Normally, a valve seat is good for the life of the engine, rarely giving a problem when the engine has normal maintenance done to it. The most common reason a valve seat comes loose is that the cooling fins on the engine become partially clogged and the engine overheats causing the aluminum to expand faster than the metal seat. The seat loosens up and can actually come completely out. When this happens it is possible to push the seat back in and peen the aluminum around the seat to hold it in. Doing this will normally change the seating capability of the valve, so the seat face should be cut and lapped to check the seat to valve fit.

If the seat is extremely loose, over .005 between the seat and block, it will need to be replaced. Replacing valve seats is beyond the scope of this article and should normally be done by a machine shop. Briggs & Stratton does offer special tools and instructions to do this in there repair manual. A graphic from Briggs & Stratton shows how to peen a valve seat. This is something that can be done without any special tools, but remember you will have to re-cut the face of the valve seat after doing this. This will take a valve seat cutter. It is also a good idea to check the valve guide for excessive wear at this time. Valve guide replacement can be done at most Briggs dealers.

Diagnosing a loose valve seat can be tricky, since when the engine cools down it will often tighten back up. If your engine runs good cold, but dies once it is hot, check the compression when hot and if it is low, you probably have a valve seat coming loose. Normally, problems with a hot engine with low compression are in the valve area. The exhaust seat usually comes loose, since most of the heat is in that area. Peening a valve seat successfully depends on how loose the seat is when you attempt the repair. Finding the problem early increases the success rate. I hope the above information helps you out and remember to keep those cooling fins clean and don't overload the engine. It's your first defense against valve problems.

Also see Complete Engine Rebuild,  Carburetor Rebuild, Starter Rope Repair, Carburetor Troubleshooting Chart or Step by Step Two Stoke Carburetor Rebuild for more troubleshooting.

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