Briggs & Stratton OHV Plastic Engine No Compression

[Tiger Small Engine]

Lapping just makes the seat and valve shiney.

Lapping valves also increases the compression. When valves and the seats are pitted and carboned up, it really affects the engine performance.

 

[RevB]

Lapping does not increase compression. Will not remove pits, not used for removing carbon, and has zero effect on performance. There are tons of marketing ads pushing this crap and it's all a lie.

The only value of lapping is to verify correct contact location of the seat on the valve and the width of the contact after the seat stones and valve grinder have done their work. That's it. Everything else is bullshit.

 

[Tiger Small Engine]

Lapping does not increase compression. Will not remove pits, not used for removing carbon, and has zero effect on performance. There are tons of marketing ads pushing this crap and it's all a lie.

The only value of lapping is to verify correct contact location of the seat on the valve and the width of the contact after the seat stones and valve grinder have done their work. That's it. Everything else is bullshit.

View attachment 71457

I don’t have a valve grinder, and surprisingly, most people don’t. Most people don’t even know what valves do, much less how to lap them. When a valve moves by turning with your fingers while under spring tension, that is what lapping helps prevent. When a valve leaks when you put gas on the cylinder head to test, that is what lapping helps prevent. Nobody said lapping valves was perfect. I believe you can buy a new set of valves for a bit less than a valve grinding machine.

 

[RevB]

I don’t have a valve grinder, and surprisingly, most people don’t. Most people don’t even know what valves do, much less how to lap them. When a valve moves by turning with your fingers while under spring tension, that is what lapping helps prevent. When a valve leaks when you put gas on the cylinder head to test, that is what lapping helps prevent. Nobody said lapping valves was perfect. I believe you can buy a new set of valves for a bit less than a valve grinding machine.

But then you still have the seat....

Agreed that not many have the stones or the tools to grind seats and/or valves....or the skill. It can be a pretty big investment and for a one time use it's better to take the parts to someone in the business of valvetrain restoration. New valves and restored seats are most likely the least expensive option if everything else is in spec. The seat to valve interface is critical as the width of the contact patch determines the amount of heat taken out of the valve head into the seat which is always cooler, relatively, especially for the exhaust valve. The intake has the benefit of a cooled intake charge. A polished valve face that is not concentric with the seat, or has slight pitting as you say, does nothing for the sealing ability, or heat transferance from the valve.

Lapping does have it's place but in the context of valves it has become the go-to to fix for problems that can't be fixed with this method. The valve face becomes concave, usually, at the seat contact area and lapping will not fix it. Only restoring the flatness and concentricity of the face will. Lapping a blind dowel pin hole to improve/increase size comes to mind to get the correct interference fit for the pin or lapping an aluminum cylinder head on a surface plate to remove warpage if the straight edge shows the surface to be not level. Lapping an iron head would take a couple of days and that is better left to fly cutting, as would the block. Generally it would be the job of the gasket to take up the slack on parts that aren't within a couple of thousandths but gaskets can only do so much.

My only point is there are expedient ways to do things that *might* get you a few more hours but since you're in there anyway, why not spend the little extra to return to as close to as-new as you can.

 

[Auto Doc's]

Lapping does not increase compression. Will not remove pits, not used for removing carbon, and has zero effect on performance. There are tons of marketing ads pushing this crap and it's all a lie.

The only value of lapping is to verify correct contact location of the seat on the valve and the width of the contact after the seat stones and valve grinder have done their work. That's it. Everything else is bullshit.

View attachment 71457

You are convinced that lapping valves does no good? Then why has it been an industry standard since the first valve engines came into production? Can the whole industry be wrong?

Why is silicon carbide lapping compound used if it does not work?

 

[RevB]

You are convinced that lapping valves does no good? Then why has it been an industry standard since the first valve engines came into production? Can the whole industry be wrong?

Why is silicon carbide lapping compound used if it does not work?

Not the whole industry....just the gullible part of the industry. Let's take Permatex 80037 as an example. This material is your silicon carbide in 4 mixed grits... 120, 150, 180, and 220. Ostensibly the grit size decreases as the paste is ground between the face and the seat...or so Permatex says. Where does that reduced grit go? Some embeds into the face of the valve and seat because it is way harder than the metal you're trying to remove. That would introduce annular rings in the seat and face because it's grinding, not polishing. You want to polish you better go to 2400 as a minimum. Most of your so-called "lapping" pastes aren't .

Now you have ground away material that is between the face and the seat but only at the contact point...exactly the wrong thing to do. They are not flat nor orthoganal any more. As stated previously, that may get you a few more hours but is that your goal...to get just a few more hours. You've already gotten hundreds of hours from that engine and it wasn't just a few hours at a time, it was continuous, unrepaired hours.

I'm saying, lapping, as a means to correct deficiencies, is a waste of time. There is no way you can control the angular interface and the contact surface by "lapping" alone. A stone, cut to the proper angle, is the only way to properly restore that angle.

 

[slomo]

These are mower engines not the space shuttle. Lapping seals valves to valve seats. Pretty important in the combustion process.

 

[RevB]

These are mower engines not the space shuttle. Lapping seals valves to valve seats. Pretty important in the combustion process.

Whatever.

 

[Tiger Small Engine]

These are mower engines not the space shuttle. Lapping seals valves to valve seats. Pretty important in the combustion process.

I don’t even know of a machinist in our area that will grind valves. When you factor in your time back and forth, and paying labor cost, it would be on the expensive side. Even new valves need to be lapped in to the old seats.