Timing Chain Tensioner Piston

[2002#3]

The timing chain tensioner piston (11311744347) appears to be NLA, at least from the several sources I checked.  Anyone have an extra new one (see Classified WTB)?

Might my old one (X # of miles; probably a lot) be OK to use in my rebuild as long as it's been soaked for hours and the ball rattles?

Thanks.

Larry

[Tommy]

If it's ok by visual inspection I wouldn't worry about using the old one.

[tzei]

+1

Never bought new one.

[TobyB]

+2- usually the timing cover takes the brunt of it.

 

t

 

[2002]

If the ball doesn't rattle you can separate the two cylinders for cleaning.

[tech71]

New after market is available from M TC but looks kinda cheesy.

No reason you can’t clean and reuse the old one

[2002#3]

The old piston was soaked, cleaned, and dried.  The ball rattles freely and seats well when air is passed through it in on one direction but not in the other direction.  This vintage piston has been paroled and is free to continue doing its job.

Thanks, guys.

Larry

[2002#3]

Steve V,

I cannot separate the piston into two parts.  I haven't resorted to using brute force or channel locks (aaagghhh) yet...  Any suggestions?

Larry

[Schnellvintage]

I used 2 deep sockets. one goes inside the tensioner and the other fits over the forked end.  I then tap the small socket (inside the piston) and it comes apart.

[2002#3]

Thanks, Steve.  Not sure I would have thought of that method, i.e., pushing out rather than twisting out.

[Tommy]

I would rather keep it intact. Only failure related to tensioner piston I have seen is that the parts separated and the smaller ones fell down the pan. That was race engine. I would think they stay better together if not separated before. I don't think you would find anything that interesting inside 😉

 

Oh, I forgot, actually not the only one. I had one of the tabs holding the slider part brake off. That may have been related to that I was avoiding the first kind of failure and had tack welded the parts together. It broke beside the tack.

Edited December 4, 2022 by Tommy

[2002#3]

Thanks, Tommy.  

 

3 questions have presented themselves during this Great Tensioner Piston Adventure, the answers to each of which will help me understand the mechanics of TC tension better. 

 

...and I know how this is going to sound...

 

1.  Sucking and blowing at the notched end:  This piston allows air flow in one direction and not in the other direction.

    Sucking and blowing at the spring end:  No air flow in either direction. 

   (i) Are these proper functions?

 

2.  (ii) Mechanically, in which direction does oil flow "through" the piston (if it does at all)? 

     (iii) Is there any oil pressure on either end of the piston? 

     (iv) How does the small oil reservoir dat the end of the piston play its part?

 

Thanks,

 

Larry

[TobyB]

1 yes.

 

2 in the direction that the ball lets it.

 

iii nope

 

iiii it supplies oil to the piston.

 

The way the thing works:  Oil's supplied by the reservoir. 

The timing chain slaps the piston back against spring pressure,

pumping out a bit of air.

The one- way valve lets oil in, but not back out.  Air can leak out

(around the internal piston fit?  Via a small relief groove?  I've never looked,

and different BMW tensioners are slightly different)

so oil replaces the air.

Eventually, the piston is full of oil, and the spring convinces it over

time to stay out against the chain tensioner, keeping the tensioner tight.

If it overtightens (thermal differential, e.g.) the pressure in the piston goes

way up, and oil is slowly bled off.

If it's loose, the chain slap against the tensioner spring and the

normal action of a 4- cylinder engine's camshaft lets

the piston move, and pump itself up a bit.

 

The piston's just a biased damper- since there isn't always tension

against the slack side of the chain, it's designed to work its way to 

about spring tightness, and then sit there.  If there's constant pressure against

it (you slowly roll the motor over in reverse for a couple of minutes)

it will eventually collapse.  But once the motor's running, it'll slowly pump

itself back into place- often, with a lot of noise that didn't seem to 

bother the BMW engineers in the slightest.

 

It's kind of elegant.

 

t

 

Edited December 4, 2022 by TobyB

[jimk]

I believe the name "tensioner" is a misnomer because it can't possible apply any appreciable tension on the chain.  It only keeps the chain from flopping (from the slack) in the non-driven side.

[Tommy]

To make it simple, the spring provides tension to the chain, not much of it but just enough and the piston acts as a damper. Oil is needed for the damper so air should be bled out by pumping it few times after assembly.