QuestionHi, Bill. I shouldn’t even have to ask such a simple question, but I have to say I’m confused on the proper way to tighten the timing chain on a ‘73 CB175. Shop and owner’s manuals simply boil down to rotating the pistons into the proper position, then--and this is what’s confusing me--loosening and then re-tightening the front adjuster bolt.
Sounds easy, but as I look at a diagram of the motor and the tensioner assembly, it LOOKS like there is no way the tensioning system would auto adjust simply by loosening up that bolt. A spring-loaded tensioner on a CB350 obviously works that way, but the diagram of the 175’s system looks like the ONLY way to tighten that tensioner is to screw in that bolt and cause pressure against the tensioner plate.
So I can’t see how loosening the bolt would do anything but loosen the tensioner.
That aside, just how tight do you screw in that darn bolt? I know it can snap easily, which is one reason I’m worried about doing the process. Do I tighten it like a clutch adjustment--that is, until I just feel some resistance? Or do I tighten it as tight as I can, short of snapping the bolt?
I’ve searched for hours on the internet and haven’t found one note on the proper way to adjust the tensioner that answers these questions!
Thanks for your help.
AnswerVince... if you look at the diagram:
http://www.cmsnl.com/honda-cb175-super-sport-175-k3-general-export_model15842/pa...
you will notice that the tensioner assembly pivots on a central shaft (guide roller pin). The spring loaded tensioner bar presses downwards on the foot of the tensioner, which in turn guides the roller end into the back side of the camchain, securing the chain from whipping back and forth during rotation.
The system functions the same as the 350 twins, however the geometry is laid out differently. Loosening the locking bolt, allows the tension shaft rod to be pushed downwards. That is all the pressure needed to tension the chain, if the chain is not be pulled by the camshaft lobes/valve springs on the back side. Turning the engine to TDC (T mark on the rotor) brings the valves into a balanced state. One side is on compression, so no valves are open. The opposite cylinder is on "overlap" stroke, where the exhaust is almost closed and the intake is just starting the opening phase. At TDC both valves are opened the same amount, which neutralizes chain slack. This is where you want to loosen the lock bolt enough to allow the tension rod to move downwards. Once it shifts, then you tighten the locking bolt enough to prevent the tension rod from moving. Tightening the bolt any more than necessary to lock the shaft in place will only break off the bolt or strip the threads in the hole. As you have noted, the 5mm bolt is already small in diameter and then is further reduced where the o-ring sits, so using "gorilla hands" results in a very bad outcome.
If you don't rotate the engine to TDC, then there is a chance that a valve is open which is putting tension force on the camchain between the cam and crankshaft sprockets on the back side. When that happens, the light spring tension on the system isn't strong enough to overcome the chain tension. Tightening the tensioner under those conditions creates chain slack when the engine is turned over further unloading the camchain stress on the back side. This creates camchain slack and whipping during operation.
Problems can arise when someone over-tightens the bolt and distorts the surface of the flat side enough to cause the shaft to bind in the bored hole. Usually that won't happen as the that much force on the tension bolt will break it off before it can do damage to the shaft surface. That can be more of a problem with 350-450-750 tensioners which have larger diameter 6mm bolts. However, in engines which haven't had regular oil changes or which have been sitting for many years, the old oil residues can cause varnish build-up in the shaft hole, effectively gluing the shaft in place which can overpower the light spring tension on the tensioner bar.
Honda designed the system to be easy to adjust... automatically, once you release the locking bolt enough to allow the system to function as designed.
Bill Silver
