Camshaft timing Part I

From pcarworkshop

In this article I will be installing the cam chains and running through the process of timing the engine.

First of all during my last article I screwed up and now had to remove the camtowers again. I had glued them down to the heads because I forgot the air baffles. Here is a shot of the seperation...you can see how well the threebond adheres and forms a gasket. Nice seal...too bad I took it all apart and redid it!

There are four chain ramps in the 993. They guide the timing chains and provide a place for the chain tensioners to push in order to keep the chain relatively tight does it does not skip. Here is the long one...it has a pivot and a pushing surface.

Just before the ramps and chain housings are put on...

In goes a nicely prepped cam...I used MPZ breakin grease on just about everything...its wierd stuff in that it very sticky but slides nice..the cams go in nice and easy. If they bind then its possible the heads are at different hieights and the cam tower has twisted.....mine were in good shape and turned smoothly. The bearings for the cam are directly on the aluminum!

Now that the chain covers are on and the cam is in the first thing is the thrust plate. This plate holds the cam from moving back and forth, its sealed with a nice big fat Viton donut.

I am inserting the thrust washer next. All the parts from now on are part of the 964 backdating of the timing gear in order to use the dial guages to time the engine as well as I like the positive lock...more on that later..

Ok, spacing. Its essential for long life of the cam chains that they be aligned with the sprockets on the layshaft in the middle of the engine. The tolerances are pretty tight so carefull measurements are taken. These shims are used to pushout the cam sprockets.

Next is the index gear. This is a funny beast, its locked to the camshaft via a woodruf key and it provides a place for the round pin to lock against to lock the sprocket to the cam... these are used ones out of an old 911 that I picked up at the junkyard!

Checking the alignment of the sprockets. You measure the offset to the cam sprockets and the main layshaft and calculate the offsets. Mine were off and I needed a 3/4 combination of spacers...exactly like the older 911's so I guess my case is very typical and correct. Luckily I picked up a set of shims at the wrecker a few months ago, these parts do not wear.

In order to calculate if the sprockets are aligned you need to measure the offset to each cam sprocket, the offset to the front of the layshaft, and use the supplied offset of the front of the layshaft to the sprockets, do some math and go from there. The trick is that you are measuring thousands of an inch so the measurements have to be square and you need reasonable measuring tools...

Finally the sprocket goes on. The number of holes in the sprocket and the number of notches in the inner gear are different so no matter what position the gear is in one hole should be very close to alignment.

You can see the shims as well as the woodruf key that holds the cam assembly in place. This is why RUF, when they update a 993, throw away all the sprocket assemblies and go back to this system.

Than then putting in the tiny locking pin. Remember most 993's do not have this, its just a single machined sprocket, no keys or pins, and the sprocket is locked to the cam via the pressure of the locking nut.

Here are both chain ramps and I am ready to put on the main sprocket. Notice the holder that I am tightening up...it provides the pivots for one of the chain ramps.

Here is my high tech chain tensioning tool.

Links to Rebuild Tour

1. Getting started, dropping the engine and superficial stripdown
2. Checking the default timing and Cams
3. Looking at the heads and the SAI passages
4. Valve guides, valves and carbon
5. Further Dissassembly
6. Exploring the SAI passages
7. Cleaning and getting ready to put it all together again
8. Rods and last minute checks before putting the case together
9. Sealing the case (or tomb)
10. Putting the pistons/cylinders back
11. Camshaft timing Part I
12. Camshaft timing Part II
13. Messing up a clean looking engine
14. Not quite there
15. Cranking it up