Timing belt conundrum

I have replaced the timing belts twice before on my car with success and no issues. But this time I seem to be having quite a time keeping all the timing marks in place. Can someone refresh my memory on how to properly adjust the tensioner bearing?

The marks for the camshafts are all lined up after installing the belt. I carefully release the tensioner. I also have tried to carefully rotate the camshaft twice clockwise to allow the tensioner to set up. However, twice now the camshaft marks are off by a belt tooth or so. What am I doing wrong?

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How do you know you are at TDC number 1?

 

I have the PM 1-4 mark lined up on the flywheel.

 

If I recall correctly, the belts are an odd number of teeth - such that, after 2 rotations (setting the tension), the belt tooth that might have been in Position X when you started with perfect cam marks will not be in Position X (after rotations) when the cams are hitting their marks. ....something about it only matches up every 3,768 rotations....

Perhaps someone will chime in and refresh my memory on this.

 

Bring the car back up to PM|1-4 (the mark is the | character).
Bring all four cam shafts back to the marks matching up their respective cam caps
Install the belts with the tensioner locked at the compressed position
make sure the belt section from the crank shaft to the first cam shaft is tight (and the shafts not moving), then release the tensioner.
Turn the engine over 2 or 4 revolutions.
Your crankshaft should be back at PM|1-4 and all cam shafts back up to their marks.

 

If both cams are going out by the same amount and in the same direction, you had too much slack (one too many teeth) in the straight run of belt between the drive sprocket and the cam sprocket. When the tensioner tightened up the belt it turned the cams to remove that slack.

You want to pull that run one tooth tighter.

Birdman’s belt change instructions include marking the drive and cam sprockets and old belt with a Sharpie before removing it. Transfer these marks to the new belts (a sort of fussy tooth-counting exercise) and you’ll know immediately whether you have the right number of teeth between the drive and cam sprockets.

 

This happens because you can't get the slack out of the belt during assembly. To "assemble" properly to a ref point you got to find TDC. pin cam shafts on marks, loosen cogs, pull pins. Tension belt allows cogs to go where they want under proper tension then repin the cogs. Everything else is just a guess on assembly. Then you decide whether to cam time.

 

Yep, that's all that matters. Same belt tooth in same pulley gap and timing will be the same as when the old belts were new to within the tolerance of belt length which comes out to less that 1 degree variation, on any car, Honda or Ferrari.

 

Is unpinning the sprockets necessary if you’re just replacing the belts on an already properly timed motor? You already have them pinned so that they will be where they belong under proper tension so the only variable is getting the right teeth onto the sprockets.

 

The difficulty is in getting the long side in position with tension. The way I do it is, with cams locked and belt clipped to cam sprocket, I turn the crank slightly CCW until the belt drops into the slot on the drive sprocket that I can't quite make by pulling with my fingers. Then return the crank CW back to the TDC mark. This will tension the long run. There is no danger in doing this as both #1 and #4 pistons are at TDC and any movement of the crank pulls both down. None of the pistons in the 5-8 bank are anywhere near TDC in this position and you only need to turn the crank a few degrees to get the tooth aligned.

Hope that makes sense. It's easier to show than tell.

 

Looks like one tooth too little on the tight side of the belt. Loosen tensioner and “roll” an extra tooth anticlockwise over the cam pulleys while keeping the belt on the drive pulleys with a couple of clips.

 

If it could be done that way properly there would be no "ferrari way". The Ferrari way is to assemble and time. People do other ways because they lack knowledge, are in a hurry, willing to take shortcuts, assume consumer acceptability, lack tools or any number of reasons. 99% can't feel a difference between a proper cam time and a dodgy belt swap. Everyone chooses their own level on the DIY continuum. Based on the question asked and problem reported I gave my best answer based on many years of doing this stuff. It is not the only answer but the best I have found and easily transitions to the next level cam timing. There are many ways to achieve a "running" car.

 

“... the belts on an already properly timed motor.”

From my limited experience and hearing from others, few of these motors are properly timed. My four cams were all 4-7 degrees off and the pins had never been moved so were that way from new.

 

No. It's done all the time. As I said above, if you start with a properly timed engine it will remain properly timed to with in the difference of belt length tolerance. The only place timing an engine is mentioned in the 308 workshop manual is when assembling the engine, not when changing belts. The 360 manual has a section on changing belts and they only state to check the timing it's off. The bit about finding TDC does not apply to a 3x8 as it is accurately marked on the fly wheel. Many 3x8s will have the TDC mark painted yellow or white by the tech why serviced the car the first time. The only question is if the engine is properly timed to start.

In certain sections of Ferrari Chat you will see people claim it is necessary to time the engine at every belt change, apparently implying that regardless who timed the engine last, they got it wrong, even if the guy doing the current belt change also did the previous one. That wouldn't give me much confidence that he would get it right this time. It's been a long running argument. Sort of like saying the depth of a hole depends on who measured it, and when it was measure. LOL.

From the 360 manual, the Ferrari way.
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It also implies that the timing regularly changes - at random(?) - between belt changes.

 

And it does. Metals parts change shape and belts stretch so much the pitch changes and they don't mesh with the pulleys and....and....and..... It's all right there is the theory of relativity. You get those cams rotating fast enough there's no telling with the timing is. Its no wonder belts break. The faster they move the shorted the get.

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LOL John... you know what I meant.

 

The method I described is the same way you'd do a chain because you can't stretch a chain over the sprocket tooth so you have to back it off a touch to get it engaged then go back to proper position. It's pretty much the same with a belt but not as extreme.

 

It's hard to get new belts on when all of the marks are lined up, because that only happens when the belts are under proper tension. I always mark the crank hub and sprocket positions before removing the old belts. After loosening the tensioner, I will back the crank up (rotate counter-clockwise) so there is slack on the long run of the belt on the left and push down between the cams so there is a little slack there as well.
The new belts will be shorter than the used (stretched) belts that you are removing, so you need the slack to get the new belts on. You should set the cams with the marks lined up, or slightly advanced (turned counter clockwise) and turn the crank hub slightly counter clockwise in order to get the belts on. Then after tensioning the belts, the marks should line up as before. Right now they appear to be one tooth off.

 

I have replaced timing chains and belts on all types of engines over many years.

I agree that checking for correct timing the first time you undertake this (or have someone else do it) is the proper action. After that...well, it becomes a personal "comfort level" thing. OTOH, it's not a big deal to turn the crank with a breaker bar and put all the marks in the TDC position before starting the procedure!

I have never locked cams in place on any car and see no reason to do so. If you set the engine/cams at TDC, you simply remove the belt and put the new one on, ensuring the cams are still at TDC when the new belt goes on. If they have shifted slightly - which they might - just rotate them back. Marking belts is not necessary. The only thing that is necessary is that the cams are in the proper position when the belt is installed. As noted, there can be a slight "misalignment" of belt teeth/cam sprocket groove. This is totally inconsequential and whatever bit of movement necessary for the belt teeth to drop in place has no effect whatsoever on engine timing.

Shops/dealers routinely install belts by simply marking the cam(s) position when the engine is stopped and pulling replacing the belts, insuring the cams stay as marked. I certainly agree that setting the engine/cams at TDC is a better way because you are actually verifying that the timing is correct as opposed to assuming that it was done correctly the last time - even if YOU did it the last time!

If you are concerned that the factory marks are not accurate, that's a whole different deal, requiring a lot more time/trouble to check and, if necessary, correct. On a full-out competition or blueprinted engine, that would be done as a normal part of the build but on a street motor it would be rather unusual to check it - not a bad thing, of course, but certainly uncommon.

 

It's not about tension or belt stretch. It's about getting cog A in slot A. The picture below is for a 355 but the same applies. Mark the pulleys and the old belts as shown at points a, b, and d. (You don't real need the mark a b.) Transfer the marks to the new belt and install so that a, b, and d on the new belt are in the correct spaces on the pulleys. Tension correctly and the timing must be the same as it was when the old belt was new to within the tolerance for belt length. Based on manufactures tolerances I calculated the maximum possible error, assuming, when new, one belt as short as possible and one was as long as possible. The error came out around +/- 0.75 degrees. Considering that most belts will be well within tolerances, you can expect the error will be less.

As for belt stretch, I cut a new belt and compared it to an old belt that had been on my 308 for 27k miles. The belts were clamped at one end and 50 lbs tension applied to each belt. The length of the belts was carefully measured. The old "stretched" belt was actually shorter than the new one. If I assumed the old belt was originally as short as it could be based on the belt length tolerance specified by the manufacture, the amount of stretch was less than the belt length tolerance.

As for why old belts loose tension, the small stretch has some effect but I have another idea that would contribute to the belt getting loose but it's a little hard to explain w/o visual aids.

Here is the picture. And before you ask about the degree wheel, it's just a picture I found on the net and marked up for illustration purposes. (Remember that group who believes the last guy to time the engine always did it wrong?)

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On a 3x8 it easy to set the engine at TDC because the flywheel is marked. If you don't trust the mark, you can check it. But with later cars, 348, 355, 360, there is not TDC mark and you have to find TDC as indicated in my post 14 or using a piston stop. If you use the dial indicator method (the "Ferrari" method) there is a judgment call on when you have reached TDC and this is a possible source of error.

Locking cam is a look idea because with the engine at TDC one cylinder on the 1-4 bank is on the compression stroke and one is at overlap on the exhaust/intake. If the cam move a little too much there is the possibility of valves contacting the pistons and it doesn't take much to bend a valve ever so slightly, thus having it not seal properly. Not that big a problem with the 3x8 due to the low compression ratio, but with higher compression engines more risk.

Personally, I don't understand why TDC was is used as the reference point for these flat crank engines. If the crank were set to 45* before or after TDC the pistons would all be down the cylinders sufficiently that the cams could be rotated without any interferences. If set to 45* and the cam pulleys and belts are marked as noted in my other post you can freely rotate the cams to get the belts positioned correctly.

 

Guys,

Again, don't confuse those Factory Marks as 'Timing' marks. They are NOT. They can be and are often 'OFF' when the Actual Timing Process is followed, and sometimes wildly OFF.

Rifledriver has posted pics demonstrating this before.

Those 'marks' are 'ASSEMBLY Marks.' They are there merely to help in (re)assembly of the motor so as to insure it is done properly and no valves and pistons collide when in rotation.

IF after Assembly AND the Valve Timing Process has been FULLY and CAREFULLY conducted, and ANY of the cam and cam cap marks align perfectly, that is just Italian Dumb Luck.

'YES' the belts stretch. However, I have read it is somewhere in the range of <2mm over the length of an intentionally cut Isoran/Dayco belt. Other belts? No idea. And take the '<2mm' number with a grain of salt. I have NEVER cut and measured 'New' vs. 'Old.' Point being, the possibility that you 'stretched' a belt somewhere while reassembling is about ZERO.

The distance between the dead centers of two adjacent belt/cog/pulley teeth or valleys is 12°. In the pics above, you may be 'OFF' by 'one,' 'a large fraction of one,' or 'one plus a fraction another' belt teeth as the cam relates to the crank pully and ACTUAL Measured TDC. Check that.

REMEMBER:
12 CRANK degrees = 6 CAM degrees.


Please come back to us with what you find.

 

Mark, you are wasting your breath.

 

"Are the marks good enough or not to assume timing" and "do you need to confirm cam timing with each belt change" are 2 distinct issues.

On the former, I believe the logic is sound that the marks don't show proper timing. On the latter, I have my doubts.