What is the average valve replacement mileage for a 308

40K miles is super, super conservative IMO -- and the vast majority of 308 go way beyond that (barring some sort of other cam drive problem causing a valve-to-piston collision). Can you give the reference where you "read" that? My data point for a US carb 2-valve for valve wear out is that at about 90K miles things got bad enough to start to notice some symptoms, and at 99K miles two of the exhaust valves got bad enough to give a leakdown of ~15% (so refreshed the top end then).

 

The sodium exhaust valves should be replaced as when they fail they brake unlike regular valves that bend. I had such a valve failure. Not fun.

This being said, I had a few damaged valve stem seals, and while you are there, replacing the valves makes sense as they are cheap. So we even trash some inlet valves that are still in reasonably good condition.

All in all these are replaced early either as part of a while you are there job or as a way to avoid catastrophic sodium valve failure.

 

I have been working on Ferraris since 1995 and I have yet to see one sodium valve fail without influence. I am sure it happens, but it is not common and I don't recommend new valves to my clients because they are sodium filled. I have had valve seat sealing issues from poor maintenance and will never put an old valve back in when the head are off. Modern technology in material and manufacturing just makes is an easy decision to install new valves. Also, the valve seat work on old Ferrari heads are poor at best. A multi angle valve job can do a great deal to help with not only sealing but low lift flow as well. Again modern technology comes through.
If a valve job and new valves are within your budget and you want peace of mind, then find an outstanding shop that has done this before and let them make your engine better.
The next one I do I am planning to run before and after on my flow bench to see what the gain actually is.

 

"Influence" is not necessarily what one would presume. My own head (400i, same valves as the 308) had a crack on the exhaust port, so we decided to perform a full rebuild: thinner valve seats, new guides, etc. As the 98.000km valves were within specifications we decided to keep them. One broke shortly after.

That's pure speculation on my side, but I presume that the new guides induced stress on a section of the valve head that was interacting differently with the previous guides. In any case that's an unfortunate way to destroy a newly rebuilt engine.

So based on my own experience, as long as you work on the head it's better to replace the $40 valves. The 308 is a revy engine, so it would make even more sense to play it safe.

 

That is a good example of outside influence. New seats & guides would definitely fall under outside influence. Raemin is correct. Valves are cheaper than pistons, sleeve and cylinder head welding.

Funny side note: I had someone call (2001) and ask when the scheduled engine rebuild is on a '77 308. I said right after the rod comes out or the valve breaks or it starts smoking.
He was under the impression that all 308s would need a rebuild very early in there life. I don't know where this comes from but it is out there.
I have had a number of 308s that are 100,000 plus miles and are still going strong. They are well maintained and driven often. I think that makes a huge difference.

 

At 50K miles my '78 308 dropped both valves in No. 8 due to faulty cam timing on front exhaust after a belt change. ( Didn't lift the valve cover using lock the cams method )

I asked them to check the wear in both heads and valves. (5 - 8 exhausts and 8 inlet were getting new valves )

All the valves were very good. The guides were slightly worn, and were serviced with inserts, and new seats cut and ground.

Will be getting the cam with the new distributer drive plug fitted back this week, then it's ll go !
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Only the 2 valve per cylinder 308s - 1974 through 1982. All the 4 valve per cylinder 308s have non-sodium exhaust valves, and don't have an inherent breakage concern.