High mileage engines have valve guide and often valve stem wear that is just expected because that’s what we’ve seen time and again. If you work in an automotive repair shop on these engines that is a common repair. It’s not that we will it, it’s just something that is.
Same for cylinder bore wear. Though since 1987 it’s less of a problem mostly because these are all fuel injected and fuel injection is a lot easier on the bore walls, rings and piston skirts because EFI is pretty balls on with mixture control where carburetors are pretty sloppy outside of steady state conditions like cruising on the interstate.
Valve guide and stem wear results in stem wobble which causes the valve to roll about its seat rather than instantly closing which offers an opportunity for the hot exhaust gasses to burn the seats. The clearance on the guide is very tight, as that widens oil gets pulled in on the intake due to operating vacuum which forms hard coke deposits on the back side of the valve and in the pocket adding weight to the valve, increasing its operating temperature and reducing port flow. On the exhaust side which sees searing heat and pressure this tends to burn oil in the guide leading to a sticking valve to blown oil black out starving the guide for lube thus accelerating guide and stem wear.
In the cylinder bore the top end lube is really thin and is subjected to combustion flame burning what little oil there is near the top of the top ring so it tends to dig into the bore around its upper travel zone leaving behind a ridge of original bore above the top end of ring travel on the bore. Again this is more pronounced on carbureted engines than those with EFI for reasons already stated about carbs being sloppy with gasoline when cold and or transitioning throttle settings. This gives the travelled bore a taper from the top of piston ting travel (especially top ring ) to the bottom of the bore which sees piston thrust forces. Piston thrust is against the cylinder wall opposite the direction of crank rotation. Looking down on the bore this gives the bore an egg shape to its diameter. Needless to say these wear patterns make it damn hard for a piston ring to follow what’s going on with worn bore shapes.
This is what to expect when taking things apart. This is why when you get deeper into the engine when you disassemble things you just can’t put them back as the act of disassembly upsets dimensions for a long list of reasons. A good example is putting new bearings in an old engine with no other work being done. As the engine wears the bearing clearances increase which sits the crank and the rods lower so the ridge making pattern on the cylinder wall moves down. Now a lot of thousands of miles later somebody puts new bearing in the bottom end to restore failing oil pressure reduce rod knock noise or whatever the reason but now the wear clearance has mostly been remove such that the crank and everything connected to it are sitting the pistons higher in their bore. Then you fire it up only to get a bunch of busted top compression rings.
For uncorrected cylinder and piston wear when doing a top end valve job restores cylinder vacuum and pressure depending on which stroke the piston is on. Here without fixing the cylinder bore, rings and piston the engine will suddenly acquire a bad case of blow by and oil consumption as the higher pressure will bleed into the crankcase on the compression snd power strokes while oil will get pulled around the worn piston rings, bore wall, etc on the intake stroke.
Basically we‘re trying to give you a high school to junior/technical college auto shop course one blog at a time. If you have a JC or Tech college in commuting distance I highly recommend you sign up. At some point you’ll be able to use the Monte as your class project. Just buy a POS commuter to get around in the mean time.
There are basically two things we here are trying to do for you by making you aware of how this stuff actually works, save you from wasting hard earned dollars and keep the Monte from going to the scrap heap when you finally have taken enough of a beating from it. Every guy here that isn’t a millionaire has been through this and I wager even more guys have been through this and in the end had to walk away from it far out number those of us still tinkering with auto’s. There just aren’t a lot of Jay Leno’s in the world that can pump the millions of dollars into this hobby. Probably the best way for the average guy to get in it is to due the time in school and on the shop floor to see if you have the skill set to open a racer’s studio that attracts the high rollers that want a rod but don’t have the time maybe not the skills but have big bucks. Get really good at this they will beat a path to your door. That also will take moving to the big city to get established once established then you can consider a country estate out of town with a barndominium.
If your not born into a wealthy family then the only way forward is by paying your dues, it ain’t easy but that’s the path.
Bogie
