Checking cam timing, here's why

I've read a lot of different opinions about degreeing the cams when changing the timing belts. I heard a number of guys say that as long as the factory marks on the cams line up with the marks on the cam caps, the cam timing will be "in the ball park" and close enough for a belt change. Paul (pad) and I are in the process of doing the belts and timing on my '78 308. We changed the belts and tensioners, valve lash was set to factory spec, and as an initial check, the factory marks on the cams all lined up with the cam caps. I'm sure there are many who would have stopped at that point and buttoned the engine up.

Instead, we degreed the cams, checking them to maximum open against the workshop manual specs. Well, when checked against the timing wheel and with the depth gauge, three of the four cams were off by as much as 20 degrees. It took us a little over 3 hours to check the cams, make the necessary changes, and bring all 4 cams to within 1 degree of spec.

Now here is the really interesting part. After setting everything up and double and triple checking it all, we looked closed at the cams. On the three cams that were out of spec, all three had secondary scratch marks, smaller than the factory marks, that exactly match the correct cam positions at TDC. It was easy to ignore those small scratches when we were focused on the deep factory marks. But those smaller scratches showed the accurate timing for those three cams.

So, seems to me the lesson here is, just because the engine seems to run okay, simply changing the belts may not be the best answer, unless you are 100% sure the cam timing is spot on. I was having a problem with the front bank running hot. Turns out, the exhaust cams were opening over 20 deg. early! Hopefully, setting the cams to spec timing will eliminate that problem and make the engine run better. With the engine out, taking the time to adjust the cams to spec was well worth the effort. Hopefully, it will prove out when the engine is back in and the car is running again.

 

Not to mess you up, but did you check indicated TDC on the flywheel marks with a cylinder stop? Many, myself included have found the marks to be off several degrees. Mine was indicating TDC when it was actually 3 degrees retarded. The slots in the indicator were not enough to slide it over, so I made marks to set it by.

 

Steve, I wonder if all 308's has that secondary scratch mark like yours?

 

Here's what I wonder....how good is the machining from lobe to lobe? Meaning, if you degree it perfectly on cylinder #1, what happens on #4?

 

Yes, we did verify TDC, and the flywheel markings were accurate.

I really don't know. It appears that these secondary markings were added later, but we really can't know for sure. What is clear is that they were purposefully made marks, not scratches from handling the cams (nice straight lines running across the rim, and they line up perfectly with the cam cap marks on the three cams where the factory marks were off.

Bird,

Wish I knew the answer to that. That, I guess, for now, I just have to take on faith. Maybe next time I'll send the cams out and have them reground and all lobes verified. For now, I guess #1 and #5 will have to do.

Regards,
Steve

p.s. I'll see if I can get Paul to post a couple of pictures of the cams showing the secondary marks, or I'll take a couple myself before I put the cam covers back on.

 

That would be cool, thanks.

 

They are quite accurate.

This is all what Dave and I have been saying for some time. The marks are intended to be assembly marks, not timing marks. Sometimes they are on sometimes they are not.

 

Seems to me a past mechanic was lazy! My question is that since you now know your cams are in spec, do you have to recheck them next belt change? Can you just lock it down, swap belts and know that it's still correct as long as nothing moved?

Ken

 

Here is a picture of Steve's front exhaust cam. The red line on the cam shows the original timing mark, the yellow line shows the partial marking we found. In all, 3 of the 4 cams had second marks, however, they are not as pronounced as the original marks. Perhaps, they were replacement cams, but there is no history with the car that suggests this. (Note: I placed the color marks via an editor).

-Paul
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Ken,

When we originally opened the covers up and spun the engine, brought it to TDC, the factory marks were dead on with the caps. What we now suspect is that at some point, the cams were degreed and given the secondary marks, but at a subsequent change, someone simply set the cams back to their factory marks thinking those were the correct valve timing marks. Paul and I were both concerned about being "off" as we were, until we spotted those secondary marks on the cams.

I agree with Brian. The factory marks are assembly marks, to get the cams in the general vicinity of correct. The only way to be sure is to degree them. I don't know if I would simply lock down the cams the next time and change the belts without popping the cam covers off and making sure that the marks we know to be correct were lining up properly. And if I had no hard evidence or first hand experience that they had been degreed the last time, I would not simply do a belt swap, even if the engine seemed to be running pretty well.

BTW, the exhaust cam on the front back was opening 20 deg. early. That may explain why I was having a problem with the front bank (5-8) temp. light coming on. The picture Paul posted is the front exhaust cam. See how close the lines are? That's how easy it is to be off by 20 degrees. Even a slight misalignment can have you off by 4, 5, 6 degrees.

 

Steve- great thread! Any more pictures of the cam timing process?

How difficult would it be if the engine was still in the car? I'm wondering about the maximun size for a degree wheel, and the easiest way to mount it to the crank.

thanks again for posting-
jwise

 

Now that it is right belts can be changed with very little possible change.

What happens is the cars get serviced and disassembled by the unknowing and just reassembled to the marks.


The vast majority of the cars we get with slow down lights and damaged, overheated cats are for that reason. The 2 valve cars were not very sensitive. The 4 valve cars more so and got even more sensitive with time as they were developed. The 5 valve motors were very sensitive. Taking a properly tuned TR and changing cam timing 2 degrees can cause overheat lights. Changing it 4 degrees is enough to cause cat destruction.

 

We probably should have taken a few pictures of the actual process, but we didn't. I think I have a couple of pictures showing the degree wheel mounted on the crank. In this case, we were lucky that Paul's engine is out of his car also, as we were able to use his crank pulley, which has three threaded studs on it holding his crank sensor wheel for his Electromotive setup. Swapping pulleys to mine made it easy. Otherwise we thought about using some double sided tape to hold it in place. Paul fashioned a pointer from some heavy coat hanger wire and we attached it to the alternator mounting bracket bolts on the front side of the engine case.

I could see that doing this job with the engine in the car would be a real PITA, especially for the front bank. You really need to be able to look straight into the cam gears to get the pin alignment set up, which would be impossible in the car (or you'd end up using mirrors), not to mention working with the depth gauge on the front bank. Since I wanted to change hoses and do a bunch of other work anyway, pulling the engine made the most sense.

 

What did you use as a depth guage to find TDC?

I heard they make a tool just for this reason, but I used a modified dial caliper when I did my 348 last year.

 

Hey Brian, you should take the time out of your busy day and do a write up on the degree wheel method of belt timing, eh? ...........

 

To find / verify TDC, we used a piston stop inserted in #1 spark plug hole, attached a timing wheel to the crank, and rotated the crank in both positions and noted the where the piston connected with the stop. The mid point between the 2 measurements gave us TDC. On Steve's car, this corresponded exactly with the mark on the flywheel next to PM1-4. I believe the piston stop came from Summit Racing - part # SUM-900189.

 

Thanks for confirming what we suspected, Brian. From the time I got the car, the front bank slow down light kept coming on. We attributed it to the carbs (too lean or too rich), the CATS being defective, bad tune, timing off, etc. Had not really thought about it being the cam timing. Now that we saw how far off it was, it makes perfect sense. Will be interesting to see how it runs with the cams in the right spot.