How do I lap or hone a cylinder?

[Kaleb]

For my planned project, I have figured out that the cylinder should not need any boring, rather it needs honing or lapping. I have made a water pump cylinder and piston by just boring the cylinder and machining down the piston, but unless it had a bit of slop, it was tight as anything. My question is, how do you lap or hone a cylinder, and how do you get a good free running piston and cylinder which seals well?

 

[Tin Falcon]

honing is done with a stone or hone. There are cylinder hone kits sold in the states for this purpose. the small ones are sold and used for car brake cylinders. I do not recall the smallest size available. There are also hones made for gunsmithing a bunch of tiny balls ons wires.

Lapping is done with a soft material and an abrasive paste. Typically lapps are made of wood, lead ,aluminum ,brass and sometimes cast iron. there are commercial lapping compounds silicon carbide powder and grease mixture clover comes to mind. or you can make your own lapps and paste. with oil and lapidary polishing compound.
Another similar technique it to split a dowel and wrap with silica carbide or other abrasive paper and use that to smooth the bore.
The softer material will olwas be the lap and the harder material will be smoothed away.
And a gentle reminder do not stick your finder inside a bore if it suddenly grabs a finger can be lost quick.
Tin

 

[kcmillin]

Kaleb,
I just finished a flame eater engine and the piston cylindert fit needed to be perfect, because no oil could be used. I used valve lapping compound from the auto parts store, with a brass lapp, then I finished up using fine sandpaper (600 grit) on a wooded arbor until I had a perfectly smooth, and square bore. I spent some time doing this, dont expect it to go fast. You can get an almost mirror loike fi nish doing this. If this engoine is going to run with oil, I woulkd go with a somewhat rough bore, by that I mean a nice crosshatch pattern in the cylinder, this will give the iol somewhere to stick to.

What type on engine are you making?

kel

 

[Kaleb]

I am planning to build a oscillating cylinder steam engine based on Stan Bray designs.

 

[kwoodhands]

Are you sure you need to lap or hone the cylinder? I am not an expert by any means and if wrong I would be glad to be corrected. Because the engine is running on steam, the cylinder and piston should be a loose fit. If it were a IC engine then you may need honing or lapping.
My first running engine was a simple oscillating steam engine. I used a 4" x3/4" brass pipe nipple with the threaded ends capped off. The caps were my cylinder heads.The piston was loose as it was a piece of brass cut with a hole saw and smoothed with a file and sandpaper.
Very crude, but works very well on air. At the time my metal working tools consisted of a drill press, hacksaw and a few other hand tools.
I got the drawing from the "Boys Book Of Engines" a Lindsay publication.
I built a boiler for it and it runs well on steam with an alcohol lamp for fuel.I only run it on air now as the boiler is not much more than a large tomato juice can. I wanted to see it run on steam at least once, I have run it twice on steam and many times on air.

mike

 

[GWRdriver]

Because the engine is running on steam, the cylinder and piston should be a loose fit.

Umm . . . I would disagree with this. Steam is no less needful of a close fit than IC although of course there are differences in what the piston is expected to do and much higher temperatures to accomodate. There are prescribed fits for model steam cylinders of various diameters and materials but none of them are what one could call loose, a couple of thou' after expansion is accounted for. In a perfect world a ringed piston in a steam cylinder doesn't touch the cylinder wall, the rings do the touching, but of course in the real world it often does touch, often at the bottom of the cylinder, due to gravity or clearances in the glands.

For some years now I've used a 3-stone automotive brake hone (3/4" min diam, fine stones) to finish my model cylinders, all of which so far have been cast iron. The results I've gotten have been very satisfactory, and those results could be bettered if finer stones were available. What a brake hone won't do is straighten a poorly made bore, one that is tapered, crooked, or out of round. This takes an automatic cylinder hone, such as a Sunnen machine. Lapping is great, and can correct some bore faults, and can be of benefit when you have very small cylinders where friction (piston or ring drag) can significantly affect the operation, but IMHO for steam cylinders above 3/4" the time it takes to lap isn't repaid in improved performance or ring life. Also, just like IC cylinders, steam cylinders need to have a little bit of texture to them to aid in retaining oil so a mirror finish bore wouldn't necessarily be a good thing.

 

[Cedge]

I've had excellent luck in lapping cylinders using rigid copper plumbing pipe. I use an under sized piece with lapping compound that self destructs as it's used. For a 3/4 bore, I'd use a 1/2 or 5/8 od pipe so that it is free within the bore. Works great here.

As for the "why" of lapping, I'll leave that debate to those who fret over hosts of angels and heads of a pins.

Steve

 

[Twmaster]

Steve, could you elaborate more about how you lap with that piece of pipe? I'm just not getting your technique.

Thanks,

 

[kwoodhands]

Thanks Harry for correcting me. I said a loose fit because I do not know how much less the piston should be in relation to the bore.
I printed out your reply for future reference. I'm an old dog but not to old to learn.
mike

 

[tel]

Harry's advise is sound Mike, as far as 'how much difference' what I aim for is;

With the bore sitting vertically, the piston and rod, when sat in the mouth, should slooooowly subside to the bottom. If it drops with a 'clang' then you are too loose. If it doesn't go anywhere then you are too tight.

Too tight can be corrected by using the actual piston as the lap, with Brasso as the medium, followed by careful cleaning.

 

[Alan J. Richer]

Too tight can be corrected by using the actual piston as the lap, with Brasso as the medium, followed by careful cleaning.

Tel, don;t you worry about getting embedded abrasive media doing it that way - or is Brasso sufficiently soft abrasive to not embed at all?

Alan

 

[Cedge]

TW....
Here is a photo of the last cylinder I lapped/honed, in progress, as well as the finished bore.

check HERE for more on the process.

Steve

 

[DICKEYBIRD]

Great pics and result Steve, thanks.

Question: In the thread you linked to, you mentioned lapping the 3/4" bore with metal polish on the 5/8" copper tubing. Are you just pressing the tubing against one side of the bore? I didn't see any means of expanding the tubing. Not being critical, just curious.

 

[ieezitin]

Cedge.

I am like dickeybird still quite not clear on your method. I noticed from the photograph that there is a little clearance between the pipe and the bore ( which makes sense as a 5/8 od pipe going into a ¾ would have 1/16 clearance ) but how does that touch the bore to lap?

I like the idea of the piping self consuming as its being worn away. Another benefit would be its very easy to reduce its diameter of the pipe just put into chuck and wet sand with emery cloth to desired size. They make copper tubing from ¼ up and its easy to acquire.

All the best. Anthony.

 

[bearcar1]

Hi Steve, I am liking your method of lapping using the CU pipe and compound, how is the pipe held in the lathe? Is it held in the tool post? I have personally never cared for the brake hone ideas as I never trusted the bloody things to hone evenly without introducing a taper. I know one has to keep the device moving but still I do not trust them. This method, if I am following correctly here would seem to assure a square bore. Thanks


BC1
Jim

 

[Cedge]

The pipe is hand held and only makes contact at a single point. I keep it sliding back and forth through the bore. The pipe doesn't wear away, but the compound is designed to wear away as it is works.

The thing that isn't show is the two straight score marks I added down the sides of the pipe to help keep the compound in the bore while it works. It took about 15 minutes to lap the bore smooth and straight. It won't remove a large taper, but it does well to even up a small one.

Steve

 

[tel]

Tel, don;t you worry about getting embedded abrasive media doing it that way - or is Brasso sufficiently soft abrasive to not embed at all?

Alan

All I can really say is that I've never had a problem with embedding using it. Some of the engines I've done this way have seen well over 20 years of perhaps higher then usual usage with no appreciable trouble.

 

[kendo]

Hi Kel and Guy
In the past i have used a piece of wooden dowel with a
vertical split in one end, to which a piece of very fine emery
or wet and dry is inserted and wrapped around, and honed
the bores that way

Ken