Valve problem

[MIKE RIGGIN]

I have a Depenbusch Hit n Miss almost complete. My valves don't seem to be sealing very well and I think my angles are probably way off. The seats were done with a 60 degree countersink and the valves are done at a 41 degree angle.Does that sound right? It seems to me that the angles should be about the same on valves and seats. What is common practice on valve angle?

 

[Longboy]

Cut your valves and seat at the same angle. I use a standard 45 degrees.

 

[awake]

Mike, one of the things that can get confusing very quickly is what the angle is measured from. If you used a 60* countersink, that could be described as a 30° angle on each side, measured axially ... but measured radially, that could be considered a 60° angle on each side. With that in mind, I suppose one could talk about different angles for the valves vs. the seats, if one is being measured axially and one radially ... but in that case, I would expect the angles to add up to 90°. The point is that the angled face of the valve should perfectly match the angled face of the seat.* I can't envisage any way to get your 60° and 41° angles to match up in that way.

*Some builders create a 1° difference between the valve and the seat, but that is a bit misleading, because they then lap the valve and the seat together - so the actual contact area is, as noted above, a perfect match between the angle of the valve and the angle of the seat.

 

[coulsea]

"Some builders create a 1° difference between the valve and the seat, but that is a bit misleading, because they then lap the valve and the seat together - so the actual contact area is, as noted above, a perfect match between the angle of the valve and the angle of the seat." Not totally correct because you only lap to get a complete seal all the way around but not over the entire width of the seat. I use 45 degrees for both but using a countersink for the seat and a D bit grinder for the valve, so using two different tools it is unlikely that the angles are actually the same. I lap with diamond paste.
If your valve guides are a good fit your valve and seat need to be equally good, a bit of slop in the guide will allow the valve to settle into the seat but is not good for the inlet valve if it is sucking air through the guide and messing with the mixture.

 

[coulsea]

I have a Depenbusch Hit n Miss almost complete. My valves don't seem to be sealing very well and I think my angles are probably way off. The seats were done with a 60 degree countersink and the valves are done at a 41 degree angle.Does that sound right? It seems to me that the angles should be about the same on valves and seats. What is common practice on valve angle?

Are you going to do a build log? I am starting a new one of these engines as air cooled, I have already done a hopper cooled one. the small one is an upshur then the depenbusch and a real 2hp buzzacott

 

[awake]

Neat collection, coulsea!

"Some builders create a 1° difference between the valve and the seat, but that is a bit misleading, because they then lap the valve and the seat together - so the actual contact area is, as noted above, a perfect match between the angle of the valve and the angle of the seat." Not totally correct because you only lap to get a complete seal all the way around but not over the entire width of the seat.

Correct - not over the entire width of the seat; just the actual contact area. That may only be .01mm, but in that tiny area, assuming one has lapped the valves, the angle should be the same.

 

[gbritnell]

A standard countersink is 82 degrees. That means the included angle. So one side of the angle is 41 degrees. While there are c'sinks with different angles I don't know if I've seen one of 60 degrees. If this is what you have used then there is no way that the 45 degree valve angle and the seat will ever seal.
If you have cut the seats this way there is probably no way to rescue the angle. By the time you would recut with a 45 degree cutter the seat will be way too large.
Valves and seats should be 45 degrees. On full sized engines there can be what's called a 3 angle valve job. That means the initial cut is made with a 60 degree cutter. (sharp angle) The seat is then cut 45 degrees. The top angle is cut with a 30 degree cutter (shallow angle) 120 included. This is meant to reduce the seat size so it's not too large. For model engines the seat angle only needs to be .020-.03 wide.
gbritnell

 

[awake]

George, I was thinking perhaps the OP meant that he cut the valve seats using a center drill - that would give the 60° angle. But you've raised an interesting point - if the OP used an 82° countersink, then cut the valves at a 41° angle to match, it could work. The problem there would be how good the finish is on the countersinked (countersunk??) hole.

 

[popnrattle]

I do 45 deg on the valve and leave my head seats sharp with a close fit on guides and stems. Especially on the intake stem/guide for no adverse affect of fuel mixture adjustment( like to run mine slow rpm). This allows the sharp seat to groove the valve head for sealing much easier/faster with lapping compound. After I get her running, the prolonged break-in period "pounds" the seats together for better sealing.

 

[MIKE RIGGIN]

Thanks for all of your helpful replies. A lot of the problem here is the fact that I have been working on this thing for a very long time. I probably made this head 15 years ago. I thought I had just used a drill bit to countersink the seats, but I noticed that the angle didn't match a drill bit. So I said " Let's just see what's in the toolbox"! Lo and behold, there was a 82 degree countersink which matches perfectly the angle of the seats, so I am not far off after all. 41 degrees on the valves now makes perfect sense. Yesterday I did some aggressive lapping and I am getting a much better seal. Still not 100%, but much closer. How much time do most of you spend lapping valve seats?

 

[awake]

Mike, what size grit are you using to lap? The fact that lapping is improving the seal suggests you are on the right track; to get as close as possible to 100%, you will likely need to move up from a more aggressive grit (maybe 320) to a finer grit (600), maybe even to a superfine (1200).

 

[MIKE RIGGIN]

I don't know, I have a small can of Clover compound that I have had for probably 40 years. One side says course and one side says fine. Where do you even get the different grits?

 

[awake]

Hmm ... it sounds like your can changes grit depending on which side you are facing!

I would venture to guess that probably what you have is relatively coarse. I should note that I am very much a newbie, with much to learn! But as I understand it, valve grinding compound for automotive use tends to be coarser than what may be needed for model engine use.

I'm aware of at least a couple of different approaches to getting different grits. Some buy the grits as a dry powder, then add in oil to make a paste to use in grinding. The powder can be diamond, silicone carbide, maybe also aluminum oxide; I don't know enough to know the advantages or disadvantages of each, though I have seen the sentiment that "diamonds are forever" and that is not a good thing - you don't want the grit to continue to grind once you are done lapping!

The other approach is to buy the grinding compound already mixed up as a paste - probably similar to what you now have. I went this route via eBay. Sometimes the ads just say "valve grinding compound" without specifying a grit, but with a little digging I was able to find listings for different grits. As I recall, it wasn't overly expensive, and using them for model engines, the containers I received will probably last the rest of my life. I am under the impression that what I found is silicone carbide as far as the grit, but I'm not entirely sure - I'll have to go back and look at the containers to see if they say.

 

[Brian Rupnow]

I have built over 20 i.c. engines, and have done just about every wrong thing that can be done at one time or another. Let me share some wisdom.--I never let my valves seat against the aluminum heads. I always use brass valve cages Loctited into the aluminum head. Why---because if you totally screw up a valve seat in a valve cage, it takes about 10 minutes to make another valve cage. If you screw up the valve seat which is machined into an an aluminum head, you may be looking at a days work to make a new head. I use common cold rolled steel to make my valves from. I have never had a valve burn or otherwise go bad. I use brass valve guides because once the engine starts to fire, even if that valve was leaking a little bit, the expansion of burning fuel in the cylinder will drive the valve face into the brass so hard that the brass deforms a bit to become a perfect seal. I learned long ago from George Britnel how incredibly important it is to have the valve seat perfectly concentric with the valve guide portion of the valve cage. George even designed a cutting tool that is guided by the valve guide portion of the valve cage and turned by hand to put the valve seat into the brass. I never use a power tool to lap my valves. I lap them once with 350 grit aluminum oxide suspended in a heavy grease, clean the valve and seat with laquer thinners, lap again with 400 grit, then clean and lap a final time with 600 grit. I make my valves to have an "included angle" of 92 degrees. I cut the valve seats to an included angle of 90 degrees x about .015" to .020" wide. When I lap a valve, I apply the lapping compound to the face of the valve and spin the valve back and forth between my fingers, to a count of about 50 times while maintaining a pressure between the valve face and the valve cage. Do not let any compound get down into the valve guide portion of the valve cage---you don't want to be lapping the valve guide area. On my engines I don't put the hole thru the side of the valve cage until after it is loctited into the cylinder head, then drill the passage thru the cylinder head and the valve cage at the same time.

 

[Cogsy]

My procedure is similar to Brians' although there's enough differences to mention. I cut my valves and seats at the same angle (nominal 45 degrees) but as the seat cutter is made in the same setup as the valves themselves it doesn't matter if it's a little off). I also use brass cages and only cut the seat after the cages are pressed in and the ports drilled.

I try to get the best surface finish I can on the valve heads when I make them and I won't use one that looks 'rough'. To lap them (a trick I learned from Brian I think) I leave an inch or so of material on the head end of the valve so I can grip it with my fingers. I only ever use standard, white toothpaste as lapping compound. I revolve the valve through about 180 degrees for 30 seconds or so while putting some pressure on the seat, then make sure there's still lapping compound where it needs to be, rotate the orientation of the valve 90 degrees and go again. I repeat this procedure 3 or 4 times and have a look at the contact line that is visible on the valve head and seat. Generally it's good to go at that point. Then I part off the extra material I was using as a handle and the valve is finished. They may leak a small amount at first but seal 'good enough' to get enough compression to start the engine, then combustion pressures finish off the sealing process fairly quickly. It works for me and not only is it cheap but it smells nice and peppermint fresh while you do it!

 

[tornitore45]

As mentioned before concentricity between valve stem and head is essential.

One way is to turn the stem and head in one chucking but then you are faced with turning a long skinny stem with good finish and no taper.
Some people can do that well.

I prefer to braze a SS head over a drill rod stem, and then machine the head.
To make sure the head has clearance for the silver solder I drill large but leave a tight fit hole for small length. That part will be turned away.
The stem is very accurately centered and center drilled with a small center.
The stem protrudes about 3/16" on the head flat side when is brazed.
Now for the trick
The stem is left pretty long like 2.5-3" and the stem protruding off the head is kept in the tail stock fitted with a dead center.

The valve head is machined while is rotating on the dead center.
Any small (0.001" max) misalignment between spindle and tailstock is divided by the ~16
The protruding stem is faced off at the end and the stem parted off at the correct length.
The valve is lapped by grabbing the stem in pin wise. Easy to rotate while applying a moderate pull on the stem.

 

[Shopgeezer]

If you can grind steel valves with toothpaste I wonder what it is doing to your teeth?

 

[tornitore45]

Toothpaste is a well known very mild abrasive.
If you have a fogged plastic lens, for example, toothpaste may be the ticket to make it transparent again.

 

[awake]

Plus, if you use toothpaste on your valves, the fluoride will help prevent any micro-cavities from forming!

I had wondered about toothpaste as a lapping compound. As I recall, toothpaste uses pumice as the abrasive - but I don't know what pumice is, chemically speaking, or how it compares to aluminum oxide or silicone carbide. I wonder if it might be something that naturally breaks down, minimizing the risk of leaving any grit behind?

 

[Brian Rupnow]

Pumice is volcanic rock, ground to a very fine powder.