Tight piston wrist pins

[viclioce]

Question for Alan.

He constantly has problems inserting piston wrist pins into the piston, mounting the piston on the rod. Says he had needed to hammer the pin in with a steel hammer EVERY time he installs a new piston.

I have never experienced this problem. He says he always oils the pin but it always resists going into the piston. Always. And he isn’t using random pins, but rather the one sent with the piston. Are their different diameter wrist pins???

Any advise I can tell him? I don’t know what to say as I’ve never had this kind of problem.

[Dale]

Yes. Don't ever hammer a wrist pin! If it doesn't slip fit with thumb pressure then the piston must be reamed to fit the pin.

[viclioce]

Thanks Dale! I’ll pass this on to Alan! Victor

[taber hodaka]

Clean the wrist pin and piston before fit. The fit cannot be loose but good firm thumb pressure should move the pin. sometimes one side of the piston seems a little tighter. Be very careful if you use a adjustable reamer. In the old days I sometimes used a nail after cutting it down the middle so I could use a strip of sandpaper in my drill. Go slow and get the correct fit. Does Alan have access to a computer? ------------- Clarence

[olddogs]

I cut a 6 by 6 piece of 400grit wet/dry sandpaper and tape it to a regular wooden pencil. It will look like a flag when done. Roll the sandpaper around the pencil and lube it with wd40. Slide and rotate the pencil as you move it back and forth through the piston. You do not usually need much pressure or time to remove the burs and make the pin slide freely. Make sure you clean the piston when finished. This has always worked for me.

[viclioce]

Thanks Olddogs. I’ll remember this for myself if I ever run into this issue.

[BrianZ]

I run into this quite often and my solution has always been to heat the piston with a heat gun (or hair dryer)and then the pin will usually slide right in. I have read here that it must be a push fit (no heat), but I figured since aluminum expands more than steel, a tight fit would be beneficial. I have never had any problems with this technique, but I am open to any suggestions as to why a finger push fit is better.

Brian

[Bullfrog]

It is relatively common to see seizures related to too tight of a fit of the piston pin in the piston. I am frankly amazed that you haven't "used up" a top end (seized) with your heating of the piston to allow insertion of the piston pin. The Hodaka recommendation is for a thumb push fit when both the piston and piston pin are at ambient room temperature.

Ed

[taber hodaka]

I have known guys that used hammers, heated pistons and dry ice the pins. Read up on procedure and follow. I have also pistons develop a crack at the pin. Some day I will get a picture of my wrist pin to rod bearing alignment tool I use it more often at my age. ------------ Clarence

[taber hodaka]

What does the manual say? Never attempt to drive the pin through piston with a hammer and punch methods --You may remove the pin but the connecting rod will have been bent in the process. page 53 shop manual. ----Clarence

[taber hodaka]

Brian good to see your post! Aluminum does expand and a tight fit is not desired because there is undue piston pressure at the pin if heat was used in order to install it. The pressure it would take to install the pin cold leaves no room for expansion, this leaves the piston to expand between the wrist pin and the cylinder wall. There is no expansion room at the wrist pin forcing and distorting the expanding piston, which applies undo pressure against the cylinder wall that can result in engine failure, seizure. Others can explain this much better than I. I would like to bid against you again at Hodaka Days. Friend Clarence

[ossa95d]

I'm with Brian that the piston will expand at a greater rate than the pin when they heat up thereby relieving the tight fit. However given the experience that Ed recites often about seizures due to the tight fit, I suspect that the problem isn't when the engine heats up for the first time, but when it cools down. The tight fit would be exacerbated by the pin also being hot as it cools down as a unit possibly making the fit even tighter. Over several cycles this could distort the piston thereby contributing to the likelihood of seizure. Makes sense to me but sometimes I overthink things.

[matt glascock]

You see, this is why I use a wooden dowel. You can easily cut and sand it to fit perfectly, and if you break one in the woods, there are replacements everywhere. Those big, fat pencils we used in first grade are also perfect and as a bonus, the smooth paint finish allows it to glide into place easily. Barella Ziti Napoletani is also the perfect diameter and can be cut to fit. Just ram it in al dente, trim the ends, and let it dry. As an added bonus, the ridges hold and distribute oil well. Super duper bonus - make extra and have lunch.

[givergas]

matt i think you got mixed up with the PINOCCHIO forum ...just saying

[matt glascock]

Oops...actually I thought I was on the Carabella forum.

[bobwhitman]

Matt
Congratulations! You just made the forthcoming Resonator “Hot Tech Tips “
Bob

[matt glascock]

Cool!!! I've always been something of an innovator. My motto: "Give me a Dremel and I'll give you a big pile of grinder dust". Actually I do have a tech tip. When is the deadline for submission to the next edition?