Yet Another Webster Begins

[fcheslop]

Make a piston ring clamp, just some shim or beer can and a hose clip
Iv still snapped a few rings though
You wont get an accurate measurement of the bore using vernier caliper as the jaw overlaps so you measure a chord
https://www.machinemart.co.uk/p/cht257-piston-ring-compressor/

 

[Brian Rupnow]

Bob--did you turn the ring grooves on the piston to the diameter recommended by the ring supplier? The rings should also be a sliding fit into the grooves in the width dimension. Not a sloppy fit, but not tight either. I found on my sideshaft engine that the rings were about half a thou over width, so I spread a bit of 600 grit aluminum oxide paste on a sheet of glass and lapped both sides of the rings, so they would be an easy sliding fit into the grooves cut by my 3/32" parting off tool.

 

[CFLBob]

I have a 45 degree lead angle there, as the drawing says, "SMALL 45 CHAMFER TO EASE RING COMPRESSION AND PISTON INSERTION," but I didn't try tapping.

As you can see, this isn't the regular Webster piston. I was concerned the cylinder was too big for the piston, and machined a handle onto it. I was going to check to see if I needed to make a new piston with as little work as I could get away with. If the piston looked like it would work, I'd do the details in the piston.

 

[CFLBob]

Make a piston ring clamp, just some shim or beer can and a hose clip

Why not just the hose clamp? It seems all the shim stock does is spread the compression a little.

 

[Brian Rupnow]

Bob--Truth is---If your piston has rings on it, you can safely make the piston .002" less in diameter than the cylinder. The expansion of the rings will seal anything trying to get past the piston. I took a look at the chamfer on the cylinder called up on the original Webster drawing, and in my opinion it's too steep an angle. The rings will just cut into the end of the cylinder, raise a burr, then break the rings if you try to force things. I stand by my 20 to 30 degree included angle about 5/16" deep into the cylinder, and lots of oil when trying to get the piston in. After turning the taper, carefully remove the abrupt inner edge of the chamfer by using some 200 grit carborundum paper--don't let it grab your finger and twist it off. I do this under power, but damned carefully. The gear clamp you show has a dirty little secret--it stays flat in the area next to the screw. It will uniformly squeeze the ring--except in the area close to the screw. In that small area, the rings will still stick out far enough past the edge of the piston to break the ring.

 

[CFLBob]

Thanks, Brian. I can modify that to be more like 20 or 30.

 

[werowance]

for what its worth, i used wider than the ring zip ties to compress my rings so i could slid the piston into the cylinder.

 

[CFLBob]

for what its worth, i used wider than the ring zip ties to compress my rings so i could slid the piston into the cylinder.

Now THAT's an idea! I'm also a ham radio operator. I have bags of zip ties to pick the perfect size for the job.

 

[CFLBob]

Well, I'm not dead and I haven't given up on this. Nothing I tried would get that piston into the cylinder, and after a self-imposed attempt to learn to measure better, I see why.

I have three measurements at the bottom and top of the cylinder. Their diameters are: 0.8831 top and 0.8819 bottom - that's .0011 bigger at the top than the bottom. That's a little concern because I figure it's probably better if the bottom were bigger, not the top. (Top is where the combustion chamber is) As it is now, if the piston gets in the bottom as I assemble the engine, it will be looser - less compression - at the top.

The piston, over the rings was 0.896 (!) No wonder it couldn't get that into the 0.882 cylinder! The piston itself is 0.878 other than over the rings.

The drawings say the cylinder should be 0.875 and the piston 0.873. There's no way to make the .883 cylinder smaller, but I could turn another piston, I just don't know what to do about the piston rings. Then I got thinking that the piston over the rings was 0.896 and that's 13 mils bigger than the cylinder, so can I take up the 13 with rings instead of the 2 that it's originally specified at? I'd make the grooves for the piston rings 13 mils deeper.

One thing I knew is that the rings were too tight in their slots, so I tried to remove the rings and destroyed them. You can see them here:

Since I'm starting over, I thought I'd ask what the rest of you think about which way to go? New piston and new rings? Modify (and finish) this piston? Are polymer rings a good option here?

 

[Brian Rupnow]

Bob--A Viton o-ring will work fine, but you will have to make a new piston to accommodate one. A ring of nominal 1/16" cross section x 0.875" outside diameter costs about a dollar. You only need one ring, and the groove should be 3/32" wide x .056" deep, about 5/16" down from the top of the piston. To make life simpler the piston can be .002" smaller in diameter than the cylinder bore. Run a 50:1 mix of two cycle oil in your fuel to keep the ring lubricated.

 

[CFLBob]

Thanks, Brian. Will a 0.875 piston ring be good for a cylinder that's 0.883 ID, .008" bigger?

 

[minh-thanh]

CFLBob !
The way I do it:
While I cut the groove for the ring, I usually insert the ring into the groove to make sure the groove is deeper than the ring's height

One more thing: I usually avoid talking about measurement results such as: 0.02 mm, 0.05 mm, 0.001 inch, 0.87 inch, or .., It is like hiding the measurement result, tolerance ... Because my measuring tools are cheap and inaccurate, sorry

 

[CFLBob]

Hi, Minh Thanh!

I like that idea of sliding the ring into the groove like you do.

I understand what you mean about measurement tolerance of measuring tools. Very important things to know.

 

[Brian Rupnow]

Bob--try it and see. Viton o-rings sold as 1/16" cross section are actually 0.070". With a 0.056" deep ring groove, that gives .014" of "squish" radially. If you have .004" of radial clearance the viton ring should probably seal that with no trouble.

 

[CFLBob]

I've been shopping at Grainger's - there's one a couple of miles from home - and they say they have rings with Nominal ID of 3/4", Nominal OD of 7/8" and actual OD of 0.879". $6.38 for a bag of 50. Since I've got an ID at the combustion chamber end of 0.883, it seems to be right on the edge.

The next size up 13/16, 15/16, and 0.941, which seems too big every way.

 

[minh-thanh]

CFLBob !
There are 2 ways: 1 Ringless . 2 Rings (buy it, make it, or O-rings) It only needs compression. Do it the way you want and be comfortable with it.

 

[Brian Rupnow]

Do not buy normal buna N o-rings. they will melt. use Viton or go back to cast iron.

 

[awake]

Bob, take what I am about to say with a BIG grain of salt, as I am still a complete novice on IC engines. But in my research on o-rings to use on my first build (in progress), it appeared that there was some wiggle room with respect to stretching the rings. IOW, if you size your piston appropriately, and size the grooves appropriately for the cross-section of the ring, then you can get away with stretching the ring a bit to fit in the groove.

I think. Did I mention salt?

But the key figure, as I understand it, is the cross section. Note how Brian has described it: nominal 1/16" rings with an actual cross section of .070"; grooves .056" deep by .094" wide; "squish" of approximately .014" (maybe actually .013" if your piston is .002" smaller than the cylinder). Even if you were to stretch the ring a bit (i.e., the diameter at the bottom of the groove is a bit larger than the actual ID of the rings), you won't change the cross section all that much; maybe you'll only get .012" of squish rather than .013" but it should still be enough.

I think. Anybody need salt?

 

[awake]

To follow up a bit more - on my build-in-progress, also of a Webster (somewhat modified), I wound up using metric Viton o-rings (that was what was more readily available). They are described as 19 x 1.5mm, which means 19mm ID and 1.5mm cross section, for an OD of 22mm. It appears that the 1.5mm cross section is pretty much spot on, not a nominal figure. So, in inches, that is .748" ID, .059" cross section, and .866" OD.

My cylinder came out .8755". I sized my pistons as follows: .874 OD, with grooves .080" wide by .052" deep. This should give me a "squish" of around .006" (maybe too little, but read on). Note what the diameter at the bottom of the grooves is: .874 - .052 x 2 = .770" - somewhat larger than the .748" ID of the o-ring - so the o-ring is actually stretched a bit when seated in the groove. And that is important, because otherwise the OD would not be large enough - only .866 in a .8755 cylinder.

But if my understanding is correct - always a dangerous assumption! - then the ring stretches when seated in the groove, and the actual OD winds up at .770 + .059 x 2 = .888, maybe a little less due to the stretching reducing the cross section slightly.

Does it work? Ultimately, that remains to be seen - I am getting close, close, close to attempting a first run, but not there yet. However, I have started to do some compression testing. First I assembled the flywheel, crank, rod, piston, ring, cylinder, and head. Even with the relatively small amount of squish and a generous helping of oil, it was quite stiff. I hooked it up to my mini-lathe and ran it for a few minutes, adding oil from time to time, and it got quite a lot smoother.

Then I added the spark plug. I pressed my finger over the hole that leads out to the valve cage, and tried to turn it over. Based on this highly scientific methodology, the compression seems to be excellent. Now I'm getting ready to add the valve cage and valves, and we shall see how badly it leaks ...

Don't forget the salt!

 

[CFLBob]

It has been another month, and while my progress moves as fast as a glacier, I'm still working on things.

Let me start with the hardest part to measure, the cylinder. In post 129, I said that I had found that the cylinder was tapered so that the combustion chamber end was wider than the entrance end, and if I could get the piston into the cylinder it would be looser in the most important zone. That told me it seemed like it would be better to open up the entrance to the cylinder (the bottom) so that it's at least as big as the combustion end. If I made the opening too big, that's better than the way it was.

I had measured it before and wanted to verify the measurements so I retook all the measurements and they were the same, which is a good thing. I set up the lathe to carefully trim the inside diameter of the cylinder. I made a cut that should have taken .001" diameter off the first 3/4" of an inch of the bottom. There was virtually no metal taken off - it looked more like dust than chips. I thought I must have made a mistake setting up, so I reset the tool to the start of the cut and set it to take off another .001". Again, no evidence of chips, just some fine scrapings of dust. The micrometer and telescopic gauge were telling me that the diameter hadn't changed, saying that it wasn't cutting. This went on a few more times until I had to think even I couldn't be that screwed up and the settings on the lathe couldn't possibly be right. Still no evidence of cuts. I stopped trying to cut, thinking I'd eventually mess it up if I kept going.

After a few days of trying to figure out what was going wrong, I took a look at the carbide insert on my boring bar with a 10x magnifier and thought it seemed too rounded; perhaps the cutting edge was pushing the steel away from the tool at the contact point rather than shearing some off? I rotated the insert to get a fresh point and started cutting without changing the lathe settings at all. As soon as the tool touched the cylinder wall, I could see chips coming off, and before the tool had cut 1/8" into the length, I saw it was taking off too much. Eventually I got a bell mouth by about .002 to .003.

Last weekend, I had ordered replacement piston rings for the two I broke - and two spares. They got here Wednesday. After a couple of days of consistent measurements of the cylinder, I took it off the lathe and put the piston back. Friday, I addressed the single biggest problem I had seen with the piston: the grooves were too tight and the ring wouldn't go to the bottom of the groove. Using the same cutoff tool I used to cut the grooves, I got my positions reset and took off .002 of width on each ring's groove. How much gap should there be on the sides? I don't have any .001 or .002" shim material, or a feeler gauge that thin.

You can see a little light under the ring in each groove (the rings are called out at 3/32" wide but measure a little short of that: 0.090". I haven't tried to install the rings, yet - I want to remeasure another dozen times since I don't know how to remove a ring if it's not right.

The drawing for the piston (sheet 6) shows the bottom of the ring grooves as a reference dimension and if you take the reference (0.771) from the nominal dimension of the piston (0.873), that says the groove should be .051 deep. My piston's OD is right around 0.878, not 0.873. Should I try to keep the depth 0.051? I'm using the piston rings from Dave Reed as called out in the drawings. I think the rings were .044 thick.

I've spent a lot of my time making myself go to school on measuring ID with the telescopic gauges and just generally measuring better.