heinrich

Canine Aviator .... toss that thing, it WILL cause belt failure in future.

FlyingDog

I've been told by a few that mine is the worst they'd ever seen. The whole piston assembly came out as one solid brown chunk with no oil.

H is right, I should get a new one, but those things are expensive. I got it cleaned up enough to work smoothly. Once I get all the other needed stuff done (see sig line for #1 on the list) I'll get to swapping the tensioner. I'm more concerned that my TB still squeeks than I am about the tensioner thermal function stuff.

dr bob

The 'thermal function' stuff is suburban legend IMHO. The oil is in there to dampen the oscillations in the belt at its point of lowest tension. Fluttering in the belt leads to premature failure of the belt itself as the reinforcing cords flex unnecessarily. Warmer oil may flow a little better, and a bigger reservoir for the oil is available if you let it up against the block. After that, the thing is a check valve and a piston pump. Oil circulates from the bottom behind the check valve to fill the space under the piston whenever the piston is pushed forward by the springs to take up a little slack in the belt. If the belt then has a tight spot in the rotation for some reason, it tries to puish the piston back down the bore and compress the springs and the oil. But by then the check valve is closed and the oil doesn't compress. Instead, the only way that the piston can slide down the bore is when oil passes by the outside of the piston in the bore, or it goes past the check valve in the wrong direction. Or it leaks past the threads on the adjusting bolt, or out throgh a bleed/fill valve left loose.

The bimetallic washer theory? I don't think the Belleville washers in mine are anything but spring steel. That's why I don't subscribe to the thermal discussion.

FlyingDog

Yeah, that too.

heinrich

I tend to side with Bob here. The washers are there IMHO to create pockets of oil in stead of a large oil pocket under constant oscillation from the plunger. One thing is certain ... if those washers or the plunger stick somewhere and do not move freely, the belt is under stress and so is the rest of the system.

Now here is the important part .... there are hundreds of dry tensioners out there and they are definitely not doing the belt, pump and rollers any good. There is a reason there must be oil in the tensioner.

WallyP

The early pour-it-in tensioners normally use gear oil. The later tensioners with the fill ports use engine oil.

It can be difficult to keep the flimsy little gasket in place as you install the tensioner. I finally gave up, and glue mine to the tensioner.

A little RTV on the tensioner bolt threads will help keep the seepage there to a minimum.

dr bob

... What Wally said.

The pour on 75W gear oil is not way different from the pour on 30wt engine oil. The slight difference makes it possible to gravity-fill the tensioner while you have lunch, as opposed to waiting overnight for it fill. If you are using the hand oil pump, the difference is only about two Advil for the cramps. After that, the thickness and temperature of the oil regulate how much will pass by the sliding piston on tensioner compression.

There is also a tendency to fill the tensioner until even the boot is full of oil. Tension the belt, and you reduce the available volume as you push the adjusting bolt into the lower cavity, while at the same time you are pushing the pin and the boot from the other end. It will ALWAYS leak when you do this. The oil has to go somewhere! Phineas Hydraulic predicts this (indirectly) in his theory of displacement reciprocity.

Plan on cleaning up the spilled oil after the first tensioning and engine run, then monitor to make sure that it doesn't continue to leak. Same deal if you decide to 'top off' the oil level, unless you fill from the bottom and stop as soon as oil comes out the upper bleed port, with the piston already compressed.

heinrich

Bob I alway use hypoid oil nd I always allow oil out under tension, and it never leaks

GlenL

What Bob siad about Wally.

And (repeating here) make sure to:

Have gasket seal well. I've used some Permatex #2 on it. Messy and sticks hard. ( but nicely pine-scented)

Don't over-fill the tensioner

lorenolson888

If you use a oil gun and a tube the pressure you can generate is enough to Pucker out the boot that seals it all togther... so it actually works well to fill it most of the way and then gravity feed it to allow the last bubble out...

Oh yeah and that rusty POS looks like it is gone...

LO

WallyP

Wow! This is a first! This is the first time that I ever remember disagreeing with dr bob!!

From:
Page 15 of "Porsche 928 Service Information" Book 4582.21, published in 1978 to acquaint 911 mechanics with the basics of Porsche's new flagship...

"Temperature Compensation of Toothed Belt Tension"
"Because of the thermal expansion of the engine block, the toothed belt tension tends to increase with increasing engine temperature.
To prevent this, 8 bimetal disc packets are installed which contract with the increasing temperature so that the tension of the toothed belt remains constant."

I guess that this means that Bob will be scrutinizing my posts with a magnifying glass now...

Ketchmi

Actually Wally and I went through this before and I'm sure we both understand how it's designed to work. The oil does provide some dampning effect but it's main purpose it to transfer heat to the bimetallic disks to relieve pressure as the block expands. There are no "oil pressure" dynamics inside the tensioner. The soft rubber boot would not hold any significant pressure.

heinrich

I don't know Dve, if you don't lube the plunger I've seen serious damage inside the tensioner tube

FlyingDog

Like this:

heinrich

like that.