Aluminum Cox .049 Pistons

Iceberg Tue Apr 09, 2019 11:07 pm

I have just now repaired an older Widow. Finally after several issues including cracked backplate lug I got it going. Then discovered also that the piston and cylinder are only about 50% compression when hot. I went into the parts I have and found one cylinder and piston that are new. The piston appears to be completely aluminum. Not having the normal copper top piston. I usually think the pistons are steel. What is with a complete aluminum piston? Are they racing style or are the a cheaper version of the same thing? Compression is good now but will run later when I can to test.

In addition the crank case had developed a bit of worn material inside in the front behind the prop plate. The crank was wobbling in the front a bit and a few droplets of black oil (worn powdered aluminum) was dripping out. I have fitted a different crankcase to the motor that doesn't have this wear. I have never had a crankcase loosen in the front like that. What could be the reason for that? The worn material might be the thickness of a paper or so enough to feel the wobble.To insert a sleeve then I guess could be done. Like a helicoil idea without thread. Any comments? I have all the lathes at my disposal however all in metric.

Like I said an older motor being brought back from the dead.

Thanks
Iceberg

edw Wed Apr 10, 2019 3:54 am

Bent crank from bad crash? If possible, use a dial indicator on it, or maybe try a straightedge along the length of crank.

coxaddict Wed Apr 10, 2019 7:00 am

When I first started running Cox engines after a 30 year layoff, the first one I rebuilt ran great with good compression. After a few runs I noticed that dark streak oozing from the nose of the crank. Now the engine would barely run. I used the wrong fuel which caused the premature wear. Some people have had success using 100 percent synthetic lubricant in the fuel. Maybe it just needs the correct additive in the mix Sad

Dave P. Wed Apr 10, 2019 7:38 am

In addition to not using the right fuel mix, an unbalanced prop will wallow out a crankcase like that in a hurry.

aspeed Wed Apr 10, 2019 9:39 am

I have never seen an aluminum piston, but maybe it works good. It would likely grow when it heats up, but maybe it has a silicon alloy or something. I have had more problems with the case wearing out near the back, but as mentioned a heavy out of balance prop could wear it badly. I have made bushings on the lathe. I turned a tube and drilled and reamed the case on a drill press or mill with a .001" press fit. The bore was a perfect fit till I pressed it in, then it collapsed a bit, and I needed to lap it again. I did a few and never really tested them, and didn't identify them, so I don't know which is which without disassembling them. I kind of got away from the reedies because of those leaking problems.

Iceberg Mon Apr 15, 2019 6:34 pm

Spun the block on the lathe and got a super nice result. Shame all blocks didn't come brass sleeved.
Iceberg

aspeed Mon Apr 15, 2019 7:33 pm

The Bees are so bad. The Tee Dees are fine, maybe because they are longer or a bigger diameter or something. Nice to see someone machining on them. You can buy new cases, but it just seems wrong to me.

1/2A Nut Tue Apr 16, 2019 2:26 am

You are the first that I know of that has taken this step. Will increase rpm no doubt.
The engine will run more steady with less heat with a good centered prop you should
yield another 600 - 800 rpm Thumbs Up

Post results when you can thank you.

getback Tue Apr 16, 2019 8:17 am

Good job on the bushing , i have heard of another person that did this with one of his replacement Porsche valve guides Very Happy

Yes do a report if you can Popcorn

aspeed Tue Apr 16, 2019 8:23 am

I have not tested mine as I am not sure which cases I had done. It was over 20 years ago, and I have got a few more Bees since. Wish I identified them. The fit has to be just right, Lots of guys liked the old aluminum case to be lapped out to go faster, then not much later the runs would be erratic. Up down up down. I would guess the bushing would be faster overall just to have a steady run. I had done a Stels .049 around the same time maybe early 1990s when they first came out. The motor would not get a steady run because there was a .003" slop in the bushing. I got the motor for a deal to play with, and made up a rear bushing. Turns out it came with a separate aluminum one already, but it was too loose fit. That motor finally turned some great rpms... for the whole tank. After that time none of us bothered with speed flying any more.

gkamysz Tue Apr 16, 2019 8:56 am

I have a sloppy TD case here, the crank is also measurably worn at the rear. I have no history of the parts though, just from an Ebay buy. Brass will work, but probably won't last as long as a bronze. I intend to try sleeving a case for some experiments as well.

Regarding the aluminum piston there was a thread recently. https://www.coxengineforum.com/t12619-cox-aluminum-piston

aspeed Tue Apr 16, 2019 9:27 am

I would use bronze if available. I could only source brass when I did mine. In regards to the aluminum piston. Your reference to the other thread mentioned coefficient of expansion for aluminum to iron and brass etc. The aluminum was higher than brass which would lead me to believe that the high silicon aluminum has much lower numbers than the normal aluminum alloys. An ABC setup gets looser as it heats up. The greatest benefit of the system. That would make me believe the aluminum piston would fall between the coefficient number of iron and brass. What aluminum Cox used is unknown too. I am sure Cox experimented thoroughly and made the right decision for the aluminum piston. Too bad the brass liners need plating. It would make things so much easier.

gkamysz Tue Apr 16, 2019 11:45 am

aspeed wrote:I would use bronze if available. I could only source brass when I did mine. In regards to the aluminum piston. Your reference to the other thread mentioned coefficient of expansion for aluminum to iron and brass etc. The aluminum was higher than brass which would lead me to believe that the high silicon aluminum has much lower numbers than the normal aluminum alloys. An ABC setup gets looser as it heats up. The greatest benefit of the system. That would make me believe the aluminum piston would fall between the coefficient number of iron and brass. What aluminum Cox used is unknown too. I am sure Cox experimented thoroughly and made the right decision for the aluminum piston. Too bad the brass liners need plating. It would make things so much easier.

Cox did the easiest cheapest thing and that was steel on steel. The fact that they didn't stick with aluminum pistons, which wouldn't need hardening, I think proves it wasn't feasible. Break out your materials books and investigate coefficients of expansion. Depending on alloy, they are all over the map and many overlap within the ranges of brass, bronze, aluminum, and steel. Pick two, then figure fits and tapers that work the best. Should be ready for racing by Saturday right? Beer Cheers

I've looked into it and done the math, it's not as simple as it sounds. If ringless aluminum on steel worked, it would be in use.

aspeed Tue Apr 16, 2019 11:47 am

F4D Phantom II Tue Apr 16, 2019 12:48 pm

I also noticed the crankshaft play when I started to break in an .049 Reedy for my plane. Used a diesel crank and noticed more play with it and aluminum on the oil residue coming from the front of the case. I checked and the diesel crank read one thousand under the factory crank reading. An out of balance prop or a wobbly drive plate can cause more vibration and wear. I made a tool to install my drive plates in hopes of improving prop tracking and reducing the vibes. I use a micrometer to make sure the distance between aluminum plates is the same before I pull the crank into the drive plate. here are pics.
Juan

Aluminum Cox .049 Pistons Index.php?action=dlattach;topic=52863

Aluminum Cox .049 Pistons Index.php?action=dlattach;topic=52863

1/2A Nut Tue Apr 16, 2019 11:20 pm

A new case with bearings would give a Killer Bee a sharp stinger.
Lug free TD case would be worth doing for the .020 / .05
No more leaky 3 part case no chance of it loosening in flight.

getback Wed Apr 17, 2019 8:31 am

That's a good looking tool you made Juan , I had bought New sometime back a Killer Bee CC and a shaft to assemble for use did it and it had so much play it would barely run , got in touch with the vendor and they sent me another it was the same,, So now i have well Parts of no good ?! Rolling Eyes

Iceberg Wed Apr 17, 2019 8:36 am

Love the bearing on the Venom. All should be bearings!!!

On the sleeve I spun out is actually bronze sorry I said Brass. My mistake. I'll post the results when I put back in the widow that started wobbling.

Thanks
Ice

F4D Phantom II Wed Apr 17, 2019 8:23 pm

Hello Iceberg
I find what you are doing very interesting,did you use a jig to place the crankcase on the lathe? I liked what you done and would like to try to do one. I went to a friends machine shop and got me a small piece of bronze to use. I`m assembling one from new parts but the case hole seems to be big and the diesel crank is one thousand under the original crank size.
Juan

Iceberg Thu Apr 18, 2019 4:20 am

Hi Juan sorry I'll be away a few days but will give some specific details on how lathed and inserted the bronze sleeve soon. I lathed it so tight needed to lap it with 1000 grit wet to allow the crank to just slide in perfectly.
Thanks
Ice

aspeed Thu Apr 18, 2019 10:20 am

I drilled and reamed and then lapped mine. Then when I pressed it in with a .001" interference fit, it was tight again, so I lapped it again. I used Silvo polish. It has to be able to get in the hole a bit first.

Iceberg Mon Apr 22, 2019 12:51 am

Hi Juan
I drilled the block front out with the wobble with a 7.04 mm flat end milling bit in the milling drill press as seen in the picture. Then with the lathe in the picture I used bronze and milled out an insert with 7.06 outside diameter. The inside diameter I milled 5.2 mm for the crank to pass and had to lap an additional .1 mm after pressing in the new bush with 1000 grit wet in a drill until it fit in perfectly. I just pressed the bush in with the .02 oversize with a bench vise to ensure it fits inside the crank nose tight enough not to ever come loose.

I will try the block soon but I have every confidence it will work excellent. I did like the inserted bearing that was posted by another last week. Very good!

Hope that helps Juan. RC Plane

Iceberg

Iceberg Mon Apr 22, 2019 12:53 am

Iceberg Mon Apr 22, 2019 12:56 am

Iceberg Mon Apr 22, 2019 12:57 am

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