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Hydraulic Cam Chain Tensioner

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K2-K6:
To the original question, I've not seen the hydraulic tensioner schematic Ash. Is it just that, pure hydraulic pressure, acting on a free floating chain wheel geometry ? Sounds like it from your rattling description, ie leaking out pressure during non running needs to be re established at startup.
If that's the case, then oil viscosity is going to be absolutely paramount to it's function, and could be the parameter that changed between field trials pre production and reality of normal service life.
What is the original oil specification for them, is it multigrade or straight listed/fixed viscosity.

The pressure "ram" would ordinarily have to have a bleed off port such that it can't air lock during initial pressure build at each start, which is sensitive to variation in oil temperature and specific viscosity for it's performance. 

There's something of a parallel in hydraulic lifters commonly used for years on American design V8 engines, but with each one inline with the pushrod for each valve individually.  They have a leak off port that will let the lifter recede if parked with that valve open,  this has to establish pressure again on start. Also, they can go over pressure (if that port can't leak fast enough) during high rpm running to give lifter pump that starts to hold the valve off it's seat and leak combustion. 
You can see the dynamic complexity in these systems as illustration for how they are designed.

The Honda one from description you've given of start up noise suggests to high a cold viscosity, such that the pump can't deliver the oil through the system fast enough to refill the chamber providing pressure. 
Their cold ambient temperature oil is only 20 viscosity as I understand it, that's below 59F from Honda specification.  Any higher and it would delay system filling, that's something that Jensen observed on the 450, going up in viscosity simply blows it out of oil pressure relief valve rather than put it through the system.  It has to do this to avoid oil pump failure, but I don't feel that proper consideration is given to how oil viscosity interacts in these engines.
Viscosity is not absolute protection as many views project, it's really just resistance same as electrical resistance. 

K2-K6:
Looking at the schematic above (didn't realise the detail initially) the lower one showing the hydraulic item, you can see where the oil port to enter the casting is at the bottom between the two bolt holes.
Looks like that runs up the diagonal to feed the far end of plunger cavity, and (detail resolution poor) appears to have a bleed out hole on the domed cap that pushes the chain wheel carriage. This hole would prevent air lock situation and control outflow to mitigate full system pressure from being unconditionally acted onto the chain.

From Honda's own oil temperature charts it would seem logical to use something like a 5w30 or 0w30 oil to give assurance in initial cold start supply (neither of these available at time of design) which would likely give reliable response and avoid chain compromise.

We were getting to this type of balance in discussion with Jensen on the 450 thread a while ago. The fixation of higher viscosity as a protector is fundamentally wrong (he effectively proved that with very detailed monitoring ) and came to the wrong conclusion in my view.
Lower cold viscosity promotes immediate supply, without which the protection arguments are simply academic.

The stated oil product viscosity numbers are not what they seem and can cause the wrong analysis to be made about what will benefit the outcome.

AshimotoK0:
You have probably hit on something there Nigel ... many people back in the day used to use 20/50 multigrade,  as used in cars of the day or 'Filtrate' .. made in Leeds with colloidal graphite or Molybdenum di-sulphide in it. Perhaps Honda tested with the 10-40 recommended on the dipstick.  Filtrate was probably fine for bikes without a centrifugal oil filter but on a 250K4 I had,  the filter housing was almost blocked with what I assumed was the additive. The original  local Honda dealer Ken Blakeys used to swear by it though.

Bryanj:
That used to happen to the filters on any oil Ash, worse on the G5 as you had to remove the whole clutch case to clean it out

K2-K6:

--- Quote from: Bryanj on March 17, 2022, 03:51:08 PM ---That used to happen to the filters on any oil Ash, worse on the G5 as you had to remove the whole clutch case to clean it out

--- End quote ---

There was certainly some suspect oils about then as working on engines (bikes and cars) late 70s found many with so much sludge in them. Contrary to how people thought about it, most of the engine material was still there  :) much of it seems to be from oil oxidation and breakdown of viscosity polymers that made up multigrade oils of the time. Speaking to a polymer chemist more recently who indicated that many of the products from this time had significant stability problems in use. I've seen some car engine head areas with sludge built up more than an inch thick.
Virtually all of currently available products have far better lifing performance than that era.

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