Valve problem

[awake]

Right - but what is that rock, exactly? I don't know the chemical composition of much of any rock other than limestone ...

Okay, you've got me curious enough to google it. According to Wikipedia:

Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It is commonly[5] but not exclusively of silicic or felsic to intermediate in composition (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but basaltic and other compositions are known.

Well, that answers all my questions ... !

And as for toothpaste, apparently pumice is sometimes used, but most toothpastes use less abrasive materials. This is from a Colgate article:

The Dental Health Foundation notes that common abrasive ingredients in toothpaste are derived from chalk and silica. Calcium carbonate, sodium metaphosphate, zirconium silicate and calcium pyrophosphate are just a few abrasives you may find on a list of toothpaste ingredients.

Now that I know much more about toothpaste and pumice, I understand even less ... !

 

[tornitore45]

In the US one can buy "Bon Amie" a cleaner for ceramic surfaces found in the kitchen. It looks and feels much like Pumice and is a very mild abrasive.
Pumice is fairly soft and breaks down fast. I doubt any residual will do much harm. I use it when the finest grit of carbide I have is too course.

 

[tornitore45]

Some toothpaste contains baking soda.
Baking soda is also used as sandblasting media.
Must be mildly abrasive.

 

[josodl1953]

I use brass polishing fluid for final lapping. I put a short piece of silicon tubing behind the head to prevent fluid entering the valve guide.

Jos

 

[tornitore45]

I put a short piece of silicon tubing behind the head to prevent fluid entering the valve guide.

Great idea, why did not think of that? Instead of wadding up some shred of paper towel.

 

[MIKE RIGGIN]

Good afternoon, Gentlemen
I have had time to devote to my valve problem since I am not working much these days. I remade a couple of valves by brazing a stem to a head and machining from there. I made sure that my compound slide was at exactly 41 degrees and mounted my dremel in the toolpost and ground the 41 degree angle. I tumble rocks and it dawned on my that I have a variety of grit on hand. I mixed 60/90 silicon carbide grit with some grease and lapped them. I then mixed 150/220 silicon carbide with grease and lapped again. Next I mixed 500 grit with silicon carbide and lapped. Finally, I did the same with TXP aluminum oxide. The seal is much better, but not perfect. My six inch chuck will not grip 1/8 inch stock so I was forced to use a 3 inch chuck gripped by the 6 inch chuck resulting in a not perfectly concentric valve face. I will keep plugging away at it until it is acceptable. I have learned that it is east to make an object shaped like a valve. Making an object that behaves like a valve is a much different story.

 

[Brian Rupnow]

Mike, all of us guys who build i.c. engines have fought that battle with valves. Concentricity between valve stem, valve face, valve guide, and valve seat are of paramount importance.---Brian ---Check out this link
https://www.homemodelenginemachinis...alve-for-model-i-c-engines.25916/#post-283262

 

[tornitore45]

If you have a eccentricity problem that should be addressed first, but
Mike, I think you are over doing the grinding.
The final seat is a ring no wider than 0.020" and 0.010" is good.
All you need is some 600 carbide grit in oil and a few rotary strokes
You already have a nice shiny finish as the fine grinding slurry does its work you can see a dull ting forming on both parts.
When the ring is wide enough to see and goes all around on both parts you are done.
Once the seat is ground too wide a sneak path has more options to develop.

I am going to say something sacrilegious to some ears:
When the piston moves at 0.5"/second the tiniest leak on piston and valve can give the feel of NO compression, zero, zilch, nada, dead.
When the engine runs and the piston moves at 300"/second nothing escapes, and the same is true at 10"/second when you are starting it.

As long as there is no hole on the piston top the engine will run.
I am not saying this to justify sloppy work, I had factory made model airplane engines that after a season flying felt dead but then it was when they run the best unrestrained by excessive friction.

 

[Longboy]

Toothpaste doe's nothing for steel. Pumice is not in toothpaste. Pumice is valve lapping compound.

 

[gbritnell]

Before I made a collet holder for my 6" lathe I needed a way to hold small rods concentric so here's what I did.
Most 3 jaw chucks will repeat their accuracy or inaccuracy for a given diameter so with that in mind tags a piece of round stock about 1/2 the length of your jaws. Chuck it up and put a scratch mark on the face in alignment with one of the jaws. Now drill and ream for the desired hole diameter. Take the bushing and split it with a saw, band saw, slitting saw or by hand. Now put it back in the chuck and align the scratch mark and insert a piece of rod. Tighten the chuck and indicate the rod. You should find that the rod turns perfectly true. Oh yes, split the bushing 60 degrees away from the scratch mark. I have a whole box full of these split bushings and the still run truer than my chuck by itself.
gbritnell

 

[WOB]

Toothpaste doe's nothing for steel. Pumice is not in toothpaste. Pumice is valve lapping compound.

Pumice is finely ground volcanic glass and is too soft for use on steel. It is used in some toothpastes. All the valve grinding compounds I have ever seen were silicon carbide powder in grease.

WOB

 

[josodl1953]

Maybe turning valves from one piece in one operation might not such a bad idea after all...
Concentricity guaranteed!

Jos

 

[mnay]

This will go against everything said above and I would only recommend it for a slow speed engine. When i built my first hit and miss i was having problems with compression. I very carefully machined my valves and lapped them in, but still had problems with compression. I talked to one of the guys in our group that had built many successful engines and he told me to make the valve stem clearance a little larger and it will let the valve move around just enough to seat. My Little Coles hit and miss runs great. This has worked on many engines.
Mike
Please don't yell at me.

 

[Cogsy]

Toothpaste doe's nothing for steel. Pumice is not in toothpaste. Pumice is valve lapping compound.

Toothpaste is about 1/3 abrasive by weight. Some of the abrasives used in toothpaste include hydrated aluminium oxides, dehyrated silica gels and silicates, which can have quite high Mohs' hardness values. Give it a go - while it won't grind away material at a rapid rate it quickly discolours from the material as it polishes the seat area. Of course if your valve face is machined well enough you only really need the brass/bronze/aluminum seat to be abraded.

 

[Shopgeezer]

Maybe toothpaste manufacturers should put grit sizes on the tube so you could start with 500 grit Crest, move up to 1000 grit Colgate and finish with 2000 grit Sensodyne.

 

[awake]

Not on the tube, but readily available: How Abrasive Is Your Toothpaste - Family Dentist Tree Rochester MN

 

[minh-thanh]

When I made the valve and the valve seat, the last step : I lap valve and the seat together with oil, YES exactly with oil !
Hello MIKE RIGGIN !
Please clean the valve and valve seat
take a permanent markers pen draw around the mouth of the valve seat, then you lap the valve and the valve seat (just a few rounds) you will easily see how the contact surface of the valve and valve seat is, more or less, good or Bad ..., if possible, take a picture and post it up here