timing belt off by half tooth

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bnam2
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Re: timing belt failure

Post by bnam2 » Wed Feb 20, 2019 3:05 am

rridge wrote:
Tue Feb 19, 2019 3:26 pm
My theory on the no-belt re-tensioning rule is that a re-tensioning upsets the wear pattern on the teeth. The tension is never exactly the same twice. Also, unless the belt is marked, the odds of reinstalling it in the opposite direction of rotation are fifty-fifty. And yes some mechanics will damage the belt just in the process of getting it off.
I don't understand the wear pattern part thought -- unless the number of teeth on the belt are an exact even multiple of the number of teeth on the cam gears, each cam gear is likely to come in contact with all teeth on the belt - whether you take it of or not, they are not in contact with just the same teeth. And one side of the belt teeth have more local stress than the other. I think an argument could be made that flipping it around is a good way to increase belt life.
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Re: timing belt off by half tooth

Post by rridge » Wed Feb 20, 2019 8:13 am

There's been a lot of virtual ink spilled on the subject of timing belt reuse over the years with precious little light generated. No reuse is a common automakers recommendation. Can't find a belt maker who make an issue of reuse per se. Their point is that the belt rubber ages and should be replace on time, typically seven years after manufacture, and that belt material ages prematurely when exposed to oil and other fluids. Many calls out prying on installation and removal as a no-no to avoid breaking the belt's tension cords.

Conti makes the point that most of their belts are designed to be run in either direction unless a direction is marked on the belt. Ford says that on some vehicles the timing belt can be reused but only if it is installed in the same direction as its original use and has not reached its expiration date.

There is a timing belt expert named "Will" that works for Gates on Guy Croft's forum. That would be a good place to poke around for more info. Drink deep or taste not the pierian spring.
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Re: timing belt off by half tooth

Post by fiatrn » Wed Feb 20, 2019 7:12 pm

It is fun to do mental gymnastics to figure out when a belt can be used, why, why not, etc. Cool thought experiments!

But at 17$ for a Gates belt, we should all just replace the thing once removed.
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Re: timing belt off by half tooth

Post by atruscott » Wed Feb 20, 2019 10:19 pm

Back on the original question..... if you install the timing belt in the order indicated by FIAT I *think* that the tensioner shouldn’t result in movement between the crank, aux shaft, and cam wheels. If you start with all the pulleys set up right, and then hold the belt tight to the crank, it should neatly snake around the aux, up and over the cams, and then around the tensioner finally.

Yes, there may be a little bit of movement of the wheels to ensure fit, and they might be a little bit off.

Tightening the tensioner should take up the remaining slack, and not move the cam wheels relative to the crank.

Oh, and I always cut the belt as I take it off. Stops me being tempted to reuse it!
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Re: timing belt off by half tooth

Post by jseabolt » Thu Mar 14, 2019 6:09 am

I was thread mining tonight!

I didn't see this mentioned but I was told by a guy who was an actual Fiat mechanic back in the day if you end up being half a tooth off when you slip the belt on, to rotate both camshafts inward a bit until the belt slips on to compensate rather than being half a tooth off on either pulley. That's the way I have always done it because I've always been off by half a tooth! Would half a tooth really make that difference in power?

Here is a good reason why not to reuse a timing belt. This maybe comparing apples to oranges but I learned the hard way when it comes to reusing V-belts. I actually had a V-belt on my Spider stretch and slip off the pulleys. Believe it or not It was still on the car but had turned itself inside out! But was stretched to the point I could not remove the slack. Luckily I was within walking distance of AutoZone. Since then I may take a V-belt off once but after a couple of times, I just get a new one. I don't know if that is why the belt stretched. It was not an old belt.

Not to hijack this thread but while we are on the subject of timing belts, I took my 2003 Subaru in to have the faulty air bag checked. Mine did not have the defective airbag but this did give the dealership an opportunity to tell me what my car needed. Like $2500 worth of work. Pretty much work I have already done. Like changing the fluids. Subaru's computer system probably thinks this car has never even had an oil change because I've never had the dealership do any work on this car. I've done it all myself.

The told me I needed a new battery. The same battery that is on it now and that was 2 years ago! And wiper blades. I just replaced them 6 months prior. New brake pads. I actually pulled the front and rear calipers off one side at 60,000 miles to check pad wear and they were only half worn.

They did tell me I needed a new timing belt simply because the car was 15 years old. At the time the car didn't even have 75,000 miles on it. Subaru recommends changing the timing belt at 112,000 miles. They said it should be changed every 10 years no matter what the mileage.

Does anybody buy that?
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Re: timing belt off by half tooth

Post by zonker » Fri Mar 15, 2019 7:44 pm

My take on timing belt replacement intervals has more to do with age of the belt, my skill set, and cost, both potential and actual. It's never a great idea to question the factory intervals on the belt that the car started it's service life.

But when the belt is the second replacement or higher, I like to exceed the factory interval recommendation, because factors the OEM does not account for come into play, like quality of the aftermarket belt, age of that same belt (just because it's new to you doesn't mean it hasn't grown old in it's box), and quality of the install as well as other factors like tensioner bearing wear or cam guide chafing.

As for being off a half tooth on the cam timing, that will make a performance difference because on a DOHC Fiat, 1 tooth = 8.5 degrees, so a half tooth of extra slack will be retarding your cam timing a little over 4 degrees.

4 degrees is noticeable enough to move the power band a little later, and if I had to armchair guess the results based on my own tuning experience with other dohc belt driven motors and stock camshafts, I'd say you'd be few ft lbs shy of torque on the lower end and a couple hp stronger at the top.

Why this happened I'd guess is previous head and block work that has shortened the distance between the cam and crank, like resurfacing the block or milling the head, or even a thinner head gasket.

To solve this, I'd consider buying a pair of adjustable cam gears so you can dial in the cams relative to the crank. Set the gears at +4 degrees and you should be all fixed up :)
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rridge
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Re: timing belt off by half tooth

Post by rridge » Fri Mar 15, 2019 9:04 pm

James wrote:
They did tell me I needed a new timing belt simply because the car was 15 years old. At the time the car didn't even have 75,000 miles on it. Subaru recommends changing the timing belt at 112,000 miles. They said it should be changed every 10 years no matter what the mileage.
Ten years is generous by Conti's standard. I think fifteen years is pushing it. Also there is the life expectancy of the bearing in the tensioner and the stability of the grease that lubricates it to consider. I'll bet the owner could change it himself.
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Re: timing belt off by half tooth

Post by jseabolt » Sat Mar 16, 2019 9:57 am

rridge wrote:
Fri Mar 15, 2019 9:04 pm
James wrote:
They did tell me I needed a new timing belt simply because the car was 15 years old. At the time the car didn't even have 75,000 miles on it. Subaru recommends changing the timing belt at 112,000 miles. They said it should be changed every 10 years no matter what the mileage.
Ten years is generous by Conti's standard. I think fifteen years is pushing it. Also there is the life expectancy of the bearing in the tensioner and the stability of the grease that lubricates it to consider. I'll bet the owner could change it himself.
I'm going to do it myself. Everything is at the front of the car on Subarus so the belts are easy to get to. The radiator fan may have to be removed but that's no big deal.
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Re: timing belt off by half tooth

Post by Jefco » Sun Mar 17, 2019 7:31 pm

"As for being off a half tooth on the cam timing, that will make a performance difference because on a DOHC Fiat, 1 tooth = 8.5 degrees, so a half tooth of extra slack will be retarding your cam timing a little over 4 degrees.
4 degrees is noticeable enough to move the power band a little later, and if I had to armchair guess the results based on my own tuning experience with other dohc belt driven motors and stock camshafts, I'd say you'd be few ft lbs shy of torque on the lower end and a couple hp stronger at the top.
" Zonker

Wouldn't it be better to be 4 degrees early than 4 degrees late? That would give better low-end torque (where we spend most of our time, like off the line) at slight expense of top-end power.
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Re: timing belt off by half tooth

Post by engineerted » Mon Mar 18, 2019 10:07 am

James, changing the timing belt is fairly easy on the Subi, Did this on my sons car not too long ago. I recommend the kit that Oak's sells. Don't get the blue belt as they have a shorter life than the standard belt for some reason. Do you have a WRX? I assume it is the same for the non Turbo???

https://www.oakos.com/GATE-TCKWP328A.htm
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