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Author Topic: Cox.049 Piston lapping  (Read 1963 times)

Online Dave Hull

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Re: Cox.049 Piston lapping
« Reply #50 on: May 15, 2019, 03:39:51 AM »
Looks really good from the pictures.

Just wanted to confirm we are talking about the same thing when discussing the oil grooves cut into the journal. There should be at least one (the stock cases had two at 180 degrees) on the face of the thrust bearing. Cut these before pressing in the bearing. There is also an axial groove. The cases I have seen have this groove in various different positions. It should be somewhere from 90 degrees before BDC to maybe 120 degrees before BDC. You want the shaft to be "climbing up" on the cushion of oil.

One way to make these would be to make a steel mandrel (dummy crank) and groove it. Then clamp it into the case and drill a small hole right down the airgap so that half of the drilled hole is in the mandrel and half in the case. The diameter should probably be around .025" by guessing. Set your quill depth to avoid breaking thru the front of the case. Done! Other alternatives would be to broach it, if you can make a broach.

If the bore was bigger and not such a high aspect ratio, you could use a handheld gouge. This was done all the time on babbit bearings. I will find and attach a link to a video that shows old time babbit bearing fabrication.

Dave
« Last Edit: May 15, 2019, 04:02:48 AM by Dave Hull »

Offline Juan Valentin

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Re: Cox.049 Piston lapping
« Reply #51 on: May 15, 2019, 07:41:08 PM »
 Hello Dave
                       Your last post is of great help. we are on the same page about the lubrication groove. I made a tool to do as you suggested. I used a grade 5 bolt for the tool, I erased all the markings in the head and turned round. Since I knew the Id of the bushing is .217 inch measuring from a small dimple I made at the center of the head I drilled a .047 inch hole at a distance of .1085 inch then I turned the shank to .2165 and half the hole was removed leaving a channel to guide the drill bit in  making the lubricating groove. I have not decided in which side of the case I will be grooving. The smallest drill bit I have is .040 inch and will try that to see how it works. Thanks for your help. Here are pics.
                                                                                                                                                                                                    Juan

Online Dave Hull

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Re: Cox.049 Piston lapping
« Reply #52 on: May 15, 2019, 08:56:19 PM »
Sweet job, Juan!

You could also cut a flat on the disk of the drill guide. Then screw in a sacrificial cylinder. Make the flat location such that the bottom of the cylinder just touches the flat. Now your drill guide will not rotate, and you do not have to worry (as much) about breaking the small drill.

You could also tap the shaft end of your tool so that you can lock it down on the case with a small screw and washer. Just like you were putting a stud into the end of the stock crankshaft.

I think a .040" drill would probably work.

I would line up the axial oil groove inside the journal with one of the two radial grooves you need in the thrust bearing. That way, there is a clear path thru the thrust bearing contact area to feed the journal bearing. As far as I am concerned, that is a design flaw in the basic Cox engine. A shame, too because doing it right is not harder and it would not have cost more.

Divot McSlow

Offline Juan Valentin

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Re: Cox.049 Piston lapping
« Reply #53 on: May 16, 2019, 09:58:05 AM »
Hello Dave
                        I went out dining with my son last night and when I came back decided to do the oil groove on the crankcase. I drilled but did not go as far as I should have. This morning I decided to drill farther beyond the point I should have stopped to break in the tool. I the first photo it shows how far I got last night and the next this mornings work. I will be trying to buy a smaller drill bit to try it out in the other side of this crankcase as I might end up removing this bushing to examine it up close. Your suggestion worked real good. You didn`t noticed that on the first tool pics I posted the washer and screw. I had cut 4-40 threads on the tool and I made the tool a little short of the end so using the 4-40 bolt and a washer I was able to lock the tool in position. I think I should run the case to see how it performs before I continue experimenting.
                                                                                                                                                              Juan

Online Dave Hull

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Re: Cox.049 Piston lapping
« Reply #54 on: May 16, 2019, 12:48:40 PM »
Aaaah.  Sorry I did not fully understand your tool. You are way ahead of me. Very nice.

I agree with trying to make the groove smaller.

As far as the groove location, all the engineering books tell you approximately where it should go. Cox apparently either knew better, or didn't care at all. I have two cases in my hand right now. One is a cast case and the groove is about where you put yours. I have an extruded case from a product engine and the groove is about 60 degrees forward of the cylinder location. The groove in the cast case is slightly larger than the one in the extruded case. Both are smaller than your first groove. The Cox extruded case groove has machining marks that suggest that they broached the groove. The groove in the extruded case is parallel-sided. The cast case groove has some taper to it. It looks like the depth gets shallower the farther forward it goes.

I would try putting the groove about 45 to 90 degrees after (in crankshaft rotation direction) the cylinder attachment centerline. You want to feed oil into the bearing before the high pressure (load) area, which is going to be opposite the cylinder. (The bottom half of the case if the cylinder is up.) The idea is that the crank rides on a wedge of oil and does not touch metal to metal. If the groove is too big, or is in the wrong place it may act like an oil scraper.

Good progress, Juan. I am very interested to see how it runs.

Dave

Offline Juan Valentin

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Re: Cox.049 Piston lapping
« Reply #55 on: May 16, 2019, 07:49:52 PM »
Hello Dave
                       I finished 2 crankcases and proceeded to asemble an engine from parts I have,new galbreath/nelson head,new crankcase with bronze, old red backplate,hi perf crank  and an old number 5 cylinder and piston. The piston is so loose it comes out thru the top of the cylinder. I had tried to do the groove on the cases with a .020 inch drill bit my machinist friend gave me but it just didn`t work with the tool I made because the groove on the tool is .020 inch. The drill started to cut into the bronze but it followed the path of least resistance so it was back to using the .040 inch drill bit. I elected to drill the holes in the 7:30 to 8:00 hour location thinking that after ignition the conrod is pushing the crank to the right and down viewing the engine from the back backplate removed. I did a vertical slot on the middle top and bottom of the bushing.
                      I ran the engine on 25% VP nitro 20% Blendzall castor oil and 55% VP 1320 Methanol. The engine turned above 19,000 Rpm which surprised me because I was more interested in seeing how the oil would come out. It came out with no traces of metal wear as you can see in one of the photos it came out amber colored. In all I`m happy that I was able to do the mod correctly with a good result. I hope you guys enjoyed the Thread and the Pictures and that they might be of use to you.
                                                                                                                                                                                       Juan

Offline Motorman

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Re: Cox.049 Piston lapping
« Reply #56 on: May 18, 2019, 04:55:44 PM »
The bushed cases that KK did had two bushings. The case was bored from the front and they left the aluminum thrust face alone. The first bushing was much the same as you have done only shorter. The nose bushing was an oilite bronze bushing right off the shelf.

The oil groove can be made by grinding the end of a small triangular file flat and sharp then simply broaching the groove by hand. The oil groove is not needed because the clearance you want to spin that fast will let plenty of oil go forward. Also a big oil groove will trap oil and it will dry in there.

The other thing is to keep your bushing thin so heat can transfer to the aluminum. A good scheme is to press in a bushing with a smaller inside diameter then bore it out once it's in place. This way you can make it as thin as you like and it won't collapse when you press it in.

Motorman 8)
There will be a sunny day and we will fly our airplanes.

Offline RknRusty

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Re: Cox.049 Piston lapping
« Reply #57 on: May 19, 2019, 01:47:34 PM »
This has been a really fun thread. I admire the work you guys can do with metal.
Rusty
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while you're doing it!

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Offline Motorman

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Re: Cox.049 Piston lapping
« Reply #58 on: May 20, 2019, 03:35:23 PM »
There's a story about a machinist talking to a guy and he says I just made a new gear. The other guy says I didn't know you could make those, I thought you had to buy them.
There will be a sunny day and we will fly our airplanes.

Offline Juan Valentin

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Re: Cox.049 Piston lapping
« Reply #59 on: May 20, 2019, 07:02:51 PM »
The bushed cases that KK did had two bushings. The case was bored from the front and they left the aluminum thrust face alone. The first bushing was much the same as you have done only shorter. The nose bushing was an oilite bronze bushing right off the shelf.

The oil groove can be made by grinding the end of a small triangular file flat and sharp then simply broaching the groove by hand. The oil groove is not needed because the clearance you want to spin that fast will let plenty of oil go forward. Also a big oil groove will trap oil and it will dry in there.

The other thing is to keep your bushing thin so heat can transfer to the aluminum. A good scheme is to press in a bushing with a smaller inside diameter then bore it out once it's in place. This way you can make it as thin as you like and it won't collapse when you press it in.

Motorman 8)


Hello Motorman
                                I used to read Joe Klause 1/2A column in Model Builder magazine and I still have those mags stored in plastic cases. I was not aware of many of the things he did,All I can remember was the NVA assy and if I remember well a Backplate mount  for Cox .049 TD. I would love to read those mags again. I collected mags for a long time and quit when they became catalogs. Is very interesting what he was doing when bushing the cases  he must had a good reason to do them that way. I have some prelubed bronze maybe I will try that some day. I made the bushings big thinking that I needed extra material to be able to do the oiling groove without the bushing collapsing on me. I could have made a smaller groove but I noticed that the newer crankcases have a big groove. I left a case with no groove and I assembled and engine but had two very short runs backwards which I stopped and then my battery for the glow plug died. I haven`t tried lately been spending time with my son and grandson.  Thanks for posting the information I saved it in my engine mods folder.
                                                                                                                                                                   Juan

Offline Juan Valentin

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Re: Cox.049 Piston lapping
« Reply #60 on: May 20, 2019, 07:14:03 PM »
This has been a really fun thread. I admire the work you guys can do with metal.
Rusty

 Hello Rusty
                    I enjoy doing it and sharing it. That way I can collect information from other guys that have done similar work. I like to post photos because that way you guys can join me on my trip of what I`m doing. I have always heard that a picture is worth a thousand words. For me doing new things on my engines is OJT (on the job training) thanks for your kind words.
                                                                                                                                                                        Juan


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