Piston tool - how loose without a micrometer?

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 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.

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..

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.

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.

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.

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.