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CB 500 Four gearshift repair
By Trike & Chiara
A usual problem for the CB 500 and CB 550 Four, after more than 20 years of perfect activity is the gearshift mechanism. Some mechanics also noticed the problem even from the seventies, on some bikes having made several ten thousand kilometres. The owners of those motorcycles accused the problem suddenly, during a normal condition running. The malfunctioning announces itself in many cases as a lost of power, similar to the clutch slipping, together with almost horrible iron shacking noises from the rear of the engine. Almost in all the cases the third gear could not be operated when accelerating, but returned to its proper activity when releasing the gas. In the worst cases, the engine definitely stops with a terrible "clunck!" and you cannot move the bike anymore, nor try to disengage the gear putting it in neutral position. In this case the only solution is ask a friend with a trolley or a van to come to take you and together raise the rear wheel when carrying the bike. In most rare cases the problem is on the fourth gear.
So, what does it happen inside the 500 and 550 four cylinders engines? The gearshift forks suffer for the consumption of the reinforced parts that stays in continuous contact with the gear itself. Significantly, the gears themselves also become consummated during the years and kilometres. We can say that the third gear is the most used (you must pass by the third, and you also remain in the third more than on the first and second) so, maybe, the problem would happen also on other gearshift forks.
Here's how the gearshift mechanism can be seen from the bottom of the CB 500 and CB 550 engine. Two of the three gearshift forks contact points marked.
What to do then? Replace everything! Forks and gears. As usually, the best solution is to buy all the component new, from your Honda shop, and follow the shop manual for the replacement. But here we suggest a good method to save money and time and having you bike running again.
1 Save money. Honda new parts are expensive. We decided to find a broken engine for spare parts. If you do not hurry up and have the patience to wait, you can have the occasion to find a CB 500 or 550 used engine taken out of the bike with the gears still working. In our case, we applied the classical "one-out-of-two" principle. We found a CB 500 engine having the distribution chain broken while running (and consequently serious damages to the valves stems and pistons): this assured us that the gearshift was still operating.
2 Save time. The method that we suggest allows you to perform the repair without dismantling the cylinders, nor the piston, nor the head, nor the crankshaft and primary transmission chain thus do not having to reassembly all (of course!). This make you save even more money from do not having to buy head seals.
This repair process should be performed by someone who has a minimum knowledge on mechanics. Let's say it's a difficult level 3 over 4 (being 1 the oil replacement!). Now, let's see the main steps for the gearshift replacement.
Step 1. Dismantle the engine from the bike (see shop manual if you do not know how to do, but you should know that if you want to go on whit this procedure!) and remove the oil pan. Clean the engine.
Step 2. Remove the clutch cover and the stator attached to its cover case.
Step 3. Remove the electric starter and all the electric wires, including the one to the oil pressure sensor. (Ok, ok, it's true... We had not yet completely cleaned the final transmission area...).
Step 4. Let's focus on the other side of the engine. First of all remove the clutch.
While removing the four bolts and springs, do not forget to say "hello" to your friends! (Note the replacement gearshift and a couple of Kick n go...).
Step 5. Remove the kick starter spring and the gearshift shaft (this one by just removing the small retaining spring and pulling it out. That's easy). Also loosen the bolts marked.
Here's what you should obtain now.
Remove the second shift mechanism (you had already loosened its bolts).
Step 6. Usually the oil seal of the gearshift shaft of the 500 and 550 Four tends to leak oil. You have a good occasion to replace it. Just take it out (with your fingers or a screw driver) and replace it with an original one or with one having same internal and external diameter (even if its slightly different in height it fits the same. Do it!
Step 7. Loosen and remove all the basement screws, both on the upper and on the lower side, front and rear. If everything has been correctly removed, try to open the back side of the lower half of the engine. If it does not, beat "smoothly" on the case, using a "gum-head" hammer. The engine can be only partially opened, since the primary transmission chain is still linked to the crankshaft. Just few degrees are anyway enough to perform all the substitution.
Step 8. Here we are! remove the two gears assembly by removing the gears and then sliding all out of the engine. Use the hardly conquered extra space to allow the shafts to exit they position. Take note of what you do or take pictures to remind you the correct order of the gears. A nice friend with little hands can really help. Take care not to allow parts to slip into the engine.
Step 9. Together you must remove the gearshift drum, by removing the bigger screw on the upper side of the crankcase cover. Carefully remove the three small cylinders that allow the gearshift forks to slide and their retainers. Those last tend to felt down and "disappear" inside the engine case. Take great care also reassembly them.
Step 10. Reassembly. At this point you should have the gearshift mechanism completely dismantled and out of your engine case. Take the replacement parts (new or dismantled in the same way from a used engine) and carefully reassembly all of them. Observe the sequence of the gears on each of the shift shaft: also carefully take note of the relative positions and orientation of the shift forks. The hardest part of the job is the positioning of the forks little cylinders and callipers. When reassembly the whole mechanism, if one of those small parts fells into the engine it is a considerable loss of time to catch it. One more suggestion is to put some wooden piece to keep the engine open during all the operation that may also last about one hour. Last, always obvious, suggestion is to have a parts list or, better, a shop manual not too far, in order to better understand the parts you're handling.
If everything went on the right way, you can close the engine case (we did not use a sealant, since it is originally closed without any); better, as always, to close the bolts the correct torque. Place back all parts in place lubricating them all, to help you reassembly. Put the engine back to the frame and reconnect everything. Give a look at the carbs condition and eventually clean them. When everything is back in place, do not forget to put the oil in the engine. By the way: change the oil filter is always a good and not expensive activity. Start your engine and have a nice trip, full of gear changing!
