probably a dumb cam-belt Question...

Good question.

I was thinking of doing mine on my 308 in my sub-freezing garage this winter too.

 

The Alfa Romeo v6 has a detensioner for when the engine warms up ...

Pete

 

But not for that reason.

I went to the factory intro school for the V6 engine many years ago. The detensioner was an interesting idea based on the idea that belt tension needed to be higher at lower RPM do to the dynamics of valve trains. It was intended to prolong belt life but for the most part all they did was leak and get oil all over the belt.

 

The thermal expansion difference you are speaking of is not a problem if the tension is set correctly. The belts do have stretch in them that degrades as they get old and hard. One more reason for the belt change intervals being set the way they are.

 

The aluminum motor will expand a handfull of thousandths of an inch (maybe even a hundredth), the belts and tensioners can easily accomodate this amount of expansion.

If the tensioniers are not set correctly, and run into a physical-limit stop, they structural rigidity of the motor and pulleys are fully capable of damaging the belt.

 

wouldn't the belts expand as well?

 

Typically plastics have far greater coefficient of expansion than metals. The change in size of the big aluminum and steel thing is probably negligible.

I've never heard of any specs regarding installation temperature for belts but it wouldn't hurt to warm it up a little just to improve flexibility a bit.

 

Well that is interesting. Every Alfa person I know thinks it is for expansion ... do you mind if I quote you on Alfa sites?

Pete

 

OK with me.

Like I said, I got it at the very first Alfa GTV6 school. Still have the books and training materials from it. I used to go to all of the Alfa schools in those days. I put hundreds of head gaskets, valve guides and shift forks in those things. The 1st generation cars ate all those things for lunch.

If you think about it, RPM goes up, oil pressure goes up, belt tensioner relaxes belt tension. If they wanted to change according to temp, oil pressure was not the way to do it.

 

Plastic????? Ferrari belts???

 

The cords in the belts are fiberglass, should have negligable temperature expansion.

 

That's actually one of the smartest questions I've ever heard anyone ask about Ferrari cam belts

The short answer is --- don't worry about it. The expansion / contraction of aluminum (especially the alloy that Ferrari uses in their engine castings) is insignificant over those temperature ranges ---- also, the geometry (relative angles of maximum expansion directions of the castings vs. the directions the belts run) cuts the movement down even further.

That said, it is always better to perform involved mechanical work (like cam belt changes) when you and the equipment are "comfortable" ---- leads to less bumbling around and a more enjoyable activity.

The only thing you don't want to do in the cold is anything that requires sealants / adhesives --- they don't stick or cure properly at low temps.

Go for it --- have fun (doesn't sound like too much fun to me, though) !

 

Correct. Because of the composites (fibers) used in the belts and the multi-directional biasing of the fiber lay-up in the construction --- the expansion / contraction (due to thermal effects) of a modern cam belt is nearly "zero". They do expand (lengthen) slightly with load (tension) and old age, but temperature during installation and at engine operating temps has no meaningful effect on the dimensions of such materials.

As an example, the glass fiber and carbon fiber composites used for aerospace applications have a CTE = (very close to) 0.00 throughout a temperature range of about -400F to 250F !

However, I would recommend keeping them (the belts) nice and warm in the house until right before you put them on --- so, they are nice and supple and easier to work with during installation. They do get significantly stiffer when cold.