Workshop: Honda CB125T part two

Honda CB125 valves

Below are the captions in full. Click on the image gallery and scroll through the pic strip for pictorial reference and captions.

Picture one: First of all I’ve opted to remove the valves from the cylinder head. My valve spring compressor has had two metal strips welded into the jaws to fit the small valve sizes frequently found on bike engines, a cost-effective alternative to purchasing a genuine Honda service tool.

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With the spring compressed the cotters can be removed and each valve slid out of its guide. Note which ports the valves come from so they can be replaced exactly the same way.

Picture two: Oily carbon deposits have built up behind the head of this inlet valve. De-coking the valves can be a time-consuming business, particularly the exhaust valves where carbon is likely to be baked hard on to the back of the valve.

I’ve found the best and fastest way to decoke valves is to use a bench grinder with a wire brush attachment. A good alternative is to clamp an electric drill into a bench vice as I’ve done here. Wear eye protection as not only carbon but also small wire bristles will fly off the brush.

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Picture three: Before and after wire brushing the valves. Now it’s possible to examine the valve seat face for signs of pitting or burning.

Remarkably, these valves were all in excellent condition and didn’t require any re-grinding, confirming my suspicions that the previous owner had done some reconditioning work.

Grinding valves on modern engines can be a time-consuming task as the valves are hardened for use with unleaded fuels. I would usually opt to have them re-faced by a specialist reconditioner.

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Picture four: As important as the valves are the valve seats in the cylinder head. The valves must seat properly without leaks or compression will be reduced and the engine will not perform properly. Again, these valve seats look fine, but if they needed work a specialist reconditioner would reface them to match their respective valves.

Check valve guide condition at this stage too. I opted to spend a few minutes decoking the ports and combustion chambers with steel wool, finishing with Solvol Autosol.

Picture five: Always fit new valve guide seals before re-fitting the valves. These seals can harden with age and allow oil to travel down the guides, leading to a smoking engine. The old seals should pop off with a screwdriver, though be careful not to damage the guide itself. A smear of grease in the new seal will let you ease it into place over the guide. Lubricate the valve stems with fresh engine oil before re-assembling them into the head.

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Picture six: To refit the valves I used my trusty home-brewed valve spring compressor again. Make sure you’ve refitted the steel washers under each valve spring pair, then fit the compressor over the spring’s top collar and wind it down until you can see the groove in the valve stem.

To refit the two split collets into the groove I first smear a blob of grease onto the valve stem, then offer each collet into place with the magnetic tip of a small screwdriver. Once the collets are engaged in their groove, unwind the compressor and check the collets are seated properly before moving on to the next valve.

Picture seven: With all the valves refitted I spent half an hour cleaning up the cylinder head with small wire brushes and steel wool before giving it a coat of VHT black to duplicate the original finish. Bead or vapour blasting the head before painting would be more effective and would give a better and more  durable finish, but where cost is a factor this approach will give perfectly adequate results.

The edges of the cylinder fins have been highlighted with fine wet and dry paper to give a shiny silver finish.

Picture eight: With the cylinder head sorted I could now turn my attention to the barrels, and here’s where the real horrors started.

Some corrosion and scuffing is apparent but, more importantly, the left hand bore has a deep groove along its length.

This had been letting oil up past the rings and is undoubtedly the cause of the oil consumption and heavy smoking. There’s only one way to repair damage like this, and that’s to re-bore the barrels and fit new oversized pistons.

Picture nine: I took the barrels along to Martyn at Serco Engineering in Brighouse. He checked the extent of the bore damage and advised a rebore to one mm oversize. Re-bore sizes are limited by the availability of suitably sized pistons and these barrels are now probably on their maximum oversize which means they cannot be rebored again. Not a major problem as they are unlikely to need further attention for many thousands of miles.

Even if they do become worn again a specialist like Martyn could fit new cylinder liners to take them back to standard. Having measured the new pistons, Martyn clamped the barrels onto the boring jig and machined them out to their new size.

Picture ten: Having rebored the barrels Martyn then honed the bores by hand. This gives the correct 'cross-hatched' surface finish and takes off any high spots from the boring process.

Reboring is actually a highly skilled job and spending a few minutes watching Martyn at work emphasised my preference for using an experienced bike engine specialist for this type of work. Motorcycle cylinders have to be rebored from the bottom upwards to ensure correct alignment and any traces of old gasket material on the cylinder base could affect the results.

Martyn also supplied the new piston and ring sets and advises that these should always be obtained before reboring to ensure correct machining to size. Piston kits will come with rings, gudgeon pin and circlips and I’ve found it easier to fit the rings onto the pistons before rebuilding the engine.

Picture 11: The lower piston ring is a composite type featuring a wafer-type ring sandwiched between two thin retainers. Here I’m fitting the wafer ring into its groove before fitting the retainers.

This composite piston ring is designed to carry oil up and down the cylinder bore and keep it lubricated.

Picture 12: Once the composite oil ring is in place the two compression rings can be fitted. This is one of the most delicate parts of the rebuild; piston rings are quite brittle and will break if handled carelessly. I carefully eased the ring apart with my thumbnails before sliding it gently it over the top of the piston into its groove.

Most piston kits come with some kind of instruction sheet for ring fitting; these didn’t. I confirmed with Martyn that the plain, cast iron, ring fits in the middle groove and the chrome ring fits in the top groove. Get them the wrong way round and the engine could burn oil.

Picture 13: The piston rings will usually have their top face marked with a tiny letter ‘N’ near the ring gap. These had the size marked instead, but in such tiny letters I almost needed a microscope to find it. Running the rings upside down may not matter but some top chrome rings have a chamfered face and could consequently lose compression if fitted upside down.

I also fit one of the gudgeon pin circlips to each piston at this stage as it will make assembly onto the con rods easier later. Finally, position the rings so the gaps are spaced equally around the piston before fitting.

Picture 14: A quick examination of the crank assembly showed no signs of any problems, and as there had been no signs of a bottom end fault I opted to refit it as it was. However, it did seem prudent to remove the camchain and tensioner blade for examination at this point as these parts can only be accessed with the crank out. Any crank or big-end problems would again require specialist attention as the unit is a pressed-up assembly.

Picture 15: With the camchain unhooked from the crank I measured its free length against the factory specification and found it comfortably within tolerance.

With an unlimited budget it would probably make good sense to replace it anyway as it has a hard life, but the little Honda unusually runs a duplex chain which is pretty tough. If replacing the camchain it’s worth examining the slipper blades too as these will wear deep ridges in the rubber surface and might need replacement at a similar mileage.

Picture 16: Turning attention to the transmission I laid out the gearbox shafts and selectors and began to carefully examine everything for signs of wear or damage. Things to watch for here are problems with the selector forks which can wear or bend, leading to poor gear selection, and damage to the dogs on the sliding gears which can cause the bike to jump out of gear under load.

Picture 17: More unusually, a gear itself might have sustained damage and I was surprised to find a broken tooth on the output fourth gear cog, probably a sign of abuse at some point in the bike’s past.

Fortunately, a trip to the local breakers provided a sound replacement for a tenner, which helped keep the rebuild within a sensible budget.

It is quite rare to have to strip gear cogs from their respective shafts, but be prepared to encounter various circlips and thrust washers between the gears. Keep a note of the disassembly order and simply reassemble it all the same way. Fortunately this gear was at the end of the shaft and simply slid into place.

Picture 18: When I stripped the engine I was a little dubious about the condition of the oil pump, partly because the previous owner had experienced a lubrication failure shortly after rebuilding his engine. The oil pump mounts in the clutch case on the these bikes and draws oil from the sump through this gauze strainer. Any foreign bodies in the oil will tend to accumulate on the strainer and in extreme cases, block it completely and starve the pump of oil with disastrous consequences.

Shortly after a rebuild it’s worth stripping the gauze strainer to clean it, as excess gasket sealant can clog it up. The strainer can be accessed without stripping the engine by removing the clutch cover.

Picture 19: With the oil pump stripped and cleaned I carefully examined the rotors and face plate for any signs of damage. Foreign bodies getting past the strainer can score the inside of the oil pump, effectively reducing the oil pressure.

Some light scoring is evident on the face plate, visible at bottom right, but in this case wasn’t bad enough to justify the price of a replacement pump.

Picture 20: As I happened to have a new set of clutch plates on the shelf I compared them to the old ones to estimate how much wear had taken place. Surprisingly, the old plates looked heavily worn compared to the new ones so Angie also got a new clutch fitted.

In practice I’ve found clutch plates can be used until the first signs of slipping under load, at which point it makes sense to fit a new set.

The clutch on this bike had shown no sign of distress but judging by the wear on the old plates, would have done fairly soon.

Picture 21: Whilst not strictly part of the engine rebuild I couldn’t help noticing how much wear there was on the sprockets and chain so I took the opportunity to add a new gearbox sprocket to the list of parts for the rebuild.

Before the bike goes back on the road I’ll also have to fit a new rear sprocket and chain, as this new replacement will wear out very quickly if run on a worn chain.

The new gearbox sprocket cost less than a fiver but check carefully when buying as some cheap far-eastern replacements have found their way onto dealers shelves without threads in the two holes for the lockplate.

Picture 22: The final item I’d always check carefully before starting to rebuild the engine is the starter clutch assembly. Three spring-loaded rollers bear on to the starter wheel boss and need to move freely and be free of any flat spots.

The boss also should show no signs of wear or ridges which could cause the starter motor to spin freely and not turn the engine over. This one looks fine.

The assembly fits behind the generator rotor so isn’t too difficult to access with the engine in the frame but on some designs the crankcases must be split to get at the starter clutch.

Picture 23: And finally, for purely cosmetic reasons I spent an hour or so cleaning and degreasing the crankcases before painting them. I used steel wool and elbow grease to clean the cases up (again, bead or vapour blasting would have been better but more expensive). If using steel wool make sure the cases are carefully cleaned afterwards as small particles could find their way into bearings and oilways, causing problems later.

I gave the cases a lovely coat of VHT black and popped each one in the oven for 20 minutes to help seal the finish. Don’t overheat the crankcases unless you’re planning to replace the main bearings. And make sure the wife has gone shopping too…

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