This style of Briggs and Stratton horizontal V-twins are notorious for overheating from grass clippings and debris lodging in the fins between the cylinders. Admittedly it is a difficult chore to keep that area clear of the build up without going through extreme measures with compressed air after each and every use. Valve guides are machined of steel and are 'interference' fit in the bore of the aluminum head to allow the valve stems to move up and down through the guides for a specific distance via push rods by the camshaft. The amount of valve opening is called 'lift' and the length of time opened is called 'duration'. The stems must fit through the guides with a specific tolerance and to avoid excessive crankcase pressure the intake valve is also fitted with a valve guide seal. Because the seals are made of rubber there are typically no seals on the exhaust valves simply because the heat generated as a byproduct of combustion.
There are laws of thermodynamics and metallurgy that dictate how much heat absorption is afforded by different ferrus and non-ferrous metals. Obviously the softer aluminum head will absorb more heat at a much quicker rate at a given temperature than the much harder and dense steel valve guide. The head is cast with fins to transfer that heat to the surrounding air. Blocking those fins with the grass clippings minimizes the efficient heat transfer resulting in a hotter interior area of the engine. All metals expand with the added heat and contract as the heat is removed....BUT....as they gain heat at different rates, so do they reject heat at different rates. Because the valve guides are 'interference' fit and not physically attached, operation during an extremely heated condition will allow the exceptionally hot and expanded aluminum bore to slightly release its grip on the pressed in valve guides. When the valve guide moves just a small amount beyond a marginal dimension, the rocker arm cannot move the valve to provide the 'lift' necessary because the tapered portion at the top of the stem comes in contact with the guide. That contact makes the valve an immovable object that cannot be moved by the push rods and immediately become the weakest link. They bend and fall off the cam rendering the engine useless. Actually a V-twin will still run and operate on only one cylinder, but it will lack a great percentage of power and will sound really odd.
Looks like a simple fix to just replace the bend push rod or rods but it will immediately bend the new ones at the first start attempt. The valve guides have to be secured back in the proper position before the engine can be started. There are several ways to accomplish this. They can be removed, knurled in a lathe with a knurling tool and re-installed. A simpler method that will work is to take a small sharp punch and peen the guide to swell it slightly. Another more difficult way is to reach in through the limited space and blob a spot of MIG weld on the bottom of the guide. Yet the most labor intensive way to repair it is to remove the guide and place a few blobs of MIG weld on the tip to make sure it can no longer be moved to cause the damage again. HOWEVER....the best way and what is suggested by the dealers is to replace the head with known good one or a new one.
Another method to minimize the effort would be to scrap the machine and go buy a new mower....and preferably one with a different engine if you have no intention of keeping it clean after each use.
This is NOT a HOW-TO video and by no means is it a suggested that you do the things I've done to make this repair. This video is for entertainment purposes only and not intended to be of instructional nature. It's just an attempt at showing another way for guys like me to remain as self-sufficient as possible. Enjoy.....
12 окт 2021
