There is the $64,000 question.
Unfortunately my experience and research would suggest that at the very least the likelihood of failure is very high. Perhaps I should explain again why I have changed my tune on this issue.
I'm a nearly 60 year old mechanic who runs his own Guzzi only business in a small rural town in Aistralia. I've been involved with Guzzi for almost forty years and unlike many of the older Guzzi owning crew I'm actually very enthusiastic about the company's current products and despite its issues, (Which now seem to have been addressed.) I think the 8V motor is an absolute gem. I was an 'Early Adopter' buying one of the very first 8V Grisos imported to Oz in 2008.
Right from the get-go there were reports coming in from parts of Europe of top end failures. I found this odd as I couldn't get mine to break and the only ones I did see that had broken had a tick-list of active fucktardery that had been inflicted on them by a variety of shops and shaved apes masquerading as 'Mechanics'! It was certainly impossible to sheet home the problems to a design issue.
By 2009 though the factory had changed the tappet material and design from a chilled cast iron component to a forged steel one with a DLC, (Diamond Like Carbon.) coating on the foot. This was supposed to be the answer to the problem that so far I was still skeptical of but I did get some failed tappets from another shop and took them in to the Australian Mint in Canberra which has one of the best metallurgical analysis labs in the Southern Hemisphere and got the failed parts tested on a Beakers machine. This DID
reveal that the tappets were soft but when I eventually, after a prolonged wait as the importer 'Forgot' to send me my upgrade kit, fitted the upgraded parts to my bike I found on replacement that my original 'Soft' tappets were still in perfect condition. Once again this pointed to there being a service and set-up issue rather than a chronic design fault and while everybody else in the world seemed to be thinking the sky was falling I steadfastly stuck to my guns and defended the product. After all, I couldn't get an engine to fail.
Continuing research and record keeping though showed that the failures were continuing, albeit at a reduced rate, and they were still far more prevalent in cooler climates but climates where people kept riding in winter when it was cold and wet but not so cold that people stopped riding at all in winter. Yes there were failures elsewhere but it was conspicuous that those circumstances led to a much higher number of failures.
A lot more data collection, pestering people and generally making myself a global, king sized, pain in the arse along with my own research using my own shitbox as a guinea-pig revealed that the 8V is grossly over-cooled! So much so that in cool damp weather it's virtually impossible to get the oil above 100*C for long enough to dessicate the water from combustion out of it! In cooler weather this will lead to large amounts of mayonnaise forming in the rocker covers and because of the angle of the cylinders it means that when the bike is parked this watery slime will drip down into the weirs the cam lobes and tappets run in. Without going into detail the results aren't good.
This alone though is unlikely
to be the only source of the problem but it is certainly, I believe, a major contributory factor.
Fast forward to last year. Last winter around here, (One of the driest areas of the driest inhabited continent of earth.) we had a remarkably cool, wet winter. This year I've had nine tappet failures through my shop, (And currently have three bikes waiting for parts with no ETA,) including my own and Mark's, (Beetle's) both of which I know both service history and useage of, (We both ride our bikes hard and have deliberately tried to ensure that our oil never stays cold enough for watery deposits to form.) and this, unfortunately, has led me to believe that my original belief was wrong and that there is a more serious underlying problem with the flat tappet design In this application
! It isn't those two cases alone but the evidence seems now to point not only to the fact that the system was inadequate but also that since the early months of 2010 the engineers knew they were on a hiding to nothing becaus from the middle of 2010 the motors started to have shims added under the inlet, (And on some models exhaust.) valve springs, the only reason these would be added would be to compensate for the extra mass of the already designed but not yet produced cure, the roller tappet system. It was pre-emptive because they knew
that down the line they were going to be having to replace a shitload of flat tappet top ends but by doing the shims early it would reduce the cost when the shit hit the fan.
The long and the short of it is that I now believe that failure of the flat tappets, while maybe not *Inevitable* is at a level of likelihood so high that it might as well be and it will also be connected on a sort of 'Bell Curve' of climate and riding conditions as to how soon it is likely to occur. Am I right? I don't know! As I freely admit I was wrong before but at least my observations were built on a lot of data and research rather than just running around screeching and panicking!
Exactly why and how the roller tappet top end manages to avoid some of the problems associated with the flat tappet design I have absolutely no idea! BUT, it has been standard fitment to all Cali1400's and all 1200 8V's since mid 2012 and I have yet to hear of a solitary failure, (And believe me, I keep my ear firmly to the ground on this because the first one that goes tits all the tearing of hair and rending of clothing will start all over again and the 8V Hate-a-thon will recommence among the beetle-browed and backwards!
Long and the short of it. If you are thinking of buying a flat tappet 8V, (And I would in a heartbeat still.)
1.) Make sure it has a full service history. This is very important, especially outside the USA, as unless you do, by an 'Authorised Dealer' you won't get the repair parts under warranty.
2.) Make sure the bike doesn't behave oddly, especially when cold. It should fire up on the button and immediately idle at 1200 RPM with no coughing, chuffing or needing for using the throttle to keep it running. When warm it should idle cleanly at 1200-1250 RPM.
3.) The 8V, especially the early A5 motored bikes, are noisy but the top end shouldn't sound like there is a rock crusher under the tappet covers!
If the PO knows nothing of the issues or denies there is a problem and it's just a myth? Be prepared to walk. It's been front page news for seven years. If there is no service history immediately drop $2 grand off the asking price. Sorry for all of those who are thinking of dumping their flat tappet bikes because of the issue but this is the reality. If it's any consolation my own bike is worth squat because it's a previous 'A5 Flattie'. (Not that I'm selling!) but it's worth is smaller, even though it's been rollerised.
So, slowing the wind a bit. By all means buy a low mileage G8 Flattie, they're a great bike and with the info and advice and talent of the likes of Paul and Beetle here you can have a bike that will run superbly BUT you should factor in the cost of a roller conversion as it is EXTREMELY likely it will need one sooner or later, usually about the 30,000 km mark but it can be earlier or later, (Mine soldiered on until 90,000Km.).
I hope that gives you a bit of info and background. FWIW mid 2012 build was when the 1200's went to rollers. Anything after that you are golden.