| The design of the 911 engine incorporates a dual-cam system that is driven by timing chains connected to the main crankshaft. One of the weak points of the early 911 motors (thru 1983) is the mechanical spring-loaded tensioners that maintain the tightness and accuracy of the chains. After many years of faithful service, these tensioners have a tendency to fail. If a chain tensioner fails, then there is the distinct likelihood that the chain will slip off of one of its sprockets. The result can be catastrophic failure as the pistons will most likely hit the valves, resulting in a complete engine rebuild and a $6-$10K repair bill.|
In 1984, Porsche developed a better tensioner. This newly designed tensioner was driven both hydraulically by oil pressure and also by a standard mechanical spring. This design was well improved over the old one and as a result, the reliability of these engines increased significantly. With the introduction of the new chain tensioners, Porsche also developed a bolt-on kit that could easily be retrofitted to all of the early cars from 1969 to 1983. Engines from 1965-66 cannot use the bolt-in upgrade because the cam tower lines attach differently. If you rebuild your early 1965-66 911 engine and use a late-style chain housing and cover, you can then use the newer chain tensioners. Engines from the 1967 year can also use this method to install the upgrade. Smog pumps from the 1968 engines will not fit unless the left cover is slightly machined as well.
The old style sealed mechanical spring-loaded chain tensioners were prone to mechanical failure. An inexpensive alternative to installing the pressure-fed Carrera chain tensioners is the addition of a safety collar around the shaft of the tensioner. While not as good as a pure replacement, the safety collar can provide some emergency help when tensioners fail. A potential pitfall is that it is not easy to detect tensioner failure, and the repeated pounding of the collar may cause it to wear and begin to lodge metal bits inside your engine. The collar is only for use on the early sealed tensioners, and not the pressure-fed ones.
Figure 1 shows the new oil lines that must replace your existing cam oil lines. There are two primary lines, and also two hard metal lines that feed the chain tensioners. Figure 2 and Figure 3 show the new chain cover housings that come as part of the kit. The primary difference between the old and new housing covers is the addition of the inlet hole for the oil line that feeds the hydraulic chain tensioners. The new chain housing covers are universal and should fit all 911s from 1969 thru 1983. The kit comes complete with all the mounting hardware that you need (not shown here).
The kit comes complete with everything that you need to perform the job on 1973-1983 cars with CIS (Continuous Injection System). The earlier cars need an oil line adapter, and also a set of chain tensioner spacers. This is because the later style tensioners had narrower bearing surfaces than the original tensioners. If your car has these narrow style idler pulleys, I'd recommend replacing them when you perform the upgrade.
The procedure for installation is quite straightforward. The first step is to drain the oil from the car. This is not an absolute requirement, but its recommended. Now, place the engine at Top Dead Center (TDC) by putting a wrench on the pulley nut and rotating until you line up the Z1 mark on the crankshaft pulley with the seam of the case. Remove the distributor cap, and make sure that the rotor is pointing to the small notch on the distributor housing. If its not, rotate the motor another 360 degrees until it does. For more information on findiing Top Dead Center, refer to our Pelican Technical Article on 911 Valve Adjustment.
Removal of the muffler is required in order to gain access to the rear of the motor. Remove the muffler by loosening up the bolts that attach it to the heat exchangers. Then loosen up the two muffler clamps. Its wise to treat the exhaust mounting hardware with some WD-40 a few hours prior to trying to remove the nuts they have a tendency to rust very easily in this area. Check the muffler clamps for damage or rust, and replace them with new ones when you reinstall the motor.
Once the muffler is off of the car, remove the rear engine shelf. Attachment of this piece varies from year to year, and may require the removal of a heater hose or two. On air conditioned cars, you need to temporarily dismount the compressor and move it out of the way. Remove the bolts that hold the compressor onto the bracket and pull it out to the side of the car. See the Pelican Technical Article on Dropping the 911 Engine for more details. Once the compressor is out of the way, remove the compressor bracket that is located right in front of the chain housing cover.
The distributor needs to be removed as well in order to gain access to the left cam oil line. Make sure that you have a timing light handy, as you will need to reset the timing on the engine when you replace the distributor.
Once you have access to the rear of the engine, remove the left and right chain housing covers. You should see the old-style chain tensioners as shown in Figure 4. If your chain ramps are looking worn, now is a good time to replace them. The upper chain ramp on the right side is shown in Figure 5. Make sure that the chain is kept tight around the cam either by wedging some wood in-between the chain and the case, or by tying the chain together near the outboard chain ramps. If you have a good pair of vise-grips, then they make excellent clamps as well. The goal is to assure that the cam doesnt move and that the chain doesnt skip on the cam gears. If the chain does come off of the cam gears, then you will have to re-time the cams.
The newer style chain ramps are manufactured out of tough plastic, and are known to stand up better than the ones used on the early cars. It is recommended to replace the ramps if they show pitting, which may be caused by the chain flapping up and down. Two odd-shaped bolts that pass through the case create the mounts for the chain ramps. Make sure that you dont install the ramps backwards the four inboard chain ramps closest to the crankshaft point with their longer end towards the shaft; the two outboard ones point their longer ends out towards the wheels. The chain ramps pull off of their mounting posts, and the new ones simply snap on.
Chain ramps should be replaced one at a time to ensure that tension is kept in the chain. Figure 5A shows an engine during assembly, and affords a unique view of how the inboard chain ramps are mounted. When removing the bolts that hold the ramps, make sure that you dont lose the ramps inside the engine, or you may have difficulty fishing them out. Some of the bolts for the inboard chain ramps may require removal of the engine mount for access. If this is necessary, support the engine with a floor jack or jack stands before disconnecting the engine mount. There are also two different types of chain ramps use the black ones everywhere except for the slightly different brown ramp which is installed on the lower right.
Removal of the chain tensioner is straightforward. Starting on the right side, simply remove the 13mm hex nut that secures the tensioner, and slide it out. Make sure that you keep tension on the chain. Once the chain tensioner is out, you can also remove the idler pulley if you are planning on upgrading to the later style pulley arms.
If you are using your original idler arms and have an early car, insert the aluminum spacer on the shaft. If you have the late-sytle idler arms, the chain tensioner should fit nicely, as shown in Figure 6. If do you upgrade to the new sprocket arms, you can use the old sprocket wheels. Remove the wheel by pressing out the small expansion pin. Installation onto the new shafts is straightforward, but make sure that the open edge of the sprocket shaft is installed facing up in order to catch oil for the lubrication of the idler sprocket bearing. Make sure that the chain doesnt some off of the cam gear when replacing the idler pulleys. Otherwise, you will have to re-time the cams.
Now, install the new chain tensioner. Do not remove the small retaining pin until the tensioner is installed and secured in place, also shown in Figure 6. Make sure that you remember to install the small, orange o-ring on the tensioner where it feeds through the chain housing, as shown in Figure 7. Wet the o-ring slightly with a bit of clean motor oil before you install it. Retighten the 13mm hex nut, pull the retaining pin, and then attach the new chain housing cover using the new gasket that was included with the kit. Figure 8 shows the new chain tensioner installed right before the pin is about to be pulled. The housing cover will offer a bit of resistance as it fits down over the o-ring on the tensioner.
The left side replacement is similar, and a little bit easier because it is less likely that the chain will lose its tension. Be extra careful securing the chain when performing the replacement on the right side. Figure 9 shows the chain tensioner removed. Figure 10 shows the new chain tensioner installed, and Figure 11 shows the tensioner with the pin pulled.
Make sure that you dont forget to pull the retaining pins out of the chain tensioners. If you do forget, there will be no tension on your chain. When you go to start your engine, the chain will slip off, causing catastrophic damage.
After you have reattached the two chain cover housings, you need to install the new cam tower oil lines. Lay them out on your workbench and make sure that you have all the fittings, gaskets and washers that you need. The lines have both banjo fittings and straight line fittings with an aluminum sealing ring that fits under a hex cap cover. Attachment of the outboard banjo fittings is straightforward. Figure 12 and Figure 13 both show the banjo fitting for the cam towers. Note the use of two sealing rings on both sides of the fitting. Figure 14 and Figure 15 show the banjo fittings that attach to the chain tensioner, on the surface of the housing cover. The inboard oil line fittings are a bit trickier to install. Make sure that you have the inboard straight oil lines placed exactly square into the inboard fittings, and the tapered sealing rings have their small edge pointing downward, as shown in Figure 16. Do not use too much force on any of the fittings they will usually seal perfectly if installed properly. You may need to remove some hoses or other additional equipment in order to obtain the necessary room that you need to tighten all the fittings. Once the lines are tightened, install the small support brackets in place. It's very important to make sure that you install all of the fittings and support brackets that hold the lines together. Failure to do so can result in the lines vibrating excessively and breaking. Figure 17, Figure 18, Figure 19, and Figure 20 all show the proper setup and orientation of the oil line brackets. Figure 21, Figure 22, and Figure 23 all show the proper installation of the brackets for the right side of the engine. Engines from 1967-72 require removal of the two upper studs on the left chain housing cover, and replacement with the two longer studs (M8 x 60).
Once you have replaced and reinstalled your muffler (use new exhaust gaskets), hoses, distributor, and sheet metal, refill the car with oil, start it up, and check for oil leaks. Small leaks around the lines can usually be eliminated with a half turn or so on the fittings. Dont forget to reset the ignition timing when reinstalling the distributor, make sure that the rotor points to the notch in the distributor housing.
Well, there you have it. Although you wont get an extra 25 horsepower from this upgrade, you will get piece of mind. The old style tensioners are almost guaranteed to fail at one time or another, and the upgrade to your car is cheap insurance against engine failure.
This technical article is brought to you by Pelican Parts, which depends upon your business and support in order to maintain this huge website. We appreciate your support and business whenever possible. The Carrera Chain Tensioner Upgrade Kit can be ordered on-line from our on-line catalog. Your support and feedback are appreciated.
To add comments or to read reviews and other users comments about this technical article, click here.
This technical article is one of many that are featured in the upcoming book, 101 Projects for the Porsche 911, due out in Spring 2001.
This information came in from one of our supplers recently:
Proper installation according to Porsche is as follows:
Submerge the tensioner in oil and pull the pin. Pump up the tensioner with your finger until no air comes out of the hole (this is done totally submerged in oil). Once the tensioner is full of oil, it can be checked for leaks, prior to installing on the car.
Porsche says that the procedure of installing the tensioner on the car and turning the car over to pump the tensioner up is 100% incorrect and should NOT be done. From talkng to XXXX Sales and our German contacts, most mechanics use this method, and have been for many years.
Regarldes of how long the "install and pump by cranking" method has been used it is wrong - air pockets in the tensioner can occur an cause uneven pressure, this will appear to be a "weak" tensioner when in fact it is a improper installation.
See this section for a complete list of the parts that you need for this upgrade.
|Want more technical articles like this one, but don't like viewing them on the computer screen? Pick up a copy of Wayne Dempsey's book, 101 Projects for your Porsche 911. See www.101projects.com for more details. Due in bookstores March 2001.|