TR Cam Belt Tooth Profile

Greetings,

I've been stalled for a few weeks trying to figure out what to do about the difference in the timing belt tooth profile I'm seeing on my '86 TR. The new Dayco belt has a tooth profile that's narrow with flat tooth faces. The old belt has teeth that are wider and with a convex face (due to wear?). The picture shows the difference to the sprocket profile and it's not due to sprocket wear. I had the belts looked at by the independent shop that rebuilt my transmission and they confirmed the new belt is the correct belt. The Ferrari parts suppliers have confirmed that it's the right belt for the car and the same as they have been selling to everybody else.

Has anybody else seen this? Has anybody else carefully checked the belt to sprocket fit? Everything looks good at a glance, but with just the right angle, the gap can be seen. The picture shows that even one tooth over, everything looks normal. I'm willing to section the belts and put them on an optical comparator to prove the point, but with the two belts in hand, it's pretty obvious there is a tooth size difference.

Any thoughts would be appreciated.
Thanks.
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Perhaps the difference in belt to cam fit may be intended to increase longevity. The tension on the belt surface is not even. It seems logical that some gap would be needed to compensate for non-linear belt wear and smoothing the belt engagement to cam during rotation as well as reducing damage from contaminants. Hopefully this is the case and that your discovery is not another design issue that we need to worry about.

 

Want an answer,

Somebody point Dave Helms to this thread ........

 

I was hoping to hear something that would help resolve the issue with the tooth profile. If this is what's in all the other cars, I'd be curious if there are any belt problems, like stripped teeth, showing up.

Should I post this in the technical forums for more eyes to see? Is this allowed? Could somebody direct this to the experts?

Any thoughts or help would be appreciated.

 

Never heard of belt faillure with testarossa's

 

The flat tooth belt is the OE design for the TR. The concave tooth belt is something we started to see in the 355's. Short answer, I dunno, but................

I have studied and plotted cam belt wear, patterns and stretch rates for coming on 15 yrs now in an effort to find a method to reduce them all. In these studies we found that the rust and debris collection on the gears was the single biggest contributing factor to belt wear. In an effort to reduce those problems I have been experimenting with developing a special plating processes on the gears. A week ago the first test mule in this research, a 355, came apart and we were able to plot the wear rate and compare this to data collected over the 15 yr time span. The results were quite enlightening and advanced us closer to extending belt change intervals. I bring this up as the 355 tooth design is the concave type and these tests show clearly where the advantages of this design lie in regard to debris collection and dealing with the rust on the gears. The TR's were just put into this test a year and a half ago so we have another 3.5 years wait until I can collect that data. The 308's using the same flat tooth design belt is showing promise but at this point the testing is inconclusive as we are only monitoring it visually and have not measured results. Rest assured, there is a great deal of tooth wear that happens to these belts so that very well could be what you are seeing.

I now have shelves full of the cam and driving gears for a number of different models, including those for a TR, 308 and a 355, 456, 550 that are already processed and plated. With the TR/308's being a flat tooth design and the 355/550 being a concave tooth design, I can compare the tooth ramp angles and design and see if there are differences. I honestly do not know as I only use belts from Ferrari and have not experimented using any different designs or manufacturer, it is not my place to risk experimenting with such a thing on customers cars. As a general observation the later concave design used a deeper tooth with rounded edges on the leading and trailing edges of the tooth to help engagement and pay out of the tooth from the gear. The earlier flat tooth design was shallow with sharp square edges on the leading and trailing edges. My first thought is I would not want a rounded edge, reduced contact area belt tooth in a shallow tooth designed gear, I would want all the surface area possible to hold the loads. As I stated earlier, I have not experimented with exchanging one design for another nor have I spoken with any engineers at belt companies about doing so.... lets just say I fell off Gates Christmas Card list when I gave up on them and built our own fuel and coolant hoses.

You raise an interesting question, one that I dont have an answer for, but one that plays in the area of my research. PM me the part numbers and manufacturer off the belts you have, both old and new. Monday I will pull gears off the shelf of both the flat and concave designs and start some measuring. I have many dozens of old belts saved and labeled from this experiment and we can look at those if need be. I think the answer will be obvious in short order.

 

Bob

I think I have given you some bum information.... that will teach me to check my notes before I answer anything early in the morning from home.

Reviewing photo's of two jobs shows the concave tooth belt coming off a 10 yr old belt change and the same going back on.

No tooth clearance issues were noted at the time and it is something we watch closely on all of these due to our research.
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Initial test mule for the SRI, TR plated gears done a few years back.

I hope this helps answer your questions.
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Hello Dave; I'm curious.....can you please rate the belt condition (1-10)(10 being brand new) of the timing belt that was 10 years old?.....in the pictures it looks like you could put a 100,000.00 miles on it...Thanks, Mark

 

Remember belts deteriorate and in two ways, physical wear and time degredation. Its the time degredation part that is not visible...but never the less is just as critical as wear. Any exposure to chemicals greatly shortens belt life, like cleaner and degreasers, oils, coolant, etc. Just rambling here....sorry...

 

Mark,

Jeff is right. That belt, exposed to what it was... it was done and then some.

If you look at the cross section of the teeth you can see the wear. Wear and stretch = retarded cam timing leading to burned up cats..... It's is not just the belt condition that one needs to be concerned of.

 

Mr. Helms,

Thank you for the pictures and explanation. Your pictures show me that the belt teeth and sprocket teeth look the same as what I have. At least now I know I have the same as everyone else out there, which seems to be working for them. I'll finish timing the cams with the cam belts as is and put it back in the car.

Maybe next time a TR comes in for service, you could put a piece of white paper behind the sprocket and see if you have the gaps.

Thank you again,
Brent

 

I don't think there will be any gaps between the belt and the sprocket. Any of the engine-out jobs I've done....the sprockets and belts have always fit together precisely.

 

I know what you are speaking about with the gaps. Some of the 348 and 355 engines that come out for service show all of 10% of the belt tooth worn away. This leaves gaps approaching .025" on either the leading or trailing edge of the tooth. In the case of the 355/360, they have a tensioner that tightens the belt further from the set point when the engine gets hot and we have no control over the final hot run tension. How many degrees this wear corresponds to in cam timing can be quite alarming and is why I have put so much time into trying to correct this.

My gut feeling is you will see most, if not all of your clearance go away once the belt tension is set and the teeth drawn into the gears. It is always possible that the incorrect belt wore the gear in an odd manner over the years leaving the new belt with a poor fit. The extent of wear on these components when rust is thrown into the equation is quite incredible. Like a river running over a rock, its not fast but it is unrelenting.

The other factor is belt stretch where teeth on the leading edge match up but by the time it gets to the trailing edge they do not. This occurs to an extreme when the belts are over tightened on initial set and once it happens it accelerates tooth wear on the belt.

Case in point:
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Mr. Helms,

Thank you for the additional pictures and information. It's really interesting to see the wear and it's really cool that you're studying this. I wonder how much the factory has watched this issue and what their thoughts are on this. Do gaps like you show lead to teeth shearing off the belt? Have you seen any of this?

My real issue is still that I see the gap with a new belt. The picture is with the new belt installed, tensioned and then turned through several rotations. The engine has 11k documented miles (the car is in near perfect condition) so I don't see it as being sprocket wear producing the gap and I would notice it on the sprocket teeth. From your previous pictures, the sprockets and belts look correct. I believe the gap is due entirely to the new belt.

The gap wouldn't be noticed unless its illuminated from behind and precisely sighted down the tooth. The gap disappears just one tooth over because of the slight angle difference in sighting, and everything looks fine at any other angle. Even a careful look doesn't reveal the gap, it's only by getting everything lined up just right that the gap shows up.

Based on what I've heard and seen, I have the right belts and sprockets.

Thank you again,
Brent
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No, I have seen no catastrophic failures from this situation. When the teeth peal off the belts, it is always on those at the driving gear and most always when the starter is first engaged. With the cams static and the bearing journals dry from sitting, this is the highest rotational resistance the belts will deal with. Every failure I have dealt with is attributed to oil soaked belts or very old and decayed belts.

My studies started with the Motronic engine management cars because those demand the cam timing is spot on and have a very narrow range of tolerance. With the addition of heat activated additional belt tensioning on the later cars, wear has been found to be somewhat accelerated. On the Boxer engine the same is found from engine growth as they heat up, but to a far lesser degree. With the TR's having very stout belt drive gear fences as an individual part rather than the failing, rolled edge drive gears like the later cars have, that are requiring welding...... its just a matter of studying all of the variations to learn from model progression.

The factory gave the tolerance of plus/minus 2 degrees on the TR's for cam timing. If the Cats are expected to live and the model meet emissions requirements, accuracy is a must but is often ignored with lock and swap belt changes. My studies are focused on holding those tolerances for a longer period of time and looking at the possibility of extended service intervals. Can one do a full cam timing one service and a lock and swap the next, thus doing the expensive major every other time with a quick, cheap one in between? Reduce the rust and debris collection on the belts and they wear less. Still a bit from done but I am making progress.... Cool is highly debatable, with each research step requiring a 5 year interval to test..... Boring is more like it but I have a plan and a quest in mind and understanding the dynamics and wear involved is a requirement.

 

Hi Bob,

I just saw your mail. I also installed the Dayco belts. I did not check the difference with the old belts, but don't worry, the Dayco belts are ok.

It doesn't matter whether there is clearance between the belt's teeth and the pulley's teeth. The thing is that the belt is "pulling" on the teeth of the cam-pulley and the crankshaft-pulley is "pushing" the belt's teeth. So any clearance between the "not active" side of the teeth is not relevant.

What is relevant is the contact surface of the belt's teeth. So, the more contact surface the belt's teeth has on the pulley's teeth, the better. A belt with very rounded angles (like your old belt) will have less contact surface, so your new belts are better.

The above is actually not so very important. The thing that is the most important is that the belt is of good quality, so that the inner cords of the belt keep it's structure and that the teeth stay in place. Dayco belts are of good quality.

I hope this answers your question.