Bjbpe

Several weeks back someone advised me to overhaul the tensioner. I do have the Porsche 928 service manuals and did look at the exploded view.

Being a very old coot, I am not inclined to buy the expensive replacement which I understand works better. However, I do not send the tach over 3500 RPM and rarely exceed 70 mph so I suspect a simple overhaul of the tensioner is sufficient.

So, what do I overhaul? Replacement of o-rings? Anybody with step-by-step instructions?

Bertrand Daoust

Dwayne's write up.
Chapter 14.
Oustanding, like everything he does.

http://www.dwaynesgarage.norcal928.o...0Procedure.htm

Pfc. Parts

Take a look at John Pirtle's instructions too for an alternate view. I used both it and Dwayne's tutorial to rebuild mine, no problems at all.

http://members.rennlist.com/pirtle/svc.html

Bjbpe

Just another question:

As I understand, and understanding is tough at age 78, the tensioner is there to compensate for the engine warming up. If a tesnioner is not overhauled (I gather that means replacement of o-rings and gear oil) what's the likelihood of catastrophic problems for an engine that rarely get's over 4000 rpm?

Landseer

Entire belt pathway would need review, not just the tensioner, regardless of how easily its driven.

When has the timing belt been changed?

the flyin' scotsman

I'll try..........the belt tension is mechanically set by a bolt and lock nut. As the engine heats up it expands so the initial mechanical setting becomes too taught. To compensate the bevel washers inside the tensioner that are heated through the oil flatten thus allow the tensioner to release some tension (de-tension). Typically the oil leaks out which causes a delayed response to the heat transfer thus the system runs at a tighter than optimum spec. There are also numerous bearings, bushings, rollers etc etc that need to be inpsected and probably replaced.

Add up all cost of the above plus at best your ending up with a 30 YO design the optional Porkensioner becomes far far more realistic.

Good luck!

MainePorsche

Well put Malcolm.

Mrmerlin

FWIW if your going to use the factory tensioner I would suggest these things be done.

Install a new boot and external clamp. NOTE Orient the internal clamp so the open end faces up.

Replace the O ring in the bottom piston.

Clean the washer stack and use the WSM to verify you have the stack correctly positioned.

Use Hondabond 4 on both sides of the new tensioner gasket . Found at the Honda dealer.

Once the tensioer is installed remove both of the bleeder screws.

Use STP oil treatment in a visene bottle to fill the tensioner using the furthest hole from the crank.
It will take about 1.4 oz to see oil come out the bleeder hole closest to the crank.
drill the tip of the bottle out just a bit to make it easier to push the oil.

DONT use a hand pump oil can, as you can blow the internal clamp off the new boot, thus requiring dis-assembly.

Once oil comes out the other hole let it sit like this for a few hours to let the oil find its level,
then add some more to be sure its filled,
replace the bleeder screws.

waynestrutt

"To compensate the bevel washers inside the tensioner that are heated through the oil flatten thus allow the tensioner to release some tension (de-tension)."

Malcom this is not exactly correct. If the 40 Belleville washers were to get hot enough to flatten your engine will have long past the melting point of the alloy. The washers retain their tension and shape through out the entire temperature range of the engine.


o Belleville Spring Features
o Flange Washer Features
o Disc Spring Features
o Material Specifications


Belleville Springs > Product Information > Material Specifications
Material Specifications


Material Types, Descriptions, Chemical & Physical Properties

AISI 6150 Alloy (ASTM A506)
Operating temperature range: -40° F to 350° F. For indoor/outdoor service, electrical and mechanical. Fully magnetic. Finish: All washers are completely scale-free and deburred,
• Mechanically zinc plated .0005" thick, minimum, with a clear chromate dip (per ASTM B695, Class 12, Type I).
• Mechanically zinc plated .0005" thick, minimum, with a yellow chromate dip (per ASTM B695, Class 12, Type II).
• Plain finish, scale free.
• Light oil protective coating.
• Black Oxide oil coat.
AISI 1074 High Carbon Steel (ASTM A684, AMS 5120H)
Operating temperature range: -40° F to 350° F. For indoor/outdoor service, electrical and mechanical. Fully magnetic. Finish: All washers are completely scale-free and deburred,
• Mechanically zinc plated .0005" thick, minimum, with a clear chromate dip (per ASTM B695, Class 12, Type I).
• Mechanically zinc plated .0005" thick, minimum, with a yellow chromate dip (per ASTM B695, Class 12, Type II).
• Plain finish, scale free.
• Light oil protective coating.
• Black Oxide oil coat.
AISI 301 Stainless Steel (ASTM A666, AMS 5519)
Operating temperature range: -400° F to 550° F. For indoor/outdoor service and use in corrosive atmospheres. Slightly magnetic. Finish: Scale-free and deburred.
AISI 17-7 PH Stainless Steel (ASTM A693, AMS 5528)
Operating temperature range: -400° F to 550° F. For indoor/outdoor service and use in corrosive atmospheres. Not recommended for chloride or fluoride applications. Suitable for cryogenic applications. Highly magnetic. Finish: Scale-free and deburred. If desired, a low stress Sulfamate Nickel plated finish from .0003" to .0005" thick per AMS 2424D can be supplied.
510 Phosphor Bronze (ASTM B103)
Operating temperature range: -400° F to 300° F. For use with copper bus silicon bronze bolts and nuts. Also for non-magnetic requirements and good electrical conductivity. Non-magnetic. Finish: Scale-free and deburred.
Inconel 718 (ASTM B637)
Operating temperature range: -400° F to 1100° F. For high temperature service in corrosive atmospheres. Indoor / outdoor service. Non-magnetic. Finish: Scale-free and deburred.
H-13 Tool Steel (ASTM A681)
Operating temperature range: -250° F to 1100° F. For Indoor / outdoor high temperature service. Fully-magnetic. Finish: Machined and lightly oiled.
Chemical Properties
Material ASTM C Mn P S Cr Si Mo Ni V Cu Al Ti Ta Co B
1074 A684 .75 .65 .04 .05 * * * * * * * * * * *
6150 A506 .5 .8 .04 .04 .95 .3 * * .15 * * * * * *
H-13 A681 .35 .35 * * 5 1 1.4 * 1 * * * * * *
301 A666 .15 2 * * 17 1 * 7 * * * * * * *
17-7 PH A693 .09 1 .04 .03 17 1 * 7.1 * 1.1 * * * * *
718 B637 .08 .35 .015 .015 19 * 3 52.5 * .03 .05 .09 5.1 1 .006
* * * * * * * * * * * * * * * * *
* * P Zn Sn Pb * * * * * * * * * * *
510 PB B103 .35 .03 5 .05 * * * * * * * * * * *
Physical Properties
Material Modulus Yield Temp. Range Finish Avail. Finish Magnetic
1074 30,000,000 210,000 -40/350F Zinc Plated Light Oil Yes
6150 30,000,000 220,000 -40/350F Zinc Plated Light Oil Yes
H-13 30,000,000 220,000 -250/1100F Machined/Oil Light Oil Yes
301 25,000,000 140,000 -400/550F Deburred * Slightly
17-7 PH 29,000,000 180,000 -400/550F Deburred Nickel Yes
718 29,000,000 180,000 -400/1100F Deburred * No
510 PB 15,000,000 100,000 -400/300F Deburred * No



A major advantage of this method is the ability to have a constant force though out the temperature range which is why it was chosen. Also should one spring break it cannot escape to do damage and the reduction in tension (2.5%) is minimal because you still have 39 more. I am not knocking the porkentensioner it is a great alternative but if the one spring were to fail what might we expect to happen. Is there any documentation showing the range of temperature it can safely provide the correct tension. Will this spring be affected by the heat of the engine over time?

Bill Ball

Wayne: I think you are not exactly correct. These are not Bellville spring washers. They are bimetallic washers. The two metals have different expansion rates in response to heat and the washers flatten when heated. It's described as such in the original 78 & 79 Service Info Tech books.

davek9

This de-tensioner has nothing to do with RPM, it's all about the Heat and engine expansion.
If the internal parts do not move freely (inside bore is Teflon coated), the T belt system will fail.

Dave

Mrmerlin

^^^ the only time i found a bad tensioner was when after taking it apart the bore was dry and the piston had worn through the teflon coating thus causing it to stick,
if you have any oil in the tensioner it should be fine for rebuilding

the flyin' scotsman

thx Bill, thats what I remembered.

I shall admit in 12 years of 928 ownership Ive never rebuilt the OEM tensioner. Ive removed quite a few and replaced them with the Porkensioner system....never a worry

Charley B

According to Mark Kibort he raced for years with a dry tensioner and suffered no ill results. This is certainly not a recommendation, but rather a friendly attempt to completely confuse the issue. I hate being the only one confused.

Dave928S

Mark always says everything's OK even when it's not. It sounds bad to me .. I think he should get a doctor to look at it ....