Piston tool - how loose without a micrometer?

Sorry John < just having a time ...I just make sure to make the ball set ( at the socket )round as possible as in tap turn tap turn tap and U k-now take the play out of Eric

John I blow it out first with brake parts cleaner so there's no oil in the ball socket to fool my finger gauge. I try to get it so I can still feel it move when I push and pull, but if it was any tighter it would have no play at all. I feel like if there is no play, yet free movement, it may be too tight when hot.

I mushroomed a few pistons before I got the hang of resetting. I try not to do it unless it's pretty sloppy and is going to be a really fast running engine. After it's reset I squirt some high castor fuel into it and work it in.

Hope that helps.
Rusty

I take all the play out. Sometimes I go to far and make it tight. When that happens I run it and check after. It's always a perfect fit after that. I run my engines hard and have never had one fail at the socket.

Basically what I am saying is it's tough to screw up.

The way you describe the fit of your best runner seems about right to me.

I took the play out 2 to 3 times and while the rods still moved freely I ended up with the ball abrasing a hole in the piston top after a few runs.The castor film and varnish could not build up in lack of the gap.

Yes, I believe that on regular sport props the rpm is too low to ever have any tension in the rod. So if there is some play in the socket developing with time, then it comes from wear in the top part. So resetting the socket on a sports engine is a moot point, it will only stop the oil from getting to where it is needed. That's how I see it at least.

John, I agree with Ron's method.. along with Eric's technique (tap-turn.. tap-turn) and will add; that placing the "piston-holder" part of the tool on a hard/solid surface (IE: a vise anvil..) and tapping very lightly with a small hammer to re-seat the socket joint will give you a more positive feel for the process. Rusty's tip on blowing the joint out with Brake Parts Cleaner beforehand seems like a smart idea.

I reset my mine with "minimal clearance".. but at a point where the rod-ball can still be rotated within the cup/socket without forcing it to turn.

Hope this helps.

I agree with Ron. I set them tight and run them in. The only failure I've had was at the crankshaft end.
For what it's worth one of my best runners is one that I set way to tight and mushroomed the piston. It initially went from 21,000 rpm to about 16,000 rpm. I decided to mix some high castor fuel and run it in to see what happened. It's the piston/cylinder  I used in the flight time contest. After 3 hours and 20 minutes at 20k+ the ball socket is still a perfect fit.

Just my ,

Jim

THANKS GUYS! Very helpful. I'll give it a shot.

I figured this was the right place to start this conversation and share some ideas on what to do if a re-set fit is too tight. I had some brass tubing with 1/4" inside diameter. This fits around the Cox .049's socket and seems to bottom squarely on the underside of the piston-face. The tubing shown inserted inside the piston is thick-walled which would offer more support around the socket to "pulling" on the conn-rod.



As for the pulling.. a #34 (.111") drill bit is a perfect fit inside the crank-pin end.



A more common 7/64" bit is .109".. and might work ok.. (you don't want to risk elongating that hole..) This fixture allows the pin to be tensioned against the tube by tweaking with a flat blade screwdriver.. or maybe even a Cox wrench.

This is a crude example.. but you can see how it would operate as a puller.



A shorter length tube with a fender-washer on top would provide a better surface for the Cox wrench to bear on.. while gently tweaking and checking. Maybe a block set in a vise would be needed. Drill a hole for the rod and install through the bottom and thread the pin through the rod to hold it in place. Slip the forks of the Cox wrench under the pin.. and apply slight pressure. The metal tube should still be used to prevent the piston-top from collapsing inward.

I haven't tried this though.. because I don't currently have any pistons that meet the criteria.

Edit: I just read back through this thread in more detail. Lots of interesting experiences.. but nothing except run-in, if you went a little too tight. Maybe this would allow you to get the exact fit you like; by having the option of loosening and checking/resetting again.

A you not concerned the rod will snap/break if you pull it too hard? Especially the crankpin end of the rod may break. The swaged socket may be too stiff to yield.

balogh wrote:A you not concerned the rod will snap/break if you pull it too hard? Especially the crankpin end of the rod may break. The swaged socket may be too stiff to yield.

Hi Andreas, Yes.. Something would naturally break if pulled too hard. Probably best to try this on a cracked/scored junk piston.. if you had one.

The "fork" on the Cox wrench seems like a sensible tool to use for tensioning the pin-end of the rod. If the pin-end is a snug fit (.111" diameter) .. I think it will provide the best support.. before yielding to fracture from tension. The tools' fork will straddle the rod and provide fairly equal tension to both sides of the pin.

Here's something to try.. Drill/tap the Cox wrench on center just behind the fork, for a #6 or 8 cap-screw to bear on the fixture/plate. Assemble/align loosely.. shim across the open end of the fork as needed.. and use a cap-screw long enough to contact the fixture/plate without bottoming in the Cox wrench. When tight.. use the proper hex-key to tweak the cap-screw slightly.. one time. Disassemble and check for play in the socket. This would make a tool that's more like an actual "puller"; having more gradient-tensioning, as compared to a prying-action.

The socket-joint's spherical movement lends itself well to equally distributing the forces of an offset pulling-load angle.. which will be present when pulling on the rod. Positioning the rod dead-center, shouldn't be critical.

bla bla bla.. I think I'll make one.. and post some pics..

I chuck the piston into the drill press with the rod into the chuck. The rod end is above the jaws and allows the rod to be firmly clamped. Seeing that the jaws are tapered down, they fit into the underside of the piston. A slurry of rubbing compound can be added into the ball socket if needed and you can hold the piston while the rod is turning. It takes a few minutes, but it will loosen up. A good hot soapy water wash followed with a little compressed air cleans the ball socket right out.

Here's my Cox .049 rod-puller that utilizes the Cox wrench. The cap-screw is 6-32 x .250", the fixture-plate is 1/8" aluminum with a .200" through-hole drilled for the rod-end to pass through. A two-screw plate in place of the wrench-forks, would work more like a puller though. I'll work on that. I wanted to use the Cox wrench because I thought it would be kinda' cool. I need a smaller tap-handle.. It was tough tapping the Cox wrench with a pin-vise. The length of the 1/4" tubing plus a steel-washer measured .600" which provided a good clearance for the materials I had on hand.

Roddie , that's a good idea but your putting the thing in a bind like that ,,, you will need to put screws only on the two legs of the handle to pull true (straight) and pressure both sides a little at a time to keep it true . Eric

getback wrote:Roddie , that's a good idea but your putting the thing in a bind like that ,,, you will need to put screws only on the two legs of the handle to pull true (straight) and pressure both sides a little at a time to keep it true . Eric

Well... There's a few ways of thinking on the forces applied. Yes, a straight pull on the socket is ideal. The rod angle can be set on center. The bridge-gap and tension-force exerted are adjusted by the screw. Getting the length of the tubing that fits inside the piston is key for locating the pin-end of the rod at the right position above the plate. Compressing the plate and bridge on the opposite end of this tool, tensions the rod.. but only as much as you want.. because there's a limit /stop when the plate and bridge make contact.

Here's some better photos.





Because there's a fork in the bridge (Cox wrench).. the piston/rod assy. "pulls right out", to check for play in the socket-joint. Leaving the .111" drill in the rod's pin-end actually helps with play-feedback in the joint, as opposed to grabbing the rod with a thumb and finger.

The Cox wrench's thin gage steel might bend before a gap can be induced in the joint.. but a stronger bridge could be easily made. As I said.. the Cox wrench seemed novel to at least show the concept.

That Looks better when held level , and the force just to bend the wrench would probably bee enough to do the job. It is a good idea if you go over tight on the rod /ball joint . Eric

getback wrote:That Looks better when held level , and the force just to bend the wrench would probably bee enough to do the job. It is a good idea if you go over tight on the rod /ball joint . Eric  

Thanks Eric. The way I see it.. why not be able to loosen-up the joint if it was possible.. and naturally; without it doing any harm. I honestly think that the force this tool would exert in tweaking-increments, would not cause a rod to break. I see the socket-joint giving-way before that point.

I also like the tool and wish it wont damage the conrod. But I saw the conrod break at the lower, crankpin bearing part. That seems to be the weak point. I would see the tool safer if the pulling force were placed on the rod itself and not on the bearing part.

balogh wrote:I also like the tool and wish it wont damage the conrod. But I saw the conrod break at the lower, crankpin bearing part. That seems to be the weak point. I would see the tool safer if the pulling force were placed on the rod itself and not on the bearing part.

That's actually possible to attempt. I measured my conn-rod to be .080" (2mm) diameter. A bridge-plate could be slotted .100"w. (2.5mm) and chamfered along the top cuts, to form a fork that would bear on where the rod flares at the pin-end. The work-pin should still be installed.. but is turned 90 degrees; in line with the slot/fork. It provides support against the pin-end collapsing in, from the compression induced by the fork.

I had this exact idea as a means to save .051 pistons that had broke rods. Rare animals there. Pull the rod entirely out of a donor piston and pull the stub rod out of the damaged set and put the good rod in the .051 piston.

Since I have only 1 .051, and it is currently good I have not explored this concept.

Phil

You can purchase a dial indicaor and stand fairly cheap. Why not get one of these to check the clearance before and after restting the rod?
In fact, why not measure the clearance of a piston rod assembly that hasn't been run?
Somewhat more scientific than guessing I'd say.

I would keep tapping it, lots of light taps, if you get past the point of too tight a drop or two of oil and twist Conrod back and forth slight usally will free it up. Aim for as little play as possible, you want it loose with no play.
As for rebuilding pistons I don't know.