Restoring a loose piston/cylinder fit?

Seeing bubbles is not uncommon. The engine still starts and this could be beneficial assuming the fit on the top end is still good. The cylinder could have a slight bit of taper in within it but a little pinch at the top . The only known way I know to fix a problem in this regards is to have the cylinder plated with nickel or chrome. No one I know would do a cylinder of that size. .049 would be the smallest. I would be absolutely certain you glow head is sealing. Lap the bottom of the plug slightly and replace the head shims with new ones to insure a non leak fit. Seeing that you have access to a machinery, taking some material off the bottom of the plug would also be another option due to changing the squish band of the head. Of course this is entirely experimental with the possibility of making it harder to start. While not really good for rpm's , upping your castor content to 25% will assist in starting and hot starts. It can possibly build up some varnish on the inside offering a slight rise in compression.

OK, I have a couple things to do.  I don't have a tach, but I can take a quick video of the engine running as is.  Can you estimate the rpm from the video sound?  While I'm at it, I can show you the bubbles.  Maybe it's not really that bad, but I didn't see that with my .020, so I'm guessing it's less than ideal.   Also I'm going to need a pair of .010 wrenches. I'll look for original Cox stamped pieces since my lasercut .020 wrench from Cox International is not a great fit and it's also too thick to slide into the cylinder top flats with the glow head on.  

And I'd like to try some thermal expansion/shrinkage experimentation on both the piston and cylinder.  I have some ideas that just might work out.  We're talking tenths of thousands or even millionths, so I shouldn't need an excessive amount of heat to make a difference.  Or maybe I would, but I won't know for sure until I try it.  Any of you gents have a couple sloppy fitting .049 pistons and cylinders you could let go cheap? 3 or 4 sets would be great.  If I come up with something that works I'll be happy to share the details.

If you have an Android phone, you can download the free app "RPM Gauge", which measures the rpm acoustically and pretty accurately in comparison with conventional RPM gauges sold on the market. Make sure though that you use this app in an open space as echos ricochetting on the walls of a room may easily falsify the results.

I wrote about it here:https://www.coxengineforum.com/t11644-rpm-test-in-flight-videos-with-android-phones

The thermal expansion you mentioned will probably not result in a permanent geometry change, in my opinion. As metals expand and contract with rising and dropping temperatures respectively, the piston and cylinder size will always return to those measured at  normal room temperature after the heating or chilling ends..

I am also a "compression-conscious" guy and am critical with engines that show bubbles when driving the piston up for compression...over the years of use of high rpm COX engines I noted though, that even if the piston/cylinder fit is already sloppy on a well used engine, at high engine rpm-s the loss of pressurized gases is minimal because the time available for the gases to escape between the piston and cylinder walls  is very short....so a blow-by seen on an engine while hand-cranking the piston slowly  is compromising engine compression incomparably less, when the engine is actually running say, 250 cycles per second i.e.  at 15k rpm..


COX once completed an experimental run on a 049 and found that even after 400 hours runtime the engine, albeit with impaired compression, could be started up with an electric starter, and performed well. (see in the framed section of the article below)


I am not sure about the work history of your tiny 010, but very few COX engines reach in their life the 400 or so hours, given the features of this hobby...I have a TD051 with near 300 hours on it, and while it shows the signs of decent ageing in terms of low compression, when it revs up it runs like a brand new one...so unless damaged by the previous owner, your 010 may still have lots of hours of enjoyment for you, even if the compression is not as new.

TexasWade wrote:...............
Here's a quick video of a Cox related project I did a while back. Made my own exhaust throttle sleeve for my Tee Dee .09.   Love messing around with these darn things =)

https://www.youtube.com/watch?v=-HO9Kthn17M

BTW your 09 throttle ring works beautifully...I also had 2 made by a machinist and sometimes use them on my Cosmic Wind and SIG Wonder R/C planes (see the Cosmic on my avatar photo) powered by TD09-s, as an alternative to the R/C carbs I also have for these engine.

TexasWade wrote:OK, I have a couple things to do.  I don't have a tach, but I can take a quick video of the engine running as is.  Can you estimate the rpm from the video sound?  While I'm at it, I can show you the bubbles.  .......

If you post a vid I will measure the rpm with my Android phone, unless you also have an Android phone and may want to download the "RPM gauge" app.....

balogh wrote:If you post a vid I will measure the rpm with my Android phone, unless you also have an Android phone and may want to download the "RPM gauge" app.....

That would be terrific.  My car would NOT read text messages off the android OR iPhones I had, but it will from the windows phone so I'm a winphone guy I guess. Also means I'm a NOAPPS guy, lol.

Good point about compression under running conditions, it really doesn't have much time to leak down and the extra clearance could just make it run more freely.  

Thermal shrinking can be a thing for sure, and some experimenting would have to be done to see how much shrinking I can get and how badly things want to go out of round etc.  The idea is not to simply heat and cool the part, but to heat it while inside a thick walled sleeve.  The part we're working on is not allowed to grow when heated, so the original room temperature OD becomes the new hot OD and the inside and outside both shrink a bit when it cools off.  Might try a similar thing with the piston, a rod inside the skirt, and cooling with dry ice in rubbing alcohol.


And good news by the way, my Cox collection just grew again!  Won this bad boy off Ebay a few minutes ago.  https://www.ebay.com/itm/372262688909  I've never even seen a Cox .15 in person so I'm super excited for this one to arrive.  I've been watching for a Tee Dee but this one is nearly as cool and seems like a really really decent price.  Maybe I'll get the carb stuff for it and make it look like a Tee Dee, then I'll just need an .049/.051 to have the whole range.  Not exactly but close enough anyway.

Windows phones - that I am not familiar with even as much as i am with the Android - may have similar applications....these phone manufacturers try to stay abreast in competition with each other...

Not necessarily a metallurgically correct observation from my side, but steel will permanently change its dimension if during the expansion the tension in its material exceeds its yield stress...it should equate some 1...3% of geometry deformation, below which the elasticity of the material would still make it act like a spring with unchanged dimensions after the tension is over....but achieving such a thermal stress by obstructred expansion in a thick wall sleeve would require such temperature rise that may compromise its metallurgical features, so the piston you want to expand may become either too soft or too brittle...maybe I am totally wrong, otherwise the idea would be a great way to restoring the tight fit of componets.

I don't necessarily disagree with what you're saying, but given some old tired parts to play around with, it would be a fairly simple thing to mess around with and see what happens in the real world.


Doggone windows phones are barely supported.  I think they actually tried to kill them a few years ago and nearly succeeded.  There are practically no apps for them...

No problem, if you post a vid, I will open it on my fartphone and run parallel with the RPM gauge so that the screen will show both simultaneously, and send you the screenshot...

...just remember to have at least 30% nitro in the fuel which is the right juice for the reference 27k rpm for 010-s...my 010-s will only exceed 27k on the bench with 30+% nitro....and the stock COX prop made for the 010-s.

I do have a grey prop that looks like probably a stocker. Will be sure to get pics of that. And I only have 25% nitro Cox brand fuel so I guess my numbers will be a smidge off..

Yes the grey one should be the stock (see here: https://coxengines.ca/cox-.010-propeller-grey-3-x-1.25.html) and the only 25% nitro will not cause too much deviation from the reference conditions. I am not familiar with fuels sold in the US (I blend my own from components sold separately) but standard fuels in my part of the world contain only synthetic oil... make sure you have the 20% or so castor in the fuel...worn cylinders seal better with the thick castor, plus these critters do not survive long on synt oil only, but you sure know that..

TexasWade wrote:...........


And good news by the way, my Cox collection just grew again!  Won this bad boy off Ebay a few minutes ago.  https://www.ebay.com/itm/372262688909  I've never even seen a Cox .15 in person so I'm super excited for this one to arrive.  I've been watching for a Tee Dee but this one is nearly as cool and seems like a really really decent price.  Maybe I'll get the carb stuff for it and make it look like a Tee Dee, then I'll just need an .049/.051 to have the whole range.  Not exactly but close enough anyway.

Hey that is an enviable catch of a .15 COX at such a deal price!!! Except for the 15-size engines I also have all COX calibers down to and including 010-s.

A TD carb may fit on the Medallion crankcase, but the front rotary port on the TD shaft is larger, and the cylinder porting is also more aggressive I suppose, so the performance of this hybrid would be a tad off the TD.

Trouble is if I really want to participate in an auction for an engine I want to buy in competiton, not just throwing a fortune into the "your max bid" box on ebay and wait till next morning to see if another COX addict has overbid me or not, I need to stay awake till small hours per my Central European time, as almost all listings for US stuff on ebay end at Pacific daytime or early evening which is 9 hours before my time either way....

Interesting thread here. I own a handful of .010 "runners" and they all vary in compression when spun by hand. A couple of them are much softer than the best poppers. The softer ones can be a little harder to start but once running, they all perform very similarly. With Sig Champion 25 fuel, a stock prop, and depending on mixture setting and weather conditions here at sea level, I generally see 27-28.5k on all of them.

That is interesting Tee Bee.  Even more so since I've also read numerous times about them getting harder to start as they loosen up.  Mine started almost by accident actually, and it doesn't even have a recoil spring.  I had soaked the whole engine in a small bowl of Cox fuel for a couple days before mounting it on a board and checking the glow plug.  Apparently having a crankcase full of fuel was just the ticket because it bumped right away when I turned it over. Gave me a cute little Brapap on about the third flip, then the next flip it actually ran for 3 or 4 seconds.  I didn't have a fuel line on it so I can't be sure yet that my needle and carb assembly are working.  Now that I said that, watch me take two hours tonight trying to get it running for the RPM video  

Looks like my Cox Super Power fuel at 25/55/10/10 is the same formula as the Champion 25 so if all is well hopefully I'll see similar numbers.

yep, all things being equal, i'd rather have a loose piston/cylinder then a tight one, they'll usually turn more rpm's.

Be sure the fuel passage in the needle valve body and the 3 in the venturi are clear. Plugging there is a common frustration with the tiny .010 and .020 TDs. I usually attach a piece of fuel line filled with alcohol or fuel to them and plug them off with my fingers. Blowing on the fuel line reveals if all the holes are clear before final assembly and test running. I love running these little engines. Good luck!