When is it time for a complete top-end?

to save you time and money i would do a warm compression and leakdown test, then plan on a valve job on both heads. drive the car awhile, then do the bottom end. or you can call TRutlands for one of thier rebuild drop in engines.
if your compression numbers are good, the oil smoke may be from worn valve guides.

 

PeterS -- What type of 308 (sodium-filled valves?; Iron or NikASil liners?; etc.).

I nursed my ex-308-2V carb to 100K (indicated ) total miles before doing the top end, but it should have been done at ~90K miles IMO (the last leakdown measurement before doing the work showed two cylinders down at ~85%).

At 70K accumulated miles, I think you can include the top end work if you're doing the rings anyway without any regret at all (and really should include it). At less than 20~30K accumulated miles, reusing a removed head (that didn't have sodium-filled valves) without any valve work might make sense, but over ~50K miles doesn't (and if it has sodium-filled valves, it's a no-brainer to update) -- JMO.

 

Peter, the smoke on start and warm up, especially at idle or other high-vacuum conditions, indicate valve guide wear. That is not going to show up in any leak-down or compression test, although you can do that if you wish. Smoke under hard acceleration AFTER THE CAR IS AT OPERATING TEMPERATURE is probably rings. Cold and throughout warm up, it's probably residual oil from the guides dripping onto the back of the head of the valve or excess fuel.

I would recommend driving it for some time as any further degradation has no ill-effect on any other engine component. It is our experience that these things last a long time. Rings and "top end jobs" are two very different scopes and expenses of work. Top end is easy, bottom is hard.

The Chicken Little's will tell you to yank it out and spend a bunch of money, but it won't generally, significantly or substantially improve performance. I certainly don't recommend prophylactic valve jobs! If you're there with the engine out for transaxle work or to reseal it, go ahead and replace the guides and do a valve job, otherwise, just enjoy it!

-Peter (remembering when it wasn't a sin for Ferraris to puff smoke...)

 

Or it could just be the valve seals, a much easier job than guides.

I agree with Lolaman, sounds like either valve seals or valve guides. A little oil burning just after startup is something that you could live with for a very very long time w/o anything serious happening.

Compression/leakdown are the tests that will indicate if something serious is going on. If they don't look too good, redo them with a teaspoon or so of oil in the cylinder to seal the rings. If still not good, then have valve problems.
If they're OK, then next major service, replace the valve seals & see if that takes care of the oil burning. If not, then it's the guides & you could choose to live with it for many thousands of miles.

Also, if you can't adjust the valve clearance using the thinnest std shim, then you've got pretty badly worn valves or valve seats & it's time for a valve job.

 

Worn guides should be taken seriously and attended to ASAP. My exhaust guides were worn at 45K mi and the result were three bent valves (since the valve rocks around while going up and down and the valve head strikes the seat unevenly. This tweaks the valve head a bit. You do that several thousand times and it'll eventually snap), I was lucky. These were sodium-filled exhaust valves. These are ticking time-bombs. Another GT4 owner I know had one go on him and it destroyed that piston and head, he spent two years (on-and-off) doing the rebuild himself before the car got back on the road. Very expensive.

This is an expensive job, but certainly less than tryng to rebuild a destroyed engine.