Can i reduce valve clearance/lash by lapping?

Hi, this is my first engine rebuild and it's a briggs 12.5 HP (285707-0113-01).

Had a broken connecting rod and am putting everything back together and am to the point of checking the valves. My intake valve is ok, but the exhaust valve is at .014 and it is supposed to be no more than .011.

Will lapping that valve some more with valve grinding compound get it down where it needs to be? Would it require a lot of grinding to get the .003 off? Or should i buy a new valve?

run it like it is , you wouldn't even notice it. .003 is nothing to worry about.

.003" is about at thick as a piece of paper.. I agree with reynoldston to just run it..

Oh. Well that's good to know, i didn't know how precise it had to be.

Also i worked on it for about 30 minutes with the lapping tool/compound and it didnt seem to be making a difference anyway. Thanks for the advice, i'll let it roll like this and see what happens.

It'll be fine..

Make sure you wash off ALL that lapping compound, don't want any of that getting into the combustion chamber.

tiresharkdbb said:

Hi, this is my first engine rebuild and it's a briggs 12.5 HP (285707-0113-01).

Had a broken connecting rod and am putting everything back together and am to the point of checking the valves. My intake valve is ok, but the exhaust valve is at .014 and it is supposed to be no more than .011.

Will lapping that valve some more with valve grinding compound get it down where it needs to be? Would it require a lot of grinding to get the .003 off? Or should i buy a new valve?

Click to expand...

Lapping valves is mostly for checking to see that they are seating well, not for adjusting clearances. When lapping valves, you should mark the valve head and the cylinder head with the same mark. Just a good scratch with a screwdriver. Apply the compound, insert the valve with the line on the valve matching the line on the cylinder head and rotate the valve back-n-forth, using the line as a reference. Go only a short turn then reverse the rotation.Do not go around and around. That will not give you an accurate reading as to wheather the valve is seating or not. After a few of the back-n-forth rotations, remove the valve, wipe off the compound, and inspect the face of the valve. There should be a definite line all the way around the face. If there is not, there is a problem with the valve or the seat or maybe both. If the problem isn't major, the valve and seat might be fixed by grinding. If you don't have a valve grinding machine, take to a shop that does. Do not attempt to do it by lapping. .003 is a lot of wrong clearance. There is a compression release that works off the exhaust valve and if the clearance is too much, it may crank over harder and start hard too. There should be adjustment screws on the rocker arm where the rocker meets the valve stem. You should roll the engine passed tdc by a 1\3 of a stroke after the exhaust stroke. This will allow for compression release holding the exhaust valve open to close and give you the correct valve adjustment. gregjo1948

Thanks everyone, great information.

I got it back on the mower to test it, and it started right up. I only let it run for about a second, just long enough to see that it was going to start, because i have some random loose ends to tie up on other parts of the mower. Very pleased to see that it actually started, haha.

gregjo, thanks for your input. i also wondered if the clearance being off was a big deal but someone on another forum informed me that the compression release was accomplished via the intake valve on this engine, and that that amount of extra clearance on the exhaust valve shouldnt really impact performance. thanks again, very informative post.