Cam Timing - Success, observations

[notmytarga]

I made my umpteenth trip to the garage to set the L cam timing as I reassemble my rebuild. I'm working with the stock cam on my 1975 911S so I used several references to verify 0.4-0.54 mm of overlap. (Wouldn't a typo here be horrible??) This worked out to 0.0185 +/- 0.0028 inches on my non-metric dial indicator. I have them set to 0.0179 and 0.0178 finally. Slightly 'retarded' but matching at midrange.

I found that the indexing pin does little to fix the timing at a desired point. It DOES fix it to a range - one that matches the spec books. I was curious whether the various pin/sprocket/toothed sprocket flange rotations would have varying amounts of play. I measured all the combinations, used an Excel graph to find a smooth increase in the size of the range as the rocker rode the cam further up. So I believe my parts are not worn out. Then again, the pin could be the culprit it I'm wrong on this range thing. It fits tightly and doesn't look worn. Do new parts have this range??

As many have described in other threads, cranking down with the 46 mm crowfoot will change the setting. Inevitably. Everytime. With only variable consistency.

In case you haven't spent hours thinking about this, here is what I figured out. If the pin only holds the timing in a range, then the spot-on timing you desire is dependent on the position and friction between the sprocket flange (indexed to cam tightly with woodruff key) and the chain sprocket. Torquing the nut while holding the cam only indirectly stabilizes this connection. What would be best is to have a way to hold the sprocket while tightening the nut. Oh wait - that is what the updated cams use! Lucky them.

Others have recommended locking the flywheel and using the crank/Int shaft/chain to hold the sprocket. This is a lot better at getting the sprocket to not move as you tighten but at 110 flt lbs applied to a 1.75 inch lever arm that gets you 750 lbs of tension on the chain which I would avoid.

I followed the advice given in other threads to lubricate the camshaft threads and washer surfaces with oil. Make sure the washer is within the depression in the sprocket. Tighten the nut as much as you comfortably can keeping a setting at a known value. NOW - rotate the crank so that the cam holding tool is held by the case somewhere and then concentrate on safely getting 110 ft lbs on the cam nut. Once you have the nut tight, bring the engine around and see how you did. Stop and check your zero. Which way did it drift (it will always want to go one way) and by how much? - this is the "cam nut drift" referenced ......only here. If it was right on your final target you did it wrong and you need more practice. Back the nut off and set yourself up again taking into account the "cam nut drift". Tighten up the nut just as firmly as before and then lock the tool against the case and go to 110 ft lbs again. Ideally you should make stepwise progress towards your goal of the timing range midpoint or what you eventually decide is close enough.

Of note is that I think it would be a mistake to set the pin where your intended timing value is at the end of and not the middle of the range. This may stop it from drifting as you tighten, but if the setting drifts during engine operation it could drift out of the range. I'm curious how well the friction interface really holds these values. I'll be checking my timing with every valve adjustment.

Another observation - when setting my chain alignment I measured my 8 shims at 0.5 mm rather than 0.25 mm. This allowed me to get to the other side of the 0.25 mm tolerance but not spot on. I also found that the thrust washers had slightly different thicknesses at 2.9 and 3.0 mm. I was able to swap them for better measurements.

[2.7RACER]

When the cam timing fails to drop into the mid point of the specified range, Simply pull the pin, slip the sprocket off the cam and rotate the cam sprocket 5 teeth in either direction.
This will re-index the sprocket and allow it be pinned in the center of the specified range.
Setting up cam timing to the retarded end or just outside of the retarded end of the cam timing spec's may work ok, but it isn't correct.
Further as the timing chain wears, the cam timing will be further retarded from the initial setting.
For this reason I set my cams slightly towards the advanced position of the cam timing specifications.
I don't see how '75 cams will slip. The semi-sprocket is keyed to the cam shaft. It should be a snug fit.
The pin through the cam sprocket into the semi-sprocket has very small, almost negligable tolerance.
Tightening the 46mm nut will move the timing very slightly, almost negligable.
Of course, a combination of worn woodruff key, in a worn semi-sprocket, with an under size or worn pin, and of course a worn out timing chain, may show the symptoms you have described.
Try rotating the cam sprocket 5 teeth. You'll find it is now possible to time the cam at the mid point of the specification. A few teeth more or less will get you exactly where you want to be.
10 teeth will put the cam back where you began.
This has been covered in the past. Maybe a year or more ago.
Good luck,

[notmytarga]

My rebuild process has allowed me to spend time at work looking for information like this as I slowly proceed. I didn't see the rotating sprocket suggestion when investigating this step. I didn't measure that out but it makes sense. In order to rotate the sprocket one would need to release the mechanical tensioner and rotate the sprocket under (relative to) the chain - right?

At 0.0178-9 inch, I am almost smack-dab in the middle of the range - just to the retarded side of the Midpoint, not the range. (Midpoint would be 0.0185 in, Range: 0.0157-0.0212 in)

You have helped direct me toward what is likely the culprit - a worn pin. If I am getting increasing ranges as I advance the cam around the sprocket and that is not supposed to be the case - then the pin would be the problem. Looks like I'll break it loose again and measure the pin - anyone have a specification for the pin?

I thought the range made sense because of the recommendations in the books to carefully hold the camshaft to maintain the setting. If the pin indexes it firmly then maintaining the setting during tightening would be a formality.

[ChrisBennet]

Instead of thinking of the holes and pins as an absolute locating mechanism, you can think of the holes as really short slots that allow you fine adjustment. I twist them one way or the other and fix them with a burp of the impact gun i.e. use the "slop" to adjust them in the direction you want.
-Chris

[2.7RACER]

Somehow, I got the mistaken impression you were setting the cams more retarded.
To answer your first question, yes, remove the sprocket after releasing the tension.
Then jump the chain.
Since it already is relatively close I would jump the chain 2 teeth, then check the results.
It will get closer or further away. If you find it is not closer, jump the chain 4 teeth the opposite direction.
You should try to advance or retard the cams by using whatever slop you find in the fit of the pin. I would be surprised if it amounts too nearly as much as has been suggested.
The pin to semi sprocket would only wear if the 46mm nut wasn't properly tightened.
I'm confident you can get the spec you desire by jumping one or two teeth until you get exactly what you want.
As you can see jumping 10 teeth returns the sprocket to the original position.
So you essentially divide the difference of moving the pin from one hole to the next by ten.
This is where the fine tuning comes from.
Pick the exact specification you want and you can get it exact or within a few thousandths.
No bull.

[notmytarga]

I went out to check the alignments - they are good - in preparation for sealing up. I checked the right timing and it got more retarded (I love that word) - reading 0.0170 now.

Time for another go. I'm going to use the advice from 2.7Racer and shoot for a final set at 0.019 inch with a pin 'slop' (another good word) range from 0.017 to 0.22 ish - hopefully this will prevent it from getting more retarded with any chain stretch, slipage etc. It'll have to wait for tomorrow.

While I have it apart I'll measure my pins.

[notmytarga]

My pins measured at 5.95-6.00 mm with the same measurement along the axis as multiple checks of the diameter. I doubt they are worn.

I found that my slop was a lot less when I set both sides at 0.009 inch rather than the 0.019 inch that I was shooting for. I was proud for a while until I discovered this mistake - very retarded!

So I systematically set about getting the 0.019 inches with symetrical pin slop around this value. Within a few chain hops, pin changes and torque downs I was at 0.019 ish on both sides.

I took the C-clamp off the left and mechanical tensioner off the right and set up the oil-fed tensioners (not new - about 5K miles). I rechecked the timing and the right was now reading 0.021 inch, which is the advanced end of the range. The gauge was zeroed, and Z1 was at the centerline. I got called to work and didn't have time to check the left.

Will this advance reduce with oil pressure? Is this normal?

[dtw]

I really feel that the pins are just an assembly aid for spinning the motor during top-end build-up. They shouldn't be a significant emphasis during the timing process. Also, keep in mind, that whatever the slop is at the pin, will cause timing measurements to vary depending on different cams, or even various positions on the same cam. Good old high school algebra, rise over run, y = f(x), and a higher coefficient (ie, more aggressive cam) will translate into a wider range of 'slop' in the pin. But honestly, I'm still an amateur - but I've timed a LOT of 911 engines with complete disregard for what the pin was doing. Stop emphasizing it. Focus on what the timing should be. With the proper touch, you shouldn't have significant timing change when torquing the cams. Alternately, if you practice enough that you realize you are getting .05 of movement when torquing, just factor that into your initial setting.

After several years of watching this forum, one thing I can take away is that people almost always tend to over-think cam timing. It is what it is. Pins are for maintaining rough-time while you build-up. Understand what the cam is doing vs. the crank when you set time. Obtain the lift you want, torque it, clear it, and forget it. It isn't rocket science.

Sorry if this comes off as a rant.

[otto in norway]

What is the overlap setting for a 964 cam. (Used on a 3 litre CIS) -Prefer mm...