Precise Engine Repair Notebook

Repairing Loose Valve Seats

I have received many request for information on what to do about a valve seat coming loose in Briggs & Stratton L-head engines. So, let's talk about it a little. 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.

Peening a valve seat

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. There is more information below about replacing valve seats. I have included a graphic from Briggs & Stratton that 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 - Bruce Perrault

The following information was submitted by a technical forum contributor. It pertains to Onan engines, but can be applied to others as well.

"As an Onan dealer we see loose valve seats a couple of times a year. We have found that many times the seat has worn the block to the point that the replacement seat will not stay in the block. Rather than replace the block we have a shop install a new seat. The automotive machine shop machines the block to receive an automotive seat that is the correct diameter for the valve. The shop finds the correct seat depending upon the engine and valve requirements. The block is then heated 350 degrees F and the seat cooled (frozen in freezer or in liquid nitrogen). The seat then drops into the block without being driven in or pressed in. The seat is then cut to the right angle" - Dwayne

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