Cam timing tolerances: how close is close enough?

I am currently degreeing the cams on my euro 308 QV. For the people that do this all the time, how close do you need to be for you to consider the result acceptable?

First, with regard to the 0.50mm clearance that is specified. Shims are in 0.05mm increments, so it's not always possible to set it to exactly 0.50mm. The method of gap measurement (feeler gauge) is somewhat subjective too. That said, how close to the 0.050mm do you shoot for before moving on?

I assume that if the gap is 0.02mm too tight (0.48mm), that it can be made up for by allowing the tappet to advance by that amount (as measure by dial test indicator) before declaring the 'event" to have occurred and reading the position at the degree wheel. Please correct me if I am wrong.

Then, when adjusting the timing itself, how close to the spec do you try to get? 1 degree? 1/2 degree? 1/4 degree? What is a reasonable target?

I ask because I want to do a good job, but I don't want to waste time achieving unnecessary levels of precision that will have no discernible impact on the end result. Any comments appreciated.

Zach

 

Anyone?

 

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.

 

There is no question regarding the need to do it. The question is what kind of accuracy to strive for. As an engineer, if I design a part with a dimension if 2.03682" and send it to the machine shop to be made I will get thrown out on my ear. Dimension that are less critical might be +- 0.005", critical dimensions might be +- 0.0005". I don't want to waste there time trying to hit a super tight tolerance where it doesn't matter. Do you see my point?

Tolerance comes into everything we do. I don't have enough experience with Ferrari engines to know what a reasonable level of accuracy is when doing this job. That is why I asked the question.

 

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

 

Surely you jest....?

 

That does help. I didn't see that in my 308 manual, but now I'll have to go back and double check. +- 1 degree sounds quite reasonable.

 

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.

 

Thanks for all the info. I checked the rear bank tonight. I checked first with the old belt installed and the timing was retarded about 3 degrees. With the new belt installed it was within a 1/2 degree of spec, so I left it alone.

 

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?

 

I haven't done the front bank yet. The rear bank was easy. I used one of the Starrett push back dial indicator sets. It comes with a clamp that I use to hold onto one of the cam cover studs. It was a bit awkward to get set up, but once in position it worked fine.

For the front bank I think I'm going to make a bridge out of flat iron so I can secure it to a stud on either side. Then I'll use my mag base Noga multi position holder to get the indicator where I need it. That's my plan anyway, we'll see how it goes.

 

Here are the pictures of the set up for the front bank. I ended up using the Starrett set-up just like on the front. It was hard to see the gage, but I was able to borrow the appropriate tool from my daughter's toy box.

The rear bank timing was dead on, but the front bank was found to be retarded ~4 degrees. I spent some time scratching me head, but I was able to figure out the needed pin/pulley/belt combination of movements on paper so I only needed to remove the pulleys and move everything once. It was a little confusing, so I was pleased when it was right on after only one adjustment.
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

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.

 

FYI, the Starrett clamp has proprietary threads so it can't be used in combination with other stuff. It's best to buy the whole set. You can usually score a used one for $50 to $100.

I actually think the bridge plate with an articulared magnetic holder and an AGD standard indicator with the proper tip would be the best set-up. I wanted to do it that way, but I didn't want to loose the weekend waiting for the stuff so I went ahead.

The main down side of the set-up I used is that the indicator stem isn't parallel to the valve movement. It doesn't really matter since I am only using it to detect movement not measure, but with a proper set-up that is aligned you could avoid re-shimming to get the .50mm gap.

 

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.

 

I set clearance to .50mm. If you want to use the loab peak method don't guess the top of the lobe. If I were going to do it I would zero the indicator at the max point, then back the crank up. Moving forward again, stop at .01" before zero and record the reading on the degree wheel then continue to zero then to .01" past zero and read the wheel again. Then split the difference of the reads to get the lobe center.

This is more or less the same approach as using a piston stop to find TDC. You can try to do it with an indicator, but at the stransition point the rate of change becomes so small that the true peak is not detectable and you end up guessing.

Easier to shim to the proper gap if you ask me.

 

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.
Image Unavailable, Please Login

 

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.