cox piston with bulge on top

Punched crown as I call it, happens when the con rod is reset too tight or the engine has a lot of time on it, also does happen with a excessively loose rod. I diagnosed one the other day with a club member, it was an old babe bee that had great compression and ran fine but suddenly started loosing power and kept quitting, he brought it over to me and I checked it out for him and found it had lost compression so I checked the head to make sure it wasn't loose, it wasn't so I popped the head off and there ya go, same issue. sucks. Only fix is a new P/C set (Bandit)

TDbandit wrote:Punched crown as I call it, happens when the con rod is reset too tight or the engine has a lot of time on it, also does happen with a excessively loose rod. I diagnosed one the other day with a club member, it was an old babe bee that had great compression and ran fine but suddenly started loosing power and kept quitting, he brought it over to me and I checked it out for him and found it had lost compression so I checked the head to make sure it wasn't loose, it wasn't so I popped the head off and there ya go, same issue. sucks. Only fix is a new P/C set  (Bandit)

Or just another piston. I wouldn't hesitate to scavenge a piston out of the parts drawer and use it without replacing the cylinder.
Rusty

RknRusty wrote:

TDbandit wrote:Punched crown as I call it, happens when the con rod is reset too tight or the engine has a lot of time on it, also does happen with a excessively loose rod. I diagnosed one the other day with a club member, it was an old babe bee that had great compression and ran fine but suddenly started loosing power and kept quitting, he brought it over to me and I checked it out for him and found it had lost compression so I checked the head to make sure it wasn't loose, it wasn't so I popped the head off and there ya go, same issue. sucks. Only fix is a new P/C set  (Bandit)

Or just another piston. I wouldn't hesitate to scavenge a piston out of the parts drawer and use it without replacing the cylinder.
Rusty

You're right *bops self in Head* have a habit of replacing p/c sets as matched sets keep forgetting that cox is one of the few that you can mix, match cylinders and pistons (bandit)

When I retired a few years back, found this site, and started up my model airplane playing after a very long absence, I went way over board not realizing Cox made millions of these lil buggers. I did not know Bernie or Jason were reselling NOS and purpose built engines.

Long way to say I bid and won a LOT of engine Lots and the occasional good looking Baby Bee, TD, or Medallion

Later I read Paul Gibeault's mouse motor set up recipe and hints

Sorted all the parts paying attention to cylinder configuration and then cleaned each Cyl and piston, finally trying each piston in a cylinder until I got good at the drop out fit

Next I taught myself how to set the pistons with what I believe were junk pistons

Then I reset each piston and found that I should have done that first as now I needed to go back and re-establish best fitting combinations

OK a lot of words (for new folks who may not know this stuff) to say that Bandits mention of "keep forgetting that cox is one of the few that you can mix, match cylinders and pistons " is partly true but proper fit may require some thought and work

ON the other hand; I remember as a kid just running the bee-gee-sus out of the Cox .049s with any old fuel we could scarf up for weekend flying. Some times we just mixed and matched parts to get one working never thinking about the engineering behind making one run the best it could or last for any length of time

Yeah that was mine. I swapped it out for one in better shape. I may put it on an old Bee and let it run until it blows. Remember to always wear eye protection.
Joe