very frustrated.....

I've just about had it with my ped...it's a 1975 motobecane mobylette, and needed a new piston, sounds easy enough right ? Well, the wrist pin is a VERY tight fit, so tight that it is impossible to push it through with your fingers. I have talked to several 2 stroke mechanics, even brought it in to a shop, and they've all said that they've never seen a pin that was such a tight fit. They all say it should go in pretty easy. So I made a homemade pin pusher, I've attached a picture of what it kinda looks like, and it works pretty well, but my problem is keeping the pin properly lined up with the connecting rod bearing when I use this tool. I can't see if it's lined up correctly when I use this tool because it blocks my view, so I have to line it up and then set up the tool and have someone hold the piston, but it's almost impossible to keep it steady while I'm tightening the nut on the bolt to push the pin through. If it moves the slightest bit, it's off line, and I end up wrecking the rod bearing if I keep going. Is there any way around this ? I'm reluctant to take it into a shop just yet because if the mechanic has as much trouble with it as I've had, it'll cost me an arm and a leg, probably more than I got the ped for. Is it just a matter of keeping the piston steadier when I'm pushing the pin through ? Please help me out, I love my ped but she's really pissin me off right now. Thanks.

piston.JPG

Re: very frustrated.....

I like your drawing.. and your pusher/puller.

I made a similar one years ago.

First check for a burr that is binding the fit.

And you are using some 2 stroke oil for lube?

If it still won't got... then >>

You could ease this situation by very carefully sanding the wrist pin bore (in the piston) with some 500 or 600 grit wet and dry sandpaper wrapped around a wooden dowel (or metal tubing).. you can slit the rod with a hacksaw to slip the paper into the slit.

I would use paper that is already partly worn out...and wet it first... then put the piston between your legs and scroll the rod or tube between your hands like you are trying to start a fire.

Do this very sparingly... then rinse the piston and try the fit with the pin.. sand lightly... try it... sand it.. try it..etc.

When you get it to slide in... then clean the parts very well... and assemble it with lube.

Don't go too far.. you still want a firm fit.

Re: Try this...

Drive the piston pin into the piston first. It should protrude just a little past the inside wall. Line up this protruding end with the bearing/bushing, and continue as you normally would.

Also heating up the piston with a propane torch makes the job easier. Squirt some WD-40 on the inside (bottom) of the piston dome. Heat the dome (outside) with a small flame from the torch til the piston just starts to smoke. Pull the torch away now and then, as the smoke is better seen with the flame away. Don't over heat the piston. Just get it to start to smoke and it should be OK.

Oh yeah, WEAR GLOVES!!!

Jim

Re: very frustrated.....

Thanks a lot Fred, I'll give that a try. Do you know why Motobecane would make it such a tight fit ? I don't understand why they would do this. I'll shoot for a firm fit but still be able to push it in with my fingers. Thanks.

Re: Try this...

Hi Jim, I've been doing it like the first method you mentioned, with the pin lined up with the wall. The pusher/puller works great for pushing and pulling the pin (by the way, you're the one who told me how to make this contraption a while ago, so thanks !!!!) but I can't seem to keep it lined up with the bearing while I'm pushing it through with the tool. I guess I just need a better helper to keep the piston steady !!!!

Re: Try this...

Ron Brown /

Moby,

As Jim says, heating the piston is essential if the pin is tight, you can also freeze the pin.

If you follow Jim's advice without a helper, the pin should never get tight in the rod end bearing, just test for play as you press the pin in place.

Ron

Re: very frustrated.....

Freeze the pin and try it, works on lots of stuff. Doug D.

Re: Try this...

I

Re: Try this...

Ron Brown /

Dave,

"Heat (moderately) the piston dome (torch) until smoke appears."

This is a direct quote from the Motobecane shop manual.

This is not an unusual way of fitting wrist pins. If you had ever seen the result of a wrist pin coming loose and popping one of the clips you would understand the reason for this method of fitting.

You should never modify the fit of OEM parts unless you know exactly why you are doing it and what the ramifications are.

Ron

Re: Try this and also...

Ron:

You are correct about modifying clearances set up by the factory. Although my experience with a Kinetic crankshaft implies otherwise.

And Also:

When installing the piston pin clips/snaprings/keepers/ whatever they are called, put the flat side out out if it's a snap ring. Look at a large washer and note how it has a flat side. This is the result of it being a stamping. Snap rings are made the same way and the flat side out prevents the risk of it popping out.

And Also Also:

The open end of a snap ring should be placed at either 12:00 o'clock or 6:00 o'clock relative to piston travel. Honda found that in racing the piston stretches and the snap ring can pop out at the other positions.

Jim C.

Re: Try this and also...

Ron Brown /

Jim,

Good call on the circlips, although I have a hard time envisioning any ped of mine reaching the rpm of a Honda Racing engine. I think if my piston stretched enough to release the circlip, that would be the least of my problems.

Also, most of the wrist pin clips I have dealt with are made from round wire and the most important part of removing and installing these is to not squish them too much and to make sure the pin is clear of the clip groove when you install the clip.

Ron

Re: Try this and also...

Actually ... It is not the piston "stretching" at all...

You put the open end of the C shape either up or down to prevent the high "G" forces at TDC and BDC from springing the C shape closed.

Re: Try this and also...

Ron Brown /

Fred,

I like that explaination a lot better!

Ron

Re: Try this and also...

Hi! Could it be the wrong piston or a bad new part? Sometimes you get something,especially foreign made like in India that is enough out of tolerance to ruin your fit.I wouldn't force it,I'd try another piston,I think. Hope you have success! You know,more and more `knock-off' parts are being imported. Fuel valves are a good example of shoddy workmanship or inferior materials being sold as replacements for good bikes like puch, tomos,etc.

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