Author Topic: Question on 6.9 chain tensioner rail design  (Read 50 times)

raueda1

  • Classic
  • **
  • Posts: 359
  • W116 Enthusiast
  • Location: Salt Lake City, Utah
Question on 6.9 chain tensioner rail design
« on: 01 October 2019, 01:10 PM »
Another thread goes into the philosophy of renewing the chain rails & guides so I won't repeat or confuse that issue here.  My question is different.

The curved guide is tensioned by tensioner.  It seems to me that, by design, it will necessarily wear.  My analysis is this: 
When new, the chain will ride over the flat surface of the curved tensioning rail on the chain links.  The links will cut 3 wear grooves in the plastic material until the chain is no longer riding on the links but on the rollers themselves.  At this point chain pressure on the rail will no longer come from the links but primarily from the rollers.  But, unlike the links, the rollers roll and are presumably far less able to cut the rail material than the links.  The links, after all, are working like a very dull chainsaw.  Hence further wear will be relatively slow after the rail contact point becomes the rollers.

If this wear model is right it suggests that the simply noting grooves on the tensioner rail does NOT indicate that it's due for replacement.  The grooves necessarily form by design.  It seems like there should be a spec on the thickness of the rail's plastic facing cause that's what's really most important.

That rail is very expensive.  It seems silly to replace it just cause there are grooves that quickly return, especially if there's still a lot of meat between the links and the metal backing, say 3 or 4 mm.  Yeah, I know, I know.  "Just bite the bullet and replace it."  Still.  Any thoughts on this?  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Squiggle Dog

  • W116 Addict
  • ****
  • Posts: 2,044
  • MBCA Member
  • Location: Surprise, AZ, USA
Re: Question on 6.9 chain tensioner rail design
« Reply #1 on: 01 October 2019, 04:40 PM »
Another thread goes into the philosophy of renewing the chain rails & guides so I won't repeat or confuse that issue here.  My question is different.

The curved guide is tensioned by tensioner.  It seems to me that, by design, it will necessarily wear.  My analysis is this: 
When new, the chain will ride over the flat surface of the curved tensioning rail on the chain links.  The links will cut 3 wear grooves in the plastic material until the chain is no longer riding on the links but on the rollers themselves.  At this point chain pressure on the rail will no longer come from the links but primarily from the rollers.  But, unlike the links, the rollers roll and are presumably far less able to cut the rail material than the links.  The links, after all, are working like a very dull chainsaw.  Hence further wear will be relatively slow after the rail contact point becomes the rollers.

If this wear model is right it suggests that the simply noting grooves on the tensioner rail does NOT indicate that it's due for replacement.  The grooves necessarily form by design.  It seems like there should be a spec on the thickness of the rail's plastic facing cause that's what's really most important.

That rail is very expensive.  It seems silly to replace it just cause there are grooves that quickly return, especially if there's still a lot of meat between the links and the metal backing, say 3 or 4 mm.  Yeah, I know, I know.  "Just bite the bullet and replace it."  Still.  Any thoughts on this?  Cheers,

This makes a lot of sense to me. While the wear can possibly affect the timing, I think a new part would wear down to that level rather quickly, anyway, and then almost stop.
Stop paying for animal cruelty and slaughter. Go vegan! https://challenge22.com/

1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 347,000+

UTn_boy

  • W116 Addict
  • ****
  • Posts: 1,087
  • Location: knoxville, tn
Re: Question on 6.9 chain tensioner rail design
« Reply #2 on: 01 October 2019, 05:06 PM »
I imagine Mercedes never published any information about the wear limit of the rails because it was a level of minuscule importance that neither the technician or repair shop needed to concern themselves with.  Though, the book states that if there is obvious/excessive wear to replace it.  But given the propensity of the plastic rails breaking apart due to age I always replace them for good measure.  Especially the ones on the bottom since they can't be accessed when the engine is together and in the car. 
1966 250se coupe`, black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3, papyrus white/dark red leather
1973 300sel 4.5, silver blue metallic/blue leather
1976 280s astral silver/blue MB Tex
1978 450sel 6.9 Euro, Anthr/velour