into the end of the shift drum (make sure that the spring/ratchet assembly is correctly fitted).
	To ensure correct alignment you need to align the line on the shifter arm with the punch mark on the shift gear as shown here, the drum itself should be in the neutral position (see previous section)
	The next thing to go on is the neutral locator - this consists of two spring loaded arms with a space in between, begin by fitting the arm with the larger wheel on the end, and then place the spacer over the locating pin. Note that the springs look the same but they do fit exactly into the recess on the arm - if you put the wrong spring on it won't fit properly
	next fit the arm with the smaller wheel - ironically we assembled this incorrectly first time round (put the arms in the wrong order) and this actually gave a good shift action until we fitted the retaining plate which stopped the shift working!
	Having fitted the arms you will need to fit the retaining plate, to do this you'll have to ease up the shfit gear as the retaining plate fits into the groove on the gear - make sure that the spring loaded parts of the shift gear don't pop-out.

The Clutch

	Before starting to rebuild the clutch you need to fit the primary drive gear, before fitting this a spacer is required. The problem with fitting the spacer is that the oil seal lip got twisted around it and took some faffing about to get right - watch this as the oil seal will fail unless the lip is seated properly
	The actual primary drive gear is a tight fit on the crank and wont fit properly until the washer and nut that hold it onto the crank shaft are fitted, we left these off so that we could fit the clutch which is easier to hold and thus tighten up the drive nut...
	Start by fitting the outer basket, which must mesh with the primary drive gear on the end of the crank. The basket looks in good condition.
	Next fit the inner clutch basket, this engages with the gear shaft and again needs to be inspected for cracks and damage. The key thing to look for is wear on the grooves around the basket - the clutch plates slide up and down the groove so they need to smooth - sometimes the clutch plates can wear ridges in the basket that can cause clutch drag.
	Having fitted the inner drum, add the washer and clutch nut and use a clutch holding tool to hold the basket and tighten the nut to the required torque
	Before fitting the rest of the clutch insert the clutch actuating arm into the opposite side of the crankcase and feed the actuating shaft into the centre of the gear shaft...
	Next fit the cap onto the end of the shaft that will actually disengage the clutch...
	Before assembling the plates and springs check that they are within the required tolerances: friction plates: min 2.45mm springs: min 30mm face plate run out: max 0.20mm If any of the components are near these limits then replace them, all our plates and springs were well within these tolerances, although a couple of the friction plates looked pretty smooth we will try before we buy.
	Insert the friction and drive plates alternately into the basket - friction plate first - until all are installed
	When fitting the outer plate you need to align the arrow on the plate to the arrow on the inner drum as shown, then the outer plate will engage with the inner drum and you can fit the springs..
	Simply place the springs into the holes provided and then tighten them down with the clutch bolts - as long as the outer plate is aligned correctly this is easy - we know because we didn't align it properly and then wondered why we couldn't get the bolts on...
	And that is it for the clutch, now you can tighten up the primary drive nut and move on to the clutch operating mechanism
	The Honda has the clutch operating arm hanging down in the clag underneath the engine - you did remember to fit it before fitting the clutch. On our bike the cable retaining point looks knackered as it isn't square and looks like it should have more meat on to hold the clutch cable...
	as we didn't have a clutch cable when we bought the bike we are not exactly sure how this works, but we made up a small plate to hold the clutch cable in place - hopefully this will work

Finishing it off

	Having closed the crankcases together and fitted the clutch you can now fit all the external bits like the clutch, flywheel, piston etc - all pretty straightforward. We had our engine rebored and are fitting an oversize piston
	To fit the piston and cylinder head its often easier to fit the engine into the frame - you will certainly find that the motor goes in easier.
	To fit the piston you'll first need to slot in the small end bearing, before doing this soak it in a bit of two stroke oil to aid assembly and ease wear when you finally start the thing up.
	Next slip the piston over the con-rod and make sure it is facing the right way (usually an arrow on the top of the piston points forward) and then slide the gudgeon pin through the piston and small end...easy peesy
	Having got the gudgeon pin central you can now slip in the retaining clips with a pair of pliers...make sure that they are firmly seated you don't want them to fall out...
	Next, the rings must be located on the appropriate pins. Note that in some engines the rings differ, or there may be only one etc. In this case they were both identical (we hope) and we fitted them with the etched letters facing upwards - we'll let you know if this is right
	Before trying to fit the cylinder make sure that you've fitted the base gasket!
	Fitting the cylinder turned out to be quite tricky. Getting the rings to stay in place was a nightmare. The trick seemed to be to check the location of the rings through the inlet port (as shown) if they aren't precisely located there is no way the cylinder will go on...once on you simply fit the head gasket (copper) and then the head. Finish off by torquing it down...
	Next fit the reed valve, gaskets (two) and the carb. We have to thanks Nils Bjelk for these items - Nils races CR125s in Sweden and looks like he knows far more about it than us... Having fitted the carb just hook up the fuel and hey, hoe here we go...well actually we haven't got it running just yet, see the testing section for more detail..

What we learnt this time...

the more engines you rebuild the easier it gets - but don't forget the kickstarter...

the next installment is the Suspension/wheels>