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The valves in an old engine will last a long time running on unleaded glass as long as you don't mess with 'em!
The engine's years of running on leaded gas will have built up a "lead memory" in the valves -- the lead deposited on them will continue to lubricate them.
Eventually it will wear off, yes. You'll notice because of increased "valve recession" -- as the seats start to wear, the valves will move up in them, and every time you adjust the valve clearances (which you need to do a couple of times a year anyway) you'll notice that the valves are always too tight, and you have to increase the clearance a bit. (Actually the valves recede anway -- it's just that loss of lubrication makes them recede faster.)
One day you'll run out of adjustment, or you'll notice that you get poor results on a compression test (which you also should do about once a year anyway.) Then it's time to remove the heads and send them out for a valve job, since by now you need one anyway. At that time you also should have hardened seats installed.
But you can go a long way before that happens, so if your compression and valve clearances are OK, there's no reason to pull the heads JUST to have hardened seats put in. So that's the simple rule: If compression and clearance are OK, leave your valves alone other than adjusting the clearance regularly. When the heads need to come off for any reason, have the hardened seats installed as part of the work.
To avoid screwing up this beautiful benign-neglect system, the one thing you must NOT do is mess with the valve seating surfaces. For example, don't do the old-fashioned trick of removing them and using valve-grinding compound as "preventive maintenance." That will remove the lead memory and make the valves wear faster, so that very soon you'll be needing to pull the !#$% heads AGAIN to have new valves and hardened seats installed.
If you've somehow unearthed a pristine-but-never-used 1970 V4 engine in a crate from some Swedish archaeological dig, that's a different can o' worms. An engine that never got used much back in the leaded-gas era won't have had a chance to build up any "lead memory," so its valves will recede faster when you run it on unleaded gas. You can still run it; just be prepared for the fact that fairly soon you'll need to redo the heads.
posted by 204.76.11...
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