I don't understand the tentioner
 

Can someone please explain how a TB tentioner on an 84 works. Why 35 little discs? why is it filled with oil?

It scares me because I cant understand it. I have to put mine back together and not knowing how it works makes it hard for me to reassemble the unit.

Thanks
kyle

If you examine the discs closely you will see they are saucer shaped. So when assembling the discs you are making several flying saucers and stacking them into a spring type assembly. If you have the repair manual CD it shows the proper stacking. The oil keeps the discs from siezing in the barrel and canceling the adjusting feature. Hope this helps. Others may have a more technical explanation.

BTW: I have the early style small diameter tensioner. I had a boot that was fried hard and cracked. All the oil was dries up and the adjuster was totally siezed. After spending several hours cleaning smoothing. and repairing a snapped and stripped adjustment bolt I discovered the replacement boot was a larger diameter. I made a rubber sleeve to take up the space between the small housing and the large boot. I was concerned about leakage of the oil so I filled mine with axle grease instead of oil. Maybe someone can tell me why this is a bad idea.

BTW: I now know there is a rubber part you can purchase to fix the NLA boot fit.

The oil doesn't just lubricate, it transfers heat from the motor quickly since it will circulate within the body and around the washers. The basic idea is the motor gets hot and expands, and the belt doesn't, so the tensioner compensates so they track each other.

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The oil has nothing to do with heat transfer to my knowledge. It is all about the dampening of motion, not unlike a gas shock with a coil over. See Tony's illustration. The copper discs absorb and dissipate heat quite easily allowing the discs to expand and contract with temperature changes. Thus keeping equal tension against the belt as constant as possible. Some have asked why this exists in place of a spring loaded tensioner. Again, to my knowledge, the action of the 928 tensioner is a slight tension of the belt that does not create undo stress on the cam shafts, bearings and seals. Porken has adapted a spring loaded late model tensioner to the 928, but I think he is still in the trial and error stages to see what kind of longevity will prevail.

So this tensioner is not a mystery, just another one of Porsches own way of doing things. They had their reasons I am sure.

Thats a new one for me. I cant imagine any oil needed to "dampen" the little movement that the discs will see. however, mine is void of oil and so is scots, (his leaks, and so does mine, but not any more, as they are both empty) and we have no belt tension issues, nor wear issues.

It would seem that the oils is used for heat transfer, because of the way it is stored in the tensioner housing. the oil is not only in the disc area, but in an channele area to , what looks like, a very efficient way to transfer heat to the oil to have the discs loosen or slightly tighten depending on the tempurature of the engine block.

I would, however, feel better, if the oil was only for damping the tensioner shaft.

MK

No oil usually means loose belt and tension light on. ( in cars with the warning). The discs are arranged over a shaft between a floating piston and a spring loaded pin. The only movement the discs get are they heat up and cool down be it deforming or expanding/contracting while they all move together as a unit on this assembly ( a function within a function). I am sure you have checked tension on your belt and found it loose if there is no oil in there....unless of course you crank the hell out of the adjuster bolt and not worry about it. :) If you say it has all to do with heat dispersion, then how is your tensioner functioning properly and keeping tension with the unit "overheating"...if that is what happens. I guess you could say that the oil does absorb heat and maybe the reason for the heavier weight oil, but I don't think that is it's sole purpose or even it's main purpose.So I take back the comment on the "nothing to do...". I think the reason for the sets of discs opposing each other is to expand and contract having voids between. 20 dics back to back seems like it will not work as well as the configuration that Porsche came up with. I am not an engineer, but I see this as a plausible function in the way I had learned it. If I am wrong, so be it.

I expect Bill Ball will join in soon enough as he has probably done more investigation on this component than most.

The oil is for heat transfer as per TSB 1983. Toothed Drive Belt Tensioning Mechanism with Hydraulic Damping: The different amounts of drive torque during a revolution of the camshafts produce oscillation in the toothed belt, which could cause belt flutter and when belt is loose, even jumping of the belt. The new belt tensioner is designed to hydraulically dampen the oscillations in the belt. The new tensioner is filled with engine oil. The pockets in the engine block and tension roller housing are used as oil chambers. This improves heat transfer between the engine block and tension roller housing and activates the bimetal diaphragm springs. This requires the sealing surfaces of both parts be in perfect condition. The sealing surface of the tensioning roller housing must be treated with Loctite 574 during assembly. Tensioning roller housings are supplied as replacement parts without the oil filling. The oil can only be added after installation on the engine. To guarantee complete filling of the chamber, it’s necessary that air be pumped out of the damping chamber by pumping action of the tensioning roller carrier with a loosely installed toothed drive belt and an open oil filler bolt and bleeder. A small air trap behind the rubber cap is desired as compressible pressure compensation. The oil level should be at checked at 12K mile intervals. Oil doesn’t have to be added, if you can see it after unscrewing the filler bolt. If oil does need to be added, also unscrew the bleeder screw in addition to the oil filler bolt. If the hydraulic damping fails because of insufficient oil, the toothed belt tensioning mechanism will function as the former system without damping. The new tensioning system can be retrofitted to older models, if the sealing surfaces of the engine block aren’t damaged.

Jim Mayzurk
93 GTS 5-spd

I think Jim M.'s text is bit flawed even though it comes from factory. There has always been oil in tensioner. In early versions (ones without filler bolts) its main purpose was to lubricate bimetal washers and other parts in it. In '83+ versions oil has access to engine block and is heated more efficiently than in early version. When oil heats up so does washers and they release belt pressure thus compensating block expansion better than earlier tensioner version.

I vaguely remember reading that you should NEVER turn the engine over by hand backwards as it will damage the tensioner ,e.g. if doing the TB & edging up to the timing marks & you miss & go past , DO NOT turn the engine back , go forward 360deg. x 2 to come back up to the same point.
Anybody know what damage can be done to the TB tensioner if it is turned over backwards ?

Tony;

I love your drawings, but damn, you have way too much time on your hands. :)


BTW, oil is also used as a lubricant to the disc. Try rebuilding a couple that have been devoid of it for a few years, you'll see why it is necessary.

IMO, there is little risk of damage to the tensioner; however, there is a greater risk of jumping a tooth on the TB and leaving slack in both the TB and the secondary cam chain drive. This can easily be verified by remeasuring the tension on an otherwise correct belt after a slight reversal.
If the covers are off, one could carefully reverse the direction past the intended goal ... and then approach that goal from the correct direction.
Being at TDC for example, has a specific unstated aspect in addition to being at that position - which is that everything is in tension at that time.

Thanks ,that makes sense , although I'm sure somebody wrote that the tensioner could be damaged.
The reason I asked is because I want to check the TDC timing mark on the damper & the most accurate way to do it is to screw in a piston stop into No.1 cyl. spark plug hole & by hand turn the engine over till it stops at xx deg. Before TDC & mark the damper at the deg. pointer, then turn the engine over backwards till it stops ,this time it will be after TDC & again mark the damper.
Then find the halfway point between the 2 marks , that is then the true TDC.

They are called bellville washers and provide more spring pressure in a smaller space. As well the tension does not change much as it compresses when stacked as opposed to a wire spring where the pressure increases with compression. The oil will provide dampening as it will fill in the space between the washers and will pressurize when compressed again.

Actually, what you propose is an excellent way to verify the precision of the TDC index mark on the damper: the problem with this verification on a 928 is the modest imprecision of the red plastic pointer screwed to a water pump that is changed out from time to time - both induce some variance: to compound that, the TB tension is often established after the center cover is bolted on ... and the reference mark becomes a black pyramid that is more or less centered over the red pointer ... :roflmao: . All in all, it is accurate enough - but could have been better thought out rather that having three variables summed in the reference point.

Still, the only technique for both precision and repeatability of setting cam timing and TB tension is to approach the mark from a clockwise direction ... which absorbs whatever lash/slack exists in the drive chain.