Cam timing tolerances: how close is close enough?

You should be able to get the cam timing +or- one degree. there is no reason to try and get it any closer. The timing belts are going to stretch over time, retarding the timing. Any more than 3 degrees is what I consider unacceptable, and would work to try and get it closer.

Brian Brown
Patrick Ottis Co.

 

I wonder: Did the factory "degree" the cams? I doubt it.

 

I'm not sure why there is any question on this. The WSM tells us what to do. In a nutshell its align the cams to cap marks THEN degree cams to confirm or adjust timing. It is really easy to do this so I don't see why there should be any issue. Anything "less" than the WSM method and you may as well just use nail polish for home made marks, cam locks and just dump the belt on. It will run but it is these kinds of things that are cumulative that make these cars screw up.

 

As an engineer you are correct. There is always a tolerance. The f355 manual specifies +-1 degree. Hope that helps.

 

Surely you jest....?

 

The TR WSM also gives +/- 1 deg as the target, and the resolution of the 308 hole-on-hole pattern adjustment method is 1 deg so that can limit you to only reaching +/- 0.5 deg best case (ignoring the rest of the fuzz).

 

The factory degreed all cams.

Plus or minus one degree is the spec.

The 308QV is just not that sensitive to cam timing but many later cars are and allowing 3 degrees is GOING to cause trouble in many of them. That is just not an acceptable range.

 

"Please, don't call me Shirley".

 

I should have known.

 

When I degreed in my QV, the engine was out so the job was relatively easy, though it did take about 8 hours start to finish (alot was just figuring out stuff). I recall there are several good write-up about the procedures. On a side note, if you are doing this while the engine is in the car, I would like to see how you set up the various instruments while working on the front bank. By any chance, did you take any pictures?

 

That's the best setup I've seen so far!!!

I've fought with various steel plates for my magnetic indicator base, could always get something to work but never really was happy with it.

Before I saw this post I was just about to either just drill a plate to mount on cam cover studs with a hole for a short indicator post, or else order a vice-grip style base


After some searching I found that that's a Starret 665G-3/8 post p/n: 52795 Dial Indicator Clamp. Seems to sell for around $50.

Found a $28 equivalent at McMaster-Carr: #20625A212

BTW, a pin type indicator contact tip with a 90 degree bend will let you pick up the edge of the cam follower cup w/o risking having the rotating cam lobe touch your indicator(been there).

 

Mounting the indicator is always the most exasperating part of the job.

 

Zach,
Which method did you use ...lobe center line or shimming to 0.50 mm ?
I just did this a few weeks ago on my 2V engine with the lobe center line method described by Fatbillybob. However, I wasn't sure when exactly the lobe center occurred. I noted the angle when dial indicator peaked. Then there was "dead band" where the cam turned about 4 degrees without any movement of the indicator. This was followed by rapid movement as the cam continued .
So my question is...where is the correct timing position ? Is it at the onset of maximum valve movement or is somewhere in the middle of the "dead band" ? ( Taking the average of the angular measurements of when the indicator stops moving and when it resumes motion ?
Any thoughts on this ?
Regards,
Mike

 

The problem with this method is that the rate of change (dlift/dangle) is 0 at those transition points so it adds more uncertainty in determining "when" these event occurs (the clearance between the lobe and shim is artificially increased for this same reason when using the "detect first motion of the tappet" method -- to get further away from the zone where the slope of the lift vs angle function is 0) . For the "lobe center" method, it would be better to determine the maximum opening value and then measure the angles on either side of the max opening where the lift is 0.5 mm less than the maximum lift -- the "lobe center" is then the calculated bisector of theses two angles.

 

Things are pretty tight under that front bank, my magnetic holders all have posts that were high enough to bump into something when I tried to mount them on a bridge plate. That's what got me thinking about just drilling & threading the bridge plate for the post. Downside of buying individual holders w/o a set of various post lengths I guess.

Didn't know about the Starrett proprietary threads, not an issue for me as I bought the McMaster-Carr one. It's 4" post seemed about the right length. for this. If not, I'll try using the post from one of my magnetic mounts, or just chuck a piece of 3/8" steel rod in my lathe & thread it.

Another advantage of a bent or 90 degree pin style tip is the tip's offset lets you align the indicator's spindle parellel to the lift axis. A 90 degree pin tip is just a bent point (see picture) with a 90 degree bend.
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That is why the lobe center is a dumb way to do it. Also you need to know where the lobe center is because what you are measuring is the point of max opening, not the lobe center. They are not necessarily the same thing.