My experience is, after bleeding the tensioner as I outlined previously, the piston is stiff the entire stroke. I could not depress it with my hand. I used a c-clamp and then it went in slowly.

Hopefully others will chime in. It would be great if more info on these tentioners could be documented.

So when you punched it out it wasn't really threads but a ribbed affair just to make a slight controlled leak?
I'm kind of thinking there's not much to rebuild, clean them with something like MEK, pop out the contact pad, clean replace and they should pump up.
The safety wire is a good idea.

Then add in the Jerry Woods mod and and it should be a failsafe set up.

Let's think of what failure modes might be?
1) the external pressure relief fails, like it is dislodged. That means that the tensioner does not have the benefit of more than the suction of the piston to get replacement oil into the cylinder. This is the only one I have experienced, and it is also the most obvious.
2) the internal check valve fails. That means the piston just pushes the oil in the cylinder back out, so little or no damping.
3) there is wear between piston and cylinder. There are no rings, O or steel, to prevent oil under pressure from doing this once the clearance gets too large somehow. Might this be the most common? Could you tell because after filling things you could compress by hand or at least more easily than with a vice or C clamp, and you could see the oil coming out around the piston, and not out at the tip?
4) an obstruction, either in the feed line or in an internal passage? But how would this happen, since engine oil is filtered, and if there is a bunch of debris in it a failed tensioner may be the least of the problems?

5) what if the threaded passage from the cylinder out the tip, whose design Gordon has uncovered to explain its observed function, were clogged? If so, air in the cylinder might find it hard to escape. That could account, for instance, for a tensioner which will compress more easily part way, and then stiffen to where hand pressure is inadequate?

6) there must be other possibilities.

It might be a bit tricky to come up with a test. You would want to know how many pounds of force would be needed to depress the piston X amount in Y time. You could insert a small load cell (like the inexpensive hydraulic gauges you can use for testing valve springs)into your vice or clamping device, and see at what force you first saw a droplet of oil coming out of the tip? Borrow a shock dyno with some kind of adapter? The tensioner is a whole lot like a coilover shock strut, but with only a single compression value on the shock, and tuned to deal with whatever frequency range chains vibrate at.

Yes, a spiral groove around the plug is more of an accurate discription. It just happens to match a 7mm screw, with the threads ground down.

In my case, the spiral groove had some crud in it effectively blocking the "orifice" which in turn prevented the air to bleed out. I removed the plug, wire brushed the spiral grooves, and replace it. The tensioner then bled out the air and seems to be working correctly now.

Seems to me that the only other point of falure would be the check valve. As you say some MEK should take care of it.

As a note. I was going to do the Jerry Woods mod. But on my initial test fit-up there isn't room for it. I only have about 1/4" of piston travel in the tensioner with my new chains. I will have to varify that once my new cams arrive.

I think the function of the external pressure relief is up for debate. From what I can tell, by blowing air through from the oil fitting, the restriction to oil flow is inside the tensioner. If you look at my previous post, I labeled a restriction orifice just inside the oil supply fitting.

I could not tell the difference, when blowing air through, when the pressure relief valve was present or missing. I compared my good tensioner to my failed one, which had the relief valve off.

I am guessing it's function is to prevent air from entering the supply chamber after the motor is shut down.

The only way to know for sure, I'm guessing, is to put pressurized oil through one with some gauges hooked up.

That's an interesting idea. It would explain its flimsy attachment, and small spring.

The check valve inside the base of the cylinder, which is similar, would never have to rely on the spring or its retention to hold any pressure, because the just a little pressure from below - the pressure line supply side from the adjacent chamber - would let oil flow into whatever space there might be for it just below the piston.

The piston trying to push down would produce quite a bit of pressure (for instance, when clamping to bleed), but that pressure would all be pushing the ball into its seat, with the spring just holding it in position when there isn't such pressure.

So if the orifice for the external check valve is so small that it delivers little force or flow upward, it wouldn't need to be sturdy in that direction. And no amount of atmospheric pressure with a vacuum in the supply chamber would cause problems.

As I understand it, Porsche didn't want to have engine oil pressure pushing the tensioner piston upward. That's not how shock absorbers work. But the tensioners don't have the wonderful seals shocks have, so just keeping oil handy to replace that which worked its way out was all that was needed?

"As I understand it, Porsche didn't want to have engine oil pressure pushing the tensioner piston upward."
Yes, it limits the PSI that the tensioner piston sees to the setting of the relief. PSI x piston area = force applied to tensioner arm adjusted for geometry of the arm = force appled to the chain.

Well, the conventional wisdom is that the spring inside the tensioner is what does the tensioning, All it has to do is to keep slack from accumulating between the cam and IS gears, as the slack side of the chain is never called upon to move anything while driving. So you don't need a particularly stiff spring to do that. But with just a spring the slack part oscillates. Those who have watched a running engine with a chain box cover off say it moves around rather a lot. Hence the tensioners acting like a damper/shock absorber. Shocks don't exert the kind of force a spring does. All that is needed by way of oil is to be sure the working cylinder always is full of oil. The earliest accomplished this with oil gathered in a cup on top of the tensioner, which flowed (via a one way valve, if I remember the diagrams of parts I don't use) into the pressure chamber whenever it needed to be topped up, so to speak.

Each time the chain depresses the piston, some oil is squeezed out. As soon as the chain moves upward in its oscillation, the piston raises and the cylinder needs to have some more oil. So the pressure fed system is just a way of doing this filling, and it may do it mainly by having the initial chamber, right next to the cylinder, full at all times, so atmospheric pressure (or, if you will, suction) will draw the needed oil from it into the cylinder.

Second set of Carrera hydraulic tensioners.........
 

Gordon & Walt,

I returned a brand new set of Carrera hydraulic tensioners to PP and got a replacement set. The newer set is now being primed with oil and will be installed in the engine. I could not get the tensioners to get firmer. I could still compress the plunger down with my bare hands considering that I am not that strong (old and weak).

PP says they don't have any problem/s with these hydraulic tensioners. So I am wondering if I am doing the correct way to prime the new tensioners. In the past, I just hooked up the oil lines and ran the engine. The tensioners in my '78 SC are more than 20 years old and working.

Any tip or suggestion highly appreciated. Thanks.

Tony

From what I know, you were correct to return them.

Let us know if the replacements firm up.

Tony - like you, I have just installed the few new ones I have used and run the engine. They come compressed with that pin holding them, and I never saw any reason to remove the pin until installed after the cams were timed. The ones I have removed when rebuilding my engines are sure hard to compress to reinsert a pin - needs a vice, cranked closed slowly.

So this hand compression baffles me.

^^^ My experience is same as Walt's above. I've only done a few Carrera tensioner installations a long time ago. But the same stiffness was experienced. Needed a giant set of channel lock pliers or the bench vise to compress them.

Maybe the issue with the new tensioners is pulling the pin before installation with no oil in the supply chamber?

Third time........
 

Gordon,

I was successful in getting the new hydraulic tensioners to get firm up and hard to compress by using the method you and JW suggested. But using the submerge method was not successful. The oil seems not to be filling up the two chambers by gravity flow while these new hydraulic tensioners are submerged in oil. The slight pressure exerted by the oil hand pump does the trick of filling the 2 chambers and expelled the air out of the system.

So what part of the hydraulic tensioner is causing the failure mode (spongy and soft to compress)?

Tony

The spongy plunger is due to air in the plunger.

IF there is oil in the supply chamber AND the check valve is working properly AND the plug orifice is not plugged up there should be a firm plunger.

The question is, are the new tensioners shipped with oil in the supply chamber?

^^This^^

New tensioners upon arrival........
 

Gordon,

The six (6) hydraulic tensioners I received from Sebro were compressed (pin installed) and not filled with oil. The first four (4) failed the tests due to failure to purge the air and filled the chambers with oil. However, the last 2 tensioners were done differently using your recommended method of injecting the oil. And the new tensioners were hard as rock. Thanks to JW and you for bringing up the procedure. The gravity flow of filling the new tensioners with oil while submerged in oil does not work very well compared to the oil hand pump method for these brand new tensioners.

Tony

So it sounds like those of us who have just put in new tensioners, pulled the pin, buttoned up the engine, and fired it up have avoided worrying? Because the pressure feed lines did their thing within a few revolutions? Especially if the practice is to pull the plugs and use the starter to turn the engine until there is some oil pressure?

I'm thinking that if you pull the pin after the tensioner is in the motor that the plunger wont extend to the point of allowing air past the check valve. Perhaps there is some oil in the supply chamber to start with. But if you pull the pin before installing the tensioner you get air in the plunger when it fully extends?
I wonder how many people check if their new tensioners are firm after installing them?

I don't know, but not me. What does extension have to do with the check valve? That would be the one between the supply chamber and the piston chamber - the one which won't allow oil to flow back into the supply chamber when the piston is depressed by some waggling of the chain.
Air in the piston chamber, one supposes, is quickly forced out along the threads of the piston tip, to be replaced by the much more viscous oil.