By Steve Grosekemper
As longtime tech advisor for PCA-SDR I get a lot of emails and phone calls, but sometimes I just get stopped at an event or scan the SDR Forum for questions I might be able to help people sort out. After writing a hundred or so tech articles since the late ’90s, I thought I would give the membership a chance to be heard. This series is about the questions I get asked as your San Diego Region-PCA technical advisor. I hope you enjoy the change-up.
While scanning the Technical forum [https://forum.pcasdr.org/index.php] index, I found a question from PCA member Fred Yip. Fred has a highly modified 1975 911 racecar that had suffered a Carrera cam chain tensioner oil feed line failure and he was looking for some technical assistance. The forum posting read: Replacing chain tensioner oil line with AN line. An AN line is an extremely durable military spec oil line commonly used in aircraft and motorsport applications. AN (Army/Navy) fittings date back to WWII aircraft use. The smallest AN fitting size is -2AN which is about twice the size of the oil line Fred needed to repair and strengthen. So read on and find out what solution I had for Fred.
First, here’s a little back story. A 911 engine has two chains that drive one camshaft each. As the engine temperature increases the chain gets tighter so there is an idler wheel and a tensioner (like a little shock absorber) that keeps constant tension on this chain. It looks a lot like the rear derailleur on your mountain bike. Early 911s had tensioners that were sealed and failed with age. When they failed, the chain would get loose and make a terrible noise if you were lucky, and engine damage if you weren’t. They got progressively better through 1983 until the design was changed in 1984 to the famous “Carrera Tensioners” that you have undoubtedly heard about.
The main difference in the design of the Carrera tensioner is that it is fed fresh pressurized engine oil whenever the engine is running so it will never suffer from loss of oil in the dampening chamber. Perfect—what could go wrong with a design like this?
Well, it is a pretty bulletproof design, but nothing is perfect under all conditions. The one small flaw with this system is that the tiny oil line that goes from the cam oil line to the tensioner can break and pour oil all over the rear of the engine. Now this doesn’t happen often but it does happen. There are a few things that will accelerate the failure rate of this line:
• High RPM engine use.
• Racing, or other track use (AX. TT, DE).
• Solid or semi-solid engine and/or transmission mounts
• Engine vibration (for multiple reasons)
• Oil feed line trauma due to over-tightening or twisting of the fittings.
• High engine mileage or line corrosion.
The problem is these very small lines have fittings attached on each end, and with the effects of age and vibration they eventually fail between the tubing and the fitting. We can’t stop the aging process on ourselves or our cars but we can stop the vibration. Porsche came up with an anti-vibration kit for these lines to reduce the likelihood of failure. I have never seen these lines fail when replaced in conjunction with the anti-vibration kit.
The anti-vibration kit consists of 6 parts (2 brackets, 2 clamps and 2 screws) and is about $25 total.
2- PCG51117402 Clamp.
2- 9001905902 Screw
1-93010734100 Bracket (Left-long)
1-93010734201 Bracket (Right-short)
Remember it is always a good idea to replace the oil feed lines as well. Just because they are not broken does not mean they have not suffered damage. If they are on a gently driven street car you are probably OK, but if your car experiences two or more issues from the above list be sure to replace the lines as well.
1-930.107.347.02 Left side short line about $40
1-930.107.348.09 Right side ling line about $50
The last thing to inspect when replacing the cam tensioner feed lines is the banjo bolt that screws into the chain tensioner. This is a hollow bolt with three large oil feed holes so it can’t be torqued very tightly before damage will occur. If it is over-torqued the bolt will stretch, the holes will elongate and the bolt will eventually snap in two resulting in a huge loss of oil.
It is imperative to replace the sealing washers on the banjo bolts when replacing them or just removing the line. There are four washers required and their part number is 90012311530. The washers should be coated in anti-seize compound when being reinstalled. If they are put in dry, the bolt could twist the banjo fitting at the end of the line and bend it, or even worse, crack or fracture the line.
The problem Fred was facing was twofold. Because the failure happened in a racecar, we know the oil lines are subjected to extreme vibration. The other problem occurred when someone replaced the right cam oil line on his car and they let it twist under the final tightening procedure, kinking the tensioner feed line at the fitting. The cam oil line twisted and fractured the tensioner feed line…this line was living on borrowed time.
Fred has corrected the cam oil line angle so as not to cause any distress on the line, and has the anti-vibration kit properly installed. This should be the last time he has to deal with this failure.
With new hard lines installed and a complete vibration kit to top it off, you will have one more thing that you won’t have to worry about on your 911. Now get out and drive that car! There is a PCA event with your name on it—whether you drive it there at the speed limit or wait for the green flag to drop at your first Club Race.
Steve Grosekemper is the San Diego Region Technical Advisor, member of the Forum Administration team, and is part of the newly expanded service team at Black Forest Porsche/BMW/Mini. He can be seen and quizzed for technical advice at just about any SDR driving event, as well as a handful of social events. Or go straight to his inbox at email@example.com and you just may find your story in an upcoming Windblown Witness article.