Re-Tensioning The Cam Belt

$60 bucks for new belts or thousands of dollars if something goes wrong? Spend the dollars for peace of mind & use the old belts for wall decoration.

 

In the context of "American Iron", worrying about the belts is silly. Of course, an F-car isn't that, but it still seems silly to worry about it. I could write much about why it doesn't make any sense to worry about it, and then I'd replace the belts anyway!
Also, check the tensioner bearings AND the drive pulley outer bearings. These, I believe, are MUCH more likely to give you trouble.
In summery: Replace the belts.

 

+2

 

Those belts are just fine. Leave them as is and put the rest of it back together and go have some driving fun.

Anyone can say "oh, you need to change those and it's not worth the risk" etc. but frankly that's not a very sophisticated technical assessment in many cases. If the belts have little to no use, are not old, haven't been abused in some way, and are properly installed then they're good.

Belts breaking or skipping teeth is as much about other factors - oil leaks/contamination, oil starvation to cams, improper tension, bearing failure, etc. - as it is the actual condition of the belts. It's worth studying these other potential causes of failure as it is obsessing over the age/condition of the belts. Just my $.02.

That said, I'd certainly pay attention to what Verrel, Rifledriver, DHelms, etc. suggest as these guys know what they're talking about.

 

I'd trust to luck that they will survive 8-9 months and two hours running time just fine!

If it was me, I'd plan on changing the belts/bearings during the winter of 2013-14.

 

If your taking them off, I guess I don't understand the question. Think of it like a sanitary napkin and ask the question again to see if it makes any more sense.

 

I am agreeing with Paul's answer(also not sure I understand the ?). Are you taking them off?, if yes then Paul's 2nd sentence applies. I don't see why you need to if you are changing the coolant hoses. OTOH I would be concerned about the bearings you said it had been 14 years since the valves were adjusted are the bearings that old? If they are they should have been changed at the time you did the belts.

 

I don't know squat so my free advice is worth half of what you are paying for it.

I'd replace the belts IF you loosen the tension for ANY reason.

It is the act of tensioning them that puts a "set" in them. Once that is done it cannot be undone or redone.

I know it seems silly but there is allot of physics going on here, some of which I'd bet even GATES doesn't fully understand (hoses anyone?) and the potential for damage is just too great.

Lementing, If they were NOT interference engines ..................... we'd never have a cam belt thread.

 

Precisely....and for the little dollars that these belts cost, why take a chance?

 

on a related note i just got some dayco 308 belts from ferrari and they dont have Ferrari stamped on them, they also say made in the ECU or something. Should i try to get "real" ferrari belts. Im serious, they were cheap and i only want to do this once!

 

Glassman- Brian Crall and Dave Helms both say if the cams have been degreed in the relatively recent past and the cam covers are not leaking, just swapping the belts without degreeing the cams is ok.

I agree the belts should not be retensioned. If necessary, time for tensioner bearing and belt replacement.

Taz
Terry Phillips

 

In normal running the cam belts are not under constant tension. They are constantly being loaded and unloaded in different sections of the belt, especially at lower engine speeds. (In fact, I believe if you go back through Helms' posts you'll find that he mentions that when giving the belts the sonic tests that the engine must be at some precise position for repeatability.) I would (and have) reuse them.

BUT it is your engine. Belts are cheap. If this will keep you up at night, or at all hamper your enjoyment of the car, by all means put new belts on.

 

I agree. The belts are not under a constant tension when the engine is running, the load on them varies with the position of the cams. There is no mechanical reason why they couldn't be re-tensioned.
However, if they lost considerable tension whilst in use, you need to find the cause, and in that case I wouldn't re-use them. Otherwise I'd be fine with the idea of using the existing belts.

 

Actually it is derived from multiple readings across spans of the belt all added together. Doing it this way the readings arrived at are quite repeatable in any static position.

Retension after two hours run time and a few heat cycles...thats a tough one and if they were not so cheap, it would merit a debate. Glassman, there just isnt enough data available for me to comfortably say yes to a re tension not knowing the tension they were set to the first time around. I do agree with your decission to remove them rather than getting them soaked with antifreeze and related debris....good call in my book.

Over the years I have come up with a very specific belt seat in procedure I use on every belt change. There are so many factors in play that I have decided it is worth the extra time and have proven to my satisfation it is justified. Each and every tech has their own opinion on this and I am not sure there is enough data to define whats right or wrong. When I asked a factory source years back why Ferrari left the tensioner bolt exposed on the TR's and 308's they said it was OK to do within 100 miles...that was multiple belt designs ago and all of the components were quite new at that time.

A recent conversation with one of my hose engineers led to some very interesting insight in cam belt design and materials. He did a stint working for Honda at one time in design and was able to explain differences in material design and how it affects overall life of any given belt under a set of given conditions. For any sane person that conversation would have been considered a real yawner...twisted as I am I was taking notes and started to wonder if we could just..... shook my head, came back to reality and chalked it up as interesting data to be looked at in another lifetime. There is so much more to all this than meets the eye.

 

Ferrari's position is that reinstalling a belt and retensioning it with less than 1000 miles is fine. The stretching that does take place happens in the last half of belt life so it is just not an issue on a belt so new. I have done it many times myself with zero trouble on motors that ran into some sort of trouble shortly after belt replacement.

I personally would replace them in your case however. For a 308 they are cheap in a relative sense and are 2 years old. For a few bucks you can delay replacement a few years.

 

To expand my comments a little, belts do stretch after installation and people worry about that. The point is IF it was properly installed and tensioned in the first place the stretching and loosening that naturally happens is fine and allowable for the expectd life of the belt and no intervention is needed. If retensioning of that properly installed, stretched belt is performed, failure can take place. Retensioning of a belt partway through its life cycle is to be avoided for that reason. If a belt is found to be loose there is no way to tell if it is from improper installation, age, or a structural problem of the belt and as such should not be readjusted, it should be replaced.

 

Tensioning the belts does not put a set in them (some stretch after time as mentioned by others but not a "set").

If they conformed permanently to some specific shape (set) after installation and tensioning how could you ever safely start the engine?

 

Eulk,

Great to see you back in the 3x8 threads. How are the two cars coming along? Very well I hope!!!!!

Back On Topic (BOT): Again, my "Idiot Disclaimer" applies.


Perhaps I am using the wrong words.

What I believe is happening is this (albiet possibily COMPLETELY Wrong):

When the belts are INITIALLY installed, they "set," "stretch" or are "shocked" ever so slightly AND Most importantly, non-uniformly across the belt due to the teeth/cogs, etc. To help "even things out," we rotate the belt around by turning the crank several times by hand before tightening the tensioner bearing as perscribed in the WSM. Those two actions must do SOMETHING to the belt. (DaveH above would not let one outta the shop unless the belt "sings" the exact same tune across the entire belt. From what he has devulged about his style of VooDoo in the Ferrari Arts, sounds like his process is quite time consuming and detailed. Him doing a valve-job or tranny rebuild makes him a lot more $ per hour than spending it putting exact tension in the rubber bands ... so that tells me something.)

Then, add centralfugal forces the belt endours, heat cycles, etc., the environment the belt lives in is no place ANY of us would want to call home.

I admit all my thoughts above maybe in error; it would not be the first time, but it does seem to make "some horse sense." I see it as akin to some applications where screws, bolts or other fasteners cannot be reused under ANY circumstances. Once tensioned, that IS IT, or get a new one. Really if you think about it, our belts are just "fasteners" holding the cams and crank together in close formation so they can all play nice with each other.

Another analogy is a helmet or bullet-proof vest. In either case once the artifact has been stressed and done its job, you throw it away (even if you just drop a helmet you should throw it away).

That is my reasoning and that is why I would not (perhaps mistakenly) reuse a belt.

And stupid question, What If ...... you accidently but the belts back on the "opposite" or "wrong" side and/or Goodness forbid, on "Backward."

Unless corrected, I'd think I'd place those two Oops-O's under the "Dept of Potential Catastopy" (?).

mwr

 

You rotate the crank a couple of times so that the spring-loaded tensioner bearings (before fully tightening them down) can "pull out" the slack from the belts as they rotate. You want to move all the belt "slop" to where the bearing is. The tensioner spring mechanisms, if in good condition and operating freely, will apply the correct tension to the belts. Very same way it's done on Fiat and Lancia twin-cam engines although they have only one belt. Probably applies to plenty other engines also.

These belts are stoopid-strong. Having said that, they could stretch a tiny bit if you put miles/heat/oil vapor on them and tension them repeatedly and at some point you might exceed the amount of slack the tensioners can take up. I think that's why they recommend only tensioning them once. I believe they're being very conservative/safe (and it doesn't hurt sales either