Article from David Bradley Honda CX 500 Cam Chain

April 28th, 2020

Noisy Honda CX500 – It must be the cam chain then!

In February1978 Honda began selling the CX500 in the UK. My local dealer had two available and I bought the first CX he sold. So mine is from the first batch and was registered on 14th February 1978. Now these first CX’s had some notable problems, big end failures,if not carefully run in – premature cam chain failure in all cases – plus Cdi ignition problems (fixed by changing the early gold coloured Cdi box to a later black one).

All the bikes were recalled for a modified cam chain tensioner support bracket but the big ends were only replaced on the ones that the owners failed to run in properly. Mine was not ridden by a hooligan so only needed the cam chain tensioner mod! All went, mostly, well for many years but apart from routine servicing it has needed the black Cdi box, a rewound alternator, new water pump seal, a head gasket replacing and eventually a new exhaust system (an after market copy of the original with a collector box under the engine).

However whilst on a ride out with mates early in 2019 after only forty one years service and just 43000 miles from new the engine became decidedly noisy, perceived wisdom says it is a broken cam chain tensioner. Do I call the breakdown service, ride slowly back home or continue the days ride out? Well since it was a fine day (the Sunshine Boys only go out to play in good weather) I decided to chance my luck and continue the days ride. So was relieved to eventually get home albeit with a very noisy Honda but without any further problems. The bike was parked in the naughty corner of the garage, where it stayed for the rest of the year. With the riding season finished and some decorating and modifications to a bedroom the Honda made it to the workbench in mid December for the expected cam chain/tensioner renewal.

Replacement of the cam chain is fairly straight forward other than the engine has to come out of the frame to get to it. So that also means remove the exhausts, radiator, carburettors and disconnect the engine electrics. Now with the engine out the rear cover can be removed to inspect the damage, well almost but to get at the cam chain the alternator rotor/engine flywheel has to be removed. This involves removing the bolt securing the rotor then screwing in an extractor giving it a sharp blow with a blunt instrument (hammer) and hey presto the flywheel is shocked from it taper, well that’s what it says in the Haynes manual. First even the rotor bolt did not want to budge, it took a fair amount of heat and a compressed air impact wrench to get it free. Then after making an extractor, with an hexagonal head, screwing it into the rotor and using the prescribed method the rotor would not budge. More heat, more use of the impact wrench but all to no avail, so give up for now. Next day with more of the same treatment there was a loud crack and the rotor came free from it’s taper on the crank shaft. Now to inspect the damage. Surprisingly the cam chain tensioners (there are two) were not broken but the cam chain (after only 43000 careful!!! miles) was badly worn, so bad in fact that it had worn/ground away (see photo’s) a large part of the head of a steel bolt plus various parts of the aluminium crank case. So where has all this metal gone but more to the point does it get trapped in the oil filter before the oil enters the crank shaft? The thought now is that the engine noise was not only the cam chain but also the big ends on the way out. This is not good news, Honda never made undersized big end shells or the thin wall bush main bearings, although standard size big end shells are still available there are no main bearings left. If necessary the crankshaft could be metal sprayed back to standard size but with no main bearing shells available this would probably not be a viable option. So lets hope the missing steel and aluminium has been trapped in the filter!!!

Now having completed the diagnosis I ordered the required new parts from Honda specialist David Silver. Having looked for advice on the world wide web I decided to do the “triple bypass” operation, that is to replace the cam chain & tensioners, water pump mechanical seal and the three starter clutch sprag springs (located within the flywheel rotor). So having stripped the engine thus far it makes sense to replace all these items. First up was to fit the mechanical ceramic disc water pump seal. This is a two part assembly, a fixed part is fitted in the rear engine cover with the end of the cam shaft passing through it. The other part, the ceramic disc, is fitted into the water pump impeller which is in turn fitted onto the protruding end of the cam shaft. The part in the rear engine cover is spring loaded, this spring keeps pressure on the rotating ceramic disc to maintain contact with the fixed part and this forms the water seal, hence the term mechanical seal. Enough of the description now lets fit the seal. Having done this previously many years ago without any problems I was confident about this part of the “triple bypass” operation. The rear engine cover has to be heated and the spring loaded part is pressed in although even with heat it is still quite a tight interference fit. Behind this seal there is a garter oil seal facing the front of the engine to stop engine oil mixing with the water. These two seals are separated by a small gap, within this gap there is a water bleed drain hole so if any water gets pass the mechanical seal it has an escape route. So up to now all is going well but then however hard I tried I could not get the mechanical seal to fit, how could this be I have after all done this before albeit many years ago. I phoned Honda UK and their advice was to contact your local Honda dealer, “would you like to know who and where they are”. So ignoring this advice I sort help again on the world wide web. This provided the reason I was unable to fit the seal. In 1978 when my bike was made the hole the seal is pressed into is 27.8mm but in 1981 the size of the hole was increased to 28.3mm and in 2020 the seal to fit the original hole size is no longer available. The part number for both seals is identical, it’s just that the seal no longer fits the earlier engines and the only solution available is to enlarge the hole the seal fits into. So a milling machine and boring head would be a help but I only have a lathe and a pillar drill! Info on Honda forums and You tube videos showed the hole can be enlarged with a drill and flap wheel. This was the route I decided to take but also wanted to ensure the new hole was concentric with the original hole. I turned two plug gauges on the lathe, one a sliding fit into the original hole and the other to the size the hole needed to be finished to. The smaller one had a shank on it to fit the chuck of my pillar drill so I could accurately centre and clamp the rear cover to the drill table. A 30mm x 5mm flap wheel was used to open the hole to the required size. This was done at a very slow speed with constant checking of the hole size with the larger plug gauge, you only get one chance to get it right. Another important point is that the hole depth has to be only just sufficient (about 8mm) for the new seal to fit into. This is because the oil seal fitted behind it is still the original size, so you will end up with a stepped hole. As well as pressing the seal in a liquid sealant should also be used (see photo’s). With the water pump seal renewed the cam chain, tensioners and finally the starter clutch sprag springs could be fitted. With the “triple bypass” completed the rear engine cover was refitted and the engine was now back together.

At this stage it seemed worthwhile to tackle the extensive corrosion to the frame before the engine was refitted. Not the full strip down and powder coating but the wire brush and emery cloth treatment of all the accessible parts. Then two coats of smooth black Hammerite rubbed down between coats, a final rub down and finish off with Hammerite “rattle” can black. I was going to use my best long haired soft brush but then thought better of it and decided to use a decorators sash brush (some say it’s a shaving brush) how unkind of them!!! The shape of the brush worked well with a part brushing, part dabbing action to get into all the nooks and crannies. Hammerite also seems to work best with a fairly thick coat and minimal brushing out. So rub down after the first coat and then apply another thick coat using the same technique as before. The following day it was time to inspect the results and wow was I surprised, no need to rub down and finish off with a rattle can the “shaving brush” finish was great! The engine finish let the newly painted frame down so I cleaned off as much dirt as possible from the engine and using a children’s small paintbrush applied “Rub’n Buff Silver Leaf” using the dabbing technique once again. This very small tube was sufficient to coat the engine as seen (in the photo) and there is still some left in the tube. There were many other originally chromed bits, but now rusty, to be cleaned up and these I mostly painted silver, this was never meant to be a back to original restoration but merely a cam chain replacement.

Not much to do now but reassemble everything into being a usable bike again, put the prescribed amount of oil in the engine, distilled water/antifreeze in the radiator and fit a new battery. Finished at last, time to remove from the workbench take it out of the shed check everything is OK and take for a test ride. These CX’s are mostly difficult to start after they have been standing for a long time unused and usually it is the case of winding them up for a good few seconds on the starter motor before they fire up and run properly. This time just one touch of the button and it fired up almost instantly I was most impressed but then let down almost straight away, the original loud mechanical noise was still the same as before the strip down!!!

The Honda was put back in the naughty corner for about another three weeks whilst I thought about how to proceed and I spent quality time with my other bikes. In earlier attempts at trying to trace the noise I had used the time honoured practice of holding the handle of a screwdriver to my ear and touching the other end onto various points of the engine but the noise was pretty much the same wherever I put the pointed end of the screwdriver. More info from the world wide web, “buy a mechanics stethoscope”. So now remove the bike from the naughty corner back into the garden and using the stethoscope listen to the engine once more, still much the same result as using the screwdriver, just a bit more clarity. By now I was in full doctor mode, lying on my back placing the business end of the stethoscope on the under side of the engine but still the source of the noise evaded me. Until, that is, I accidentally placed the listening device onto the exhaust collector box, the sound was deafening in the earpieces, I had found the pot of gold at the end of a rainbow, something was loose and rattling in the collector box so should be much easier to fix than a broken engine! Now off with the collector box and give it a good shake. But still no rattle until that is by holding it up with one hand and give it a good slap with the other then hey presto there was the noise. The next move was to cut a patch out of the box with a disc cutter and investigate further. Inside there are two crossover pipes, the left hand cylinder gases mostly exit into the right hand silencer and the right hand cylinder gases mostly exit into the left hand cylinder. These crossover pipes are spot welded into the mouth of the collector box where the exhaust pipe enters. The spot welding of the crossover pipe on the right hand side had come adrift and it was this pipe rattling about causing the noise. It would be difficult for me to weld back in place but a very simple fix was to drill a 5mm hole through the box and pipe then join them together with a 5mm nut and bolt (see collector box photo’s). Now it just remains to weld back the unnecessary patch cut in the collector box, paint it with cylinder black paint and fit the exhausts back onto the bike.

The cam chain was obviously well overdue for replacement but the noise problem was mostly the collector box rattle but alls well that ends well. So for now start up the bike and listen, sounds good to me and alls well with the world again. Just the best bit to come now, take it for a test ride however that can wait until tomorrow. But then tomorrow didn’t come and the worlds not alright again. Boris has just announced we have all been locked down, so no test ride, unless of course we consider ridding a motorcycle essential travel and beneficial to our health and well being!!!!!!!