"Bearing" noise in 3/4/5 Explained!

LWJ · 02-28-2010, 10:28 PM

Hello,

I have had what I described as Bearing Noise at low RPM's only in 3rd, 4th, and 5th gear. This is at an RPM that I would have guessed would possibly induce knocking, but my car has to be really lugged out to knock. I noticed this at 1500 to 2000ish RPM.

Conventional wisdom has it that the bearing on my pinion (I believe) was failing. My first gear failed and I pulled the trans out and sent it to a very respectded P-wrench. I really was not concerned as I figured that he would find and fix the problem.

Fast forward to the garage yesterday. I decided to replace my chain ramps and tensioners as I have the original tensioners. Where am I going here? Collapsed tensioner! One side was significantly extended. The other? Totally failed. If not for the over-extension of the one side, it would have been catastrophic.

I have driven this car daily for the last 6 months. You would think it would be more noticible, but it wasn't. In talking with Steve @ Rennsport Systems, he said that low RPM's induces a harmonic wave on the chain that really works the tensioners. I believe that I heard the chain slap during this wave. Higher RPM absolutely no problem.

Let my close call be educational for all.

Larry

PS This is an 80 SC with 110K on it.

sc_rufctr · 02-28-2010, 10:33 PM

Well done on the find and thanks for posting. It could have been a lot worse.

It never ceases to amaze me how our aging cars keep surprising us.

safe · safe's Garage

Quote:

Originally Posted by LWJ

Collapsed tensioner! One side was significantly extended. The other? Totally failed. If not for the over-extension of the one side, it would have been catastrophic.

The chains are independent. If one tensioner is over extended, it has nothing to do with the collapsed tensioner, is due to a worn (elongated) chain.

DanielDudley · 03-01-2010, 04:50 PM

Nice save.

LWJ · 03-01-2010, 05:31 PM

Here is the bad one:

Here is the good one:

safe · safe's Garage

The good one looks OK, pretty normal extension.

Change or rebuild both.

LWJ · 03-02-2010, 06:01 AM

Given SAFE's observation about the two chains, I do wonder why I didn't experience a total failure? Until pointed out, I was under the incorrect assumption that there was one chain. Maybe I am just that lucky?

safe · safe's Garage

You probably have to be unlucky to have a catastrophic failure from a collapsed tensioner, that's what I have heard.
Timing will be of a little, so depending on the compression and cams there could be higher or lower risk of pistons hitting the valves.

That is my guess at least.

You should check your cam timing, it could have skipped a tooth or so.

Walt Fricke · 03-02-2010, 01:43 PM

LWJ

Agree with Safe. That left side tensioner looks about normal. I am looking at the distance between the chain housing and the idler arm. And while that right side tensioner collapsed, it still limited the excursion of the idler gear to way less than full droop. I think the slack present would not allow the chain to skip a tooth absent some other factor.

The 1960s 911s had rubber chain ramps, which became brittle and broke off in pieces. The pieces (per the old timers who had to deal with this) could get wedged between chain and gear, causing a tooth skip, leading to bending all the exhaust valves.

That stopped with the advent of our plastic ramps.

Since you are a man after all our DIY hearts, you might invest in some literature. Wayne's 911 Rebuild book, or Bruce Anderson's classic, or a Bentley, or even the humble and inexpensive, if from time to time confusing Haynes.

But thank you for the post. Despite what I think I know about 911s, I'd not have thought you could drive it for so long with a collapsed tensioner with so little indication or trouble. I do know that a mechanic once told a friend that they could safely drive the 100 or so miles home from a race track with one collapsed. Just take it easy, he said. So I have given the same advice on occasion. Now I know I wasn't giving bad advice.

One more thing - as part of your pressure fed chain tensioner upgrade did you purchase the upgraded idler gear carrier? If you didn't, and have to use that thick aluminum spacer which came with your tensioner kit, I suggest you do. Porsche solved tensioner failures by this upgrade with its wider bearing surface for the pivot, and then sort of double solved it with the pressure feed system.

I was just helping a friend with a rebuild, and saw that he had purchased the pressure feds but had the old idlers. I felt the thick cylindrical pivot posts. One side felt fine, but the other felt rough, so I smoothed it with some emery cloth. And the old idlers looked like they had no bushing. The newer ones are bushed. Take a look in our host Pelican's parts catalog.