The CB125TD is pretty much the same engine as all the other 'Benley' series engines, as used in CB125, CD125 'Benley', the CM125 'Custom', CD200, and the 233cc CB250 Nighthawk, amongst others, as well as many licence built Chinese bikes.
There are differences, but the only one of note to this job, is that the older bikes, mostly the ones with 6v electrics, have a different cam chain tensioner arrangement, so barrels and heads aren't directly interchangeable.
As illustrated here, the rebuild was performed with new barely & piston kit, procured very reasonably from e-bay, rather than trying to have the old barrels re-bored, and fitting over sized pistons to them. Ecconomically, it worked out about even either way, but the kit meant less running around, and the job could be done in one shot, without having to strip the barels off, and have it hanging around in bits however long the machinists took to do the barels, then however long it took for the pistons the right size to fit however big a hole the machinists had had to bore, to arrive.
So, getting on with the job. First thing, engine BACK out the frame! AGAIN. Then down to business.
Part One - Stripping Down
On the Bench, Rocker cover removed
Breaker bar to 'crack off' each of the cylinder head studs, in sequence, then ratchet to wind them off.
Then the rocker assemblies can be removed to expose the camshaft, and the cam chain sprocket loosened off.
Removing the cam chain is err... fiddly! But then the cam can be withdrawn from the sprocket and head.
Then the Cylinder Head can be lifted off.... then the barrel. (Makes it all sound SO! easy!)
Cylinder Head studs loosened off with clever 'cam' action stud wrench, then removed.
Gudgeon pin removed from piston, and the pistons removed
and the base gasket face cleaned up.
Part 2 - The Cylinder Head
The camshaft & rocker assemblies were removed from the had casting to get the barrel off the bottom end. All that remains in the head are the valves. This can be the awkward part and where a valve spring compressor can be very helpful. If you don't have a valve spring compressor, or one that is small enough for the deminutive little Benley cyclinder head, then its possible to improvise using a G-Clamp and a piece of tube with a couple of slots or holes drilled in it, to allow access to the collets in the top of valve spring cap, which need to be removed, when the spring is compressed down the valve stem, to allow the cap top be removed and the valve slid out the valve guide in the head.
The valves need to be removed for inspection, and if nothing else, lapping back in, to clean up the sealing faces between valve seat and valve. If the engine has been left derelict for a long period or has a high mileage, its likely that the valve seats will be eroded, or pitted, and both the valves and the seats will need re grinding. This is a machine shop job. The valves set in a machine like a lathe, but with a grinding wheel, instead of a cuting bit, set at an angle to put a clean flat, or often two flats on the sealing rim of the valve. The valve seats, depending on wear may need to be removed and replaced, or similarly reground, using a machine like a pillar drill, but with a complicated multi stone grinding head that lets the tool setter set the correct angle and diameter to grind the valve seat.
I advise, removing one valve at a time, for inspection, and 'lapping' as I do here, so as not to mix up valves and springs and collets from their original location. If the need fully regrinding, then they all need to be removed, and it doesn't matter too much which valve they originally came from (provided you don't mix up inlets and exhausts!)
In this case, the valve seats weren't in bad shape, and the lack of compression, and high oil consumption of the engine, was as likely to be due to worn piston rings, and hard valve stem seals.
Valve spring removed, the valve stem seal on the end of the valve guide, and its replacement in the little bag. Before fitting new valve stem seals, worth checking that the valve guides themselves aren't excessively worn. The 'rule of thumb', is actually a rule of thumb! Remove the old valve stem seal, dip your thumb in a little old engine oil, put valve back into guide, then push it down with your thumb, so that the ball of tour oily thumb is sealing the end of the valve guilde, with the valve stem inside it. Should be able to pick the head up and hold it with the valve stuck in the guide purely from the vaccum in the guide, and if its good, pulling the valve, you should feel it 'suck' against your thumb! If the valve falls out... then the valve guide is probably worn, and will need replacing (machine shop job). In this case, rule of thumb worked, and the guide was in pretty good shape.
Valve head can then be cleaned up with a little fine wet and dry or soft wire brush, and we can get on with inspecting the seats.
This Hi-Tec tool is a lapping stick, and once upon a time there was one in every shed in the land, with a little tine with two lids of 'lapping paste', one end of the tin having the first 'course' polish, the other the second shot 'fine' polish. Tool has multiplicity of uses however, and if you are eagle eyed, you may have noticed I used it to hold the cam chain from dropping into the crank case during strip-down!
However. Vale LAPPING. Often bigged up and called a 'valve grind' in adverts, which it most certainly isn't, is basically polishing the valve and its seat together, to get a smooth surface and better seal. Argument rages as to whether its really 'necessary' these days, though. In times past it was a standard service procedure, undertaken as part of 'user maintenance'! during a routine 'Decoke', when the head would be taken off and the pistion top polished, the combustion chamber cleaned and the valves cleaned of all hard combustion deposits. Oil and fuel back then didn't have the 'deturgents' modern petrol products do, and it was something people would do annually, or every couple of oil changes. These days, its not so common, and many people suggest its not worth doing, engines not getting as coked up, as they used to, and if its in question, better to have the valves and seats properly reground. However, in this case, on an older low miles engine, where the head doesn't really warrant reconditioning, its a worthwhile thing to do.
On a cylinder head that has been reconditioned, and has reground valves and seats, or new valves or guides, there is some debate as to whether to 'lap in' new valves. Some say that it helps 'bed in' the valves before running. Others suggest that it's as likely to wear the knife edges of newly ground seats, as offer real benefit. Personally I am in the latter camp, but many engine builders still insist on the practice of lightly lapping new valves. I leave it to your judgement!
Getting on with it... pretty simple, you dib a little grinding past onto the valve rim, stick the lapping stick's sucker on the valve head, and put valve into guide and rub, rolling the stick back and forth between the palms of your hands so it rotates the valve back and forth, lifting occasionally to move the lapping paste about, and moving the stick around a little so you aren't rubbing the same spots continually. Pressure between valve and seat need be minimal, your polishing, not grinding! and you keep going, until you have nice shiny surfaces on valve and seat! Re-application of lapping past from time to time may be required, and worth wiping old paste off with a rag between applications. Jobs done once with the first 'coarse' lapping paste, then that's cleaned off, then job repeated using the 'fine' lapping paste, and all I can say it it takes a lot of time and patience!
Apart from to warn that the valves in these engines are peculiarly tiny, and the sucker on the end of the lapping stick may be a bit big on the head! Mine worked for me, though.
With the first valve 'lapped' the valve can be reassembled, replacing the spring, by much the same process as it was removed, compressing it over the valve stem, replacing the spring cap, and fitting the collets. The port in the head, the valve seat, valve and guide should be carefully cleaned to remove any deposits of lapping paste first though. Cotton bud and thinners works well. then with one down, three to go... and eventually the head is ready to go back on. Time to look at.....
Part 3 - Pistons & Rings
So, first picture, pistons, old and new. Second picture, before doing anything else, lubing up the new slugs with two stroke oil. BUT it's a FOUR STROKE! You don't yell in unison! Well, two stroke oil is very useful for this kind of thing, and the main property is that its disposable, combustible lubrication. Putting together a new engine, very useful for putting some oil between moving parts for when the engine is first turned over, or run up, but once the engine gets warm, it burns off, and 'clears' fairly easily without leaving too much by way of residues and deposits, as four stroke engine oil, or grease would. If you don't have any, Kitchen vegetable oil is a useful substitute.
The CB125 has a three ring piston. Top and middle rings are 'sealing' rings, bottom ring is the oil control ring. This is a niggly little bludger, and is actually three rings in one groove. Two thing rings top and bottom, that dont have the tapered form of a sealing ring, often called 'scrapers' they wipe the oil off the cylinder wall. Between them is a concertina ring that holds the two scrapers apart, and provides venting holes to let the oil wiped by the scrapers escape through the piston.
Take Note: this is another reason NOT to neglect the oil strainer in a Benley series engine, or the oil changes in any bike! If the oil goes 'thick' these rings often easily clog up, reducing the amount of cylinder wall lubrication, aiding premature bore wear, and increasing possibility of snapped or snagged rings.
Care is needed to fit the piston rings, they are thin, they are cast iron, so brittle, and they have to be stretched over the SOFT aluminium piston, walked down the skirts, and seated in their respective groove.
Starting at the top, working them over the piston crown, you start with the lowest ring first, which will be the first scraper ring, next is the concertina ring, followed by second scraper ring, then lower sealing ring, then top sealing ring.
All rings installed, CHECK which way round the piston sits in the bore. The pockets in the piston crown are different sizes for the different sized valves. Piston is normally marked with an arrow or stamped 'front' to indicate which way it should be fitted. Then ONE gudgeon pin circlip can be fitted to the inside circlip groove. Ie the side that will be next to the cam-chain, so different on each slug. This makes fitting the pistons easier.
Piston can then be fitted to con rod, sliding the gudgeon pin through till it buts against pre fitted circlip. (rather than going in and coming out the other end!) and the outer circlip fitted to make sure the gudgeon pin stays put!
Repeat for other piston, and you can put the barrels back over them, then put the lid back on.
Part 4 - Building Back Up
Base gasket and Cylinder head studs replaced, and the barrel fitted on.
Care is needed to wiggle the barel over the pistons without snagging the piston rings, risking snapping a ring, or scoring the bore.
Head Gasket goes on, and then the cylinder head.
Recycled photo of 'removing' the cam chain, because I forgot to take one putting it back on! Likewise refitting the cam chain sprocket! ALL after the idgery nadgery job of timing the cam in!
And more recycled photos, reversing the sequence, fitting the rocker assemblies, and the cylinder head bolts, after MUCH muggering about getting the cam chain tensioner aligned and bolted up! Cylinder head bolts given 'final' tightening with torque wrench, to book setting, of I think 20ft lb. Not shown because my capable camera woman had gone to bed!
One more reycled, photo, with the rocker cover back on, and jobs a good'n! Engine rebuilt and ready to go back in the frame.
Worth while making sure that the engine has good oil before it's run up, though, and also winding the engine over by hand for a few cycles to make sure that it IS timed in correctly and valves don't touch pistons or anything daft.
Incredibly, hands on spanners time for this job was barely a working day. From the arrival of the barrel kit to having the engine back in the frame, and running, was barely 36 hours!