Camshaft Timing Chain Tensioner Service, and More Stripped Threads

One of the first things I did when I bought Gretchen was inspect the timing chain and wear pads on the tensioner. The car's documentation told me the pads were only a few years old, and upon inspection they showed very little wear.

I probably could have left those pads in for another few years, but as I already had the new parts, and the cam cover gasket was leaking, I decided to replace them after the first month or two of driving.

 

Chain Tensioner Service

When I removed the cam cover to inspect the timing chain pads I saw this, and I was shocked! From that moment on, I've been absolutely 100% sold on synthetic oil, which I had never used in any of my engines before. This head has seen approximately 20,000 miles of service since it was rebuilt and installed 9 years and 8 months prior to this picture being taken. Only synthetic has ever been used in this engine, according to service records going back to 2006. Except for the discoloration on the chain tensioner pads, everything still looks factory-fresh. Amazing!

 

Close-up of the old timing chain pads and the absolutely spotless engine.

 

Exhaust cam on the left, intake on the right. The tensioner can be seen between the cam sprockets, and uses an internal spring along with pressurized oil to maintain tension on the timing chain.

 

Pictured here are the tensioner's oil supply line, mounting bolts, oil supply banjo bolts, lower pad mounting screws, and the "special tool" (I say that jokingly), made from a short piece of coat hanger wire. It's used to hold the tensioner's upper pad in a retracted position to allow removal of the tensioner. The S-shaped oil line is the original version; later units that appeared on S2 and 968 cars are J-shaped.

 

This is the cast iron chain tensioner sitting on the bench, with the upper pad removed. The oil port is visible on the right side. The upper side of the tensioner pushes up on (tensions) the chain, as that is where the chain slack is. The lower side of the chain is the driven side, that is, driven by the exhaust cam to turn the intake cam. The exhaust cam is driven by the timing belt and adjustable cam timing pulley behind the distributor.

 

Here is an end view of the upper pad. Although the picture is blurry, you can see that there is hardly any chain wear at all on the surface of the pad. I could only just barely catch a thumbnail on the double row of shallow grooves. This is about 20,000 miles of wear.

 

Compressing the tensioner in the vise to install the special retainer tool (the coat hanger wire). I quickly loosened the vice once I realized that the round dowel was beginninig to put a flat spot in the Delrin (or Teflon, or whatever it is). Note the piece of aluminum angle on the other vise jaw to protect the lower pad.

 

This worked out much better, as the flat piece of wood spread the load over more surface area. The internal spring is surprisingly strong; with an additional few bar of oil pressure behind it, this assembly would be rather solid and very tight on the chain.

 

You can see how the special tool holds the upper shoe of the tensioner in a retracted position so you can get it under the chain to install it. Once the unit is installed, the upper pad is kept from sliding off the upper shoe by the two tabs that the special tool is inserted into. The pad can only be removed once the upper shoe raises above the tabs, and while it's installed in the head under the chain, it can't do that. Note the machined surface of the oil port; this is necessary for the crush washer to seal against.

 

The tensioner ready to install. The ends of the special tool have been carefully rounded to avoid scratching the pad's surface when it is extracted.

 

The tensioner is installed to the head and is ready to have the special tool removed. I made another special tool (not shown) to depress the tensioner without damaging the new pad. It hooks under the intake camshaft and uses leverage to press down on the pad in between the retainer tabs.

 

Here you can see how once the special retainer tool is removed, the upper pad is prevented from coming off the upper shoe by the two retainer tabs. The oil feed line is installed but not torqued yet. You have to be very careful not to drop a copper washer or a bolt down the oil return passage just below the tensioner. All in all this job is very simple and straightforward, if you're meticulous and use the correct tools. Everything is tightened with a 1/4 inch drive metric torque wrench according to Porsche torque specifications.

 

Cam Timing Adjustment

While I was in here, and since I had the necessary tools, I decided to make sure the cam timing was set up properly. The adjustment range of the cam pulley is fairly small, probably 4-6 degrees maximum advance or retard, and it's not something that will make your engine run "better" or "worse", or fix a drivability problem. Cam indexing in this range is just a technique to move the engine's power band up or down, rpm-wise, and following the factory service manual procedure puts the power band where the Porsche engineers decided it should be. Generally speaking, advancing the cam timing shifts the power band lower in rpm, while retarding it moves it higher. Street driving usually benefits from a slightly advanced setting. For now, I just wanted to set it to the factory spec.

 

I attached a piece of flat bar across two cam cover bolt holes to serve as  a solid mounting point for the magnetic dial indicator base. The bolts are M6 x 1.25, if I remember correctly.

 

You need to start at top dead center to do this. Because I can't read the mark on the top position of my flywheel, I have to get under the car and use the lower mark to set TDC. A better way is to dial indicate the top of the #1 piston crown through the spark plug hole, but you need a long extension for your dial indicator, which I didn't have.

 

You have to find a spot on either one of the #1 intake lifters and vertically position a dial indicator there in order to measure the valve lift. It must be vertical, or your measurement won't be accurate. Even though these lifters are huge, it's really hard to find a good spot. I ended up using a 45 degree angled extension probe on my indicator and placed it just to the left of the left-hand cam lobe. It was extremely tight, because the camshaft can't touch the indicator in any way, only the lifter can be in contact with the indicator tip.

 

The dial indicator is set up and zero'd. At this point, I got wrapped up performing the procedure and forgot to take any more pictures! 

Basically, what you are attempting to do is adjust the cam timing pulley so that when the crankshaft is at TDC (on #1), the intake valve is already at 1.4 mm of lift. You do this by loosening the cam pulley adjustment bolts, which allows the pulley to move independent of the camshaft. You then turn the crankshaft backwards slightly,  which also moves the pulley, but not the cam, and this results in the cam position being advanced a bit relative to the crank position. Now you tighten the cam pulley adjustment bolts, locking the cam to its pulley again, and advance the crank until you get 1.4mm of valve lift. At this point, loosen the bolts again, then turn the crank back to TDC (meanwhile the cam doesn't move), then lock the bolts down. Now, the cam is sitting at 1.4 mm of lift and the crank is back at TDC. Theoretically, you're done, but in my case it took a few tries to get the adjustment right on the mark. Rotate the engine a few times and re-check the lift at TDC. If it stays at 1.4 mm, you're done. If not, go to step 1...

 

Oil Pressure Reducing Valve service

I thought it would also be a good opportunity to inspect the oil pressure reducing valve and replace the o-rings. This turned out to be a good move, because I fished a nasty piece of old gasket or RTV out of the oil gallery below the valve. Sorry, I didn't take a pic of that or of the removed valve. The valve is the round thing with the small hole in the center, just to the left of the #4 left hand exhaust lobe. It lowers the oil pressure to the valvetrain, and therefore the oil flow rate to the valvetrain, so that there is more oil available down at the main bearings. The manual doesn't give any information about how much it reduces the pressure, or whether or not it also acts as an anti-drainback (check) valve as in the other 944 heads, but on inspection it appears that it does indeed act as a check valve as well. This maintains oil in the head at all times to reduce cold start wear on the cams and lifters.

 

You can see the top of the valve from the side here. Just visible is the groove in the body where it meets the head. This is what you pry up on gently to remove it. Once the valve is out you clean it with solvent and replace the two o-rings that are mounted on it.

 

This set screw must be removed before you try to extract the reducing valve. When you put the valve back in, put a good coating of Loctite 222 on the screw to seal the threads or it will leak. Don't use anything stronger or you may never get it out again without destroying it. You can easily damage the valve if you jam this screw in there. Screw it in gently until it just bottoms on the valve, then unscrew 1/8-1/4 turn.

 

Stripped Cam Cover Thread Repair

I finished everything I planned to do inside the cam cover, and while carefully tightening the cover bolts, this hole stripped out. Bummer. I've never seen another 944S head up close, so I don't know for sure, but to me these holes sure look like they have Time-Serts installed. Interesting. Did they come from the factory like that, or was this done by a machine shop when the head was rebuilt 9 years ago?

 

I thought long and hard about how to do this for about a day. I didn't want to pull and disassemble the head for this one hole, but I also didn't want metal shavings getting in here. I came up with a plan to use window sealing putty to form a large dam that I could work in, and drill very slowly and carefully, with grease on the drill bit.

 

This is after drilling and vacuuming away the chips. The putty was pretty good at releasing them; you can see only a few sticking in there at the bottom left.

 

Much like when I drilled, tapped, and Heli-Coiled the intake holes a few posts back, I used the coupling nut tap guide trick to tap the hole for the Heli-Coil STI perpendicular to the gasket surface.

 

Here's a shot of the tapping process from above. From here, this is exactly the same as the manifold repair. This time I didn't need to use specially coated STI's ; the normal stainless steel ones are fine in aluminum (or was it a steel Time-Sert? I still have no idea). You can see it was a very clean operation and no chips got inside the head. The putty cleaned away easily afterwards and when it came time to break off the tang at the bottom of the Heli-Coil, I just vacuumed it out of the hole.

The cam cover is back in place and torqued down, although I don't tighten these bolts to 10 Nm as the manual specifies, I take them only to about 5 Nm, which is still quite tight. Yes, this cam cover looks like hell, courtesy of someone painting over the magnesium some years before without having a clue. The cover, distributor housing, and intake manifold will all be stripped and refinished properly in a future post.

Stay tuned for more mechanical adventures with Gretchen! I'm still trying to figure out a way for people to sign up to be notified when a new post comes out. Hopefully by then I'll have it figured out.