Valve Lapping

[Mek-a-nik]

Slow season; I'm going through a Honda GX340 powered pressure washer a customer gave me back in May. It was under salt water after Hurricane Sandy in Oct. 12. He let it sit all winter before he called me. I haven't gotten to the pump yet, but the engine only needed a piston and rings, as the rings were stuck good. Also needs a carb, it was past ugly.
Upon lapping the valves, the intake is requiring quite a bit of labor. I've been trying to lap it until no gas leaks through it when poured in the port with the spring installed. Is this the standard for a good valve job? Been a long time since I was taught the basics, and haven't used them much.

[bluemower]

about 50 years ago, I was taught this procedure when I attended an aircraft mechanic school near St Louis. Kerosene was used instead of gasoline. I doubt if anyone uses this procedure today. High performance shops probably have equipment to trap air pressure against a renewed valve seat. Regular production shops do not normally use the lapping compound.

you will probably not achieve this top level of perfection due to slight wear on the valve face, valve stem and valve guide.

[bgsengine]

Thats what we do with OHV valve jobs (the few that we actually do - frequently it's cheaper to just throw in a complete head set and have done with it.)

Finish the valves, install valves, springs, retainers as usual, install plug and snug it up, gently clamp head assembly in vise, fill combustion chamber with mineral spirits (parts cleaner) and see if anything leaks past. - have found cracked heads that way too (cracked from spark plug threads, snugging up the plug put enough strain on it that the crack opened enough for the leak to show up through the crack!)

The mineral spirits are thin enough that they'll leak anywhere that air will, but they are not as explosive as having raw gas laying around. (You could, of course, use denatured alcohol or rubbing alcohol as well, if you don't have mineral spirits, but I'd NEVER recommend using gasoline.)

On L-Head engines, it's mainly a visual inspection with a dab of machinist's blue.

[bobodu]

I actually used the gas trick last week on the K241.

[Deere2me]

... Is this the standard for a good valve job? Been a long time since I was taught the basics, and haven't used them much.

Not really. One little bit of debris on the valve/ seat and ya get gas leakage,right? In my learned opinion, its more important to achieve a uniform matte/dull seating area, one that doesn't have any shiny streaks, rings , or spots. Also, if you have the time, you should pay attention to the seat width, though ya really can't change that much by lapping. I just go for the dull finish. It ain't a race motor!

[madmantrapper]

I've always used the gasoline method, at least for the last 50 years, never knew any other way unless you have a flow bench available. I wouldn't throw it out for some seepage.

[wristpin]

... Is this the standard for a good valve job? Been a long time since I was taught the basics, and haven't used them much.

Not really. One little bit of debris on the valve/ seat and ya get gas leakage,right? In my learned opinion, its more important to achieve a uniform matte/dull seating area, one that doesn't have any shiny streaks, rings , or spots. Also, if you have the time, you should pay attention to the seat width, though ya really can't change that much by lapping. I just go for the dull finish. It ain't a race motor!

I go along with all of that and would add that if the valves or seats are bad enough not to respond to the fine end of the paste tin they need refacing and recutting. Forget the coarse paste it will only dig a trench in the valve face and round off the edges of the seat.
Being fortunate enough to have the kit I routinely give valves a quick spin on the refacer and a light skim on the seats. This reduces lapping time to less than a minute to achieve the text book continuous even matt finish. If the seat gets a bit wide it can be narrowed down using different angled cutters. Use of the cutters also allows the contact patch to be "moved" to the centre of the valve face.
Quote from my favourite manual
" The technician who is valve critical will always have a better record of success than the one who takes a near enough approach"

[Mek-a-nik]

I'll lap it some more, I guess. The finish on the valve and seat is a smooth, matte type. So it's close. I've only been using the fine compound.
Has anyone ever chucked the valve in a drill and done it that way (at a slow speed)? My old suction cup lapper doesn't schtick too well and my hands constantly move done the handle, kind of a pain.

[Deere2me]

I'll lap it some more, I guess. The finish on the valve and seat is a smooth, matte type. So it's close. I've only been using the fine compound.
Has anyone ever chucked the valve in a drill and done it that way (at a slow speed)? My old suction cup lapper doesn't schtick too well and my hands constantly move done the handle, kind of a pain.

On most OHV units I use a short piece of fuel line to connect the wood dowel to the valve stem and a couple of hose clamps , if needed. Obviously, you have to exert an upwards pressure using that method. I wouldn't use a drill!

[bgsengine]

I'll lap it some more, I guess. The finish on the valve and seat is a smooth, matte type. So it's close. I've only been using the fine compound.
Has anyone ever chucked the valve in a drill and done it that way (at a slow speed)? My old suction cup lapper doesn't schtick too well and my hands constantly move done the handle, kind of a pain.

We actually take *advantage* of that to make lapping valves quicker and better quality.

Roll lapper til either hands slide down too low , or cup comes unstuck, then re-stick cup if needed, lift valve and rotate a quarter turn. 30 seconds will have seen 1-1/2 rotations of the valve while lapping, and you end up with a more even finish , and a final check with machinist's blue (a dab of colored grease can work too if the color is heavy enough, or just shine a light over the surface) - a very thin smear of grease over the valve, then drop it in and a firm tap on the valve head with a finger , lift it straight out without any rotation, and you can see any possible high spots on the face or seat and see where exactly it seats. Usually takes me less than a minute per valve for the complete lapping operation.