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Life of Hi........................Vo

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K2-K6:
Topic as title,  often comes into focus in different threads with detail embedded but a little difficult to find.

The primary drive chain of smaller Honda 4 cylinder engines appears to be one of the most frequently worn  components, even if the other parts show very good lifing for the mileage they've completed. It may not show a good condition with even the most diligent of servicing, and so to consider some of the influences that could bring this about.

It's a Morse type chain design, developed into "Hi-Vo" as a defined product by Borg Warner transmission company.  A combination that's split between very clever design and mechanical performance but opposed by very simple materials (could be considered crude) and construction that gives some fairly unique properties.

Essentially could be considered as a linear gear drive with soft engagement to rotating gear pairs,  it can mask mechanical condition exceptionally well from new to old tolerances.  High torque capacity but with the trade off being it's relatively heavy for given size that has an effect of centrifuge at high rotation speeds. This aspect of the design seems to be one of the contolling parameters in Honda using two small diameter gears,  then changing ratio onwards to gearbox by using differential clutch drive gears to complete the primary route.

Still,  it's capability has to be rotating at 10,000rpm crank speed and keep together with that attendant centrifugal force acting on it.  This does in reality  provide the "tensioning" with this type of drive, and why they are usually used without such devices.

K2-K6:
To divert a little,  tensioning,  of a chain, is really the wrong description.  Tensioned in it's true sense would simply add excessive load to the whole drive train,  and then wear the chain bearings until it creates enough gap to run without that tension.

Our use of the word can more easily be explained as controlling the slack side of chain run.  Usung camchain as example,  the drive / pull side of the chain is run over it's guide in a gentle arc to prevent a straight pull and leave that chain run vulnerable to being exited (like a double base string) from differing frequencies coming from both crankshaft and camshaft pulsed rotation.  The "tensioner" side run, then has to be arranged that the unloaded section doesn't flap and cause unwanted effects while returning to the crankshaft to go round again. It can't use a cush drive as such which would cause timing to oscillate, so effectively has to involve a more consistent guide (that curved run) to prevent frequencies getting into a problem phase that could potentially destroy the chain. 

The primary has a much greater torque loading than the  camchain,  with it's tension/pull side unlikely to live with the duty cycle it has when pulled across a guide as the camchain does. But then it uses the first cush drive to dissipate torque frequency pulses input by the crankshaft.  This results in the highest peaks of the crankshaft pulses being absorbed by the cush drive rather than shocking the  chain components,  additional to that is preventing those severe peaks passing further down the drive train into the gearbox.  If the cush drive fails to respond in the correct frequency band as designed (deterioration to hard) then the surrounding components still have to absorb that unwanted oscillation.  Ultimately that's going to shorten the lifespan of these components. 

Putting anything on the non pull side has difficulties as when the throttle is shut the over run torque coming back through the gearbox is also potentially high,   with the same durability problems as above. A jockey wheel here may keep it quieter if idle is uneven, but can't ultimately change the loading the chain receives.

taysidedragon:
In short, Honda got it right. 😉

K2-K6:

--- Quote from: Oddjob on March 16, 2021, 02:00:20 PM ---Which makes me wonder why Honda didn't fit a tensioner like they did with the 650 which was essentially the same lower engine as the 500/550.

I'm adapting my restoration 1972 500 with a 650 tensioner, we'll see how long it lasts when that's fitted although reports say it seems to last extremely well from those who have done the mod.

--- End quote ---

It "looks" like they've responded to customer observation,  and perhaps service workshop feedback about noise at tickover and the risk of wearing the oil gallery.  Quite plausible and possibly effective but appears more to deal with symptoms than why the chain wears faster than most of the other components in the same environment.
It'll be interesting to see a first hand experience when you get yours running.

K2-K6:

--- Quote from: taysidedragon on March 16, 2021, 11:57:22 AM ---In short, Honda got it right. 😉

--- End quote ---

It looks close to right as so many aspects of it look to be a very good answer to provide a solution for their drive design needs.

As any engine is though, a collection of bits that given correct research and development, will live well together. Some of those components may be much closer to their wear margins than the others. This one sticks out and selects itself for scrutinizing as virtually all that come apart seem to show fairly advanced wear of this chain over and above the other parts.

What I'm curious about is that this type of chain has extremely long lifing in other transmission systems, it's easily capable of getting to 200,000 miles in many 4WD transmission with sizeable loads in comparison to it's dimensions.  But here it wouldn't appear to get anywhere near that service life.

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