Valve Seat Lapping - Is it Necessary for New Valves and Seats?

[Jocko_51_B3B]

 

I just had new valve seats professionally installed by a machine shop and I have a new set of valves ready to go in.

Is lapping necessary with new seats and valves? Or is lapping just an old archaic process usually reserved for well-used / pitted valves and seats?

[mechresto]

With the modern tooling, 3 and 5 angle cuts, in a word....no.

BUT...

It's always a good practice to double check the work of any machine shop. Seat width and placement can be checked with a sharpie marker, place 4 lines on the valve face, parallel to the stem equal distance apart, then "slap" the valve against the seat, the resultant wiping away of the marker will tell you what you need to know.

 The other option is a light lap (which I always use), it gives extra insurance against possible errors in machining.

 

[Elwood]

15 hours ago, Jocko_51_B3B said:

 

I just had new valve seats professionally installed by a machine shop and I have a new set of valves ready to go in.

Is lapping necessary with new seats and valves? Or is lapping just an old archaic process usually reserved for well-used / pitted valves and seats?

What mechresto said.  I, too, always lap new valves/seats after they've been replaced and cut or ground.

 

I'd add that a liquid leak test is always a good practice, too.  This test is easier to do on an overhead valve cylinder head removed from the engine, but it can be done on a valve-in-block flathead.  You'll need to have the valves and valve springs installed, but the cylinder head, tappets, and camshaft should not be installed.  Invert the block (easier to do this if it's mounted on an engine stand), and then pour a small amount of kerosene (or something similar - ATF might work) into each intake and exhaust port pocket against the valve head backside.  Let the block sit overnight (and put a drain pan underneath it).  If any of the kerosene leaks past any of the valves, then that leaking valve seal is not good enough.  Lapping might be enough to fix it, or the valve and/or valve seat may need to be trued up.  There are measuring tools for checking the concentricity of the valve seat to the valve guide, and of the valve face to the valve stem.

 

Good valve-to-seat contact is not just about sealing the combustion chamber and getting power out of the engine.  The contact area between the two, especially on exhaust valves, is important for heat transfer and engine longevity.

 

Lapping of used, pitted valves and seats will likely not do anything except use up some time, effort, and lapping compound.

 

If you do decide to lap your valves, be sure to clean off ALL of the lapping compound when done.  Lapping compound is an abrasive, and not good for engine internal parts.

Edited April 8, 2018 by Elwood

[Dave72dt]

I guess i'm old school but I'd want to check them before everything got put back together and then find out  you've got a leaker.

[MBF]

Yea, I'd lap them, but prior to doing that check for the stem to guide fit too.

[Jocko_51_B3B]

Great advice from all you guys so a light lapping it will be. I have my block on a stand which will allow me to invert it. After the lapping, I think I will try the "upside down leak test" suggested by Elwood (Smart idea. Never woulda thought of it.) 

[48Dodger]

On 4/7/2018 at 4:34 PM, Dave72dt said:

I guess i'm old school .......

 

No guessing........we are old school.

please make a note of it. lol

 

48D