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.

 

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.

 

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.

 

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.

 

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.