piston replacement

Well guys, I'm about to make my first engine repair. I'm replacing the piston on my moby. In the manual it says to rotate the crankshaft until the piston is at TDC. Do I need to remove one of the crankcase halves to do this ? Also, it says to pour boiling water on the piston to remove the gudgeon pin. Do I simply pour a pot of water on the piston or can I remove the pin without doing this ? One last thing is how can I support the piston during removal, because the book says this is very important but doesn't really explain. Thanks!

RE: piston replacement

No you don't have to split the engine to rotate the crankshaft, just turn the flywheel.

Pooring hot water on the piston will cause the piston to expand (because of the heat), the pistonpen will be much easier to remove if you do so.

If you dont support the piston the crankshaft will turn while you're working on it.

Also you should cover the hole in the engine where the driveshaft is with a piece of cloth to prevent anything from falling into the engine, it's one hell of a job to get small parts out of the engine again.

RE: piston replacement

Well im not sure if this is what your manual is talking about, but i know in my peugeot's manual, it shows a 'tool' to support the piston when the cylinder is removed. It's simply a rectangular board with a slit in it. the connecting rod goesin the slit and the bottom of the piston rests on the wood.

like this(if my ascii art works)..


| _________|

| |_________

| |


somethin like that? simple but kinda hard to describe..

RE: piston replacement

Well, I put the new piston in and I have to say it was quite a bitch putting the gudgeon pin in, even though I put the piston in boiling water to allow it to expand. That was the only problem I had. The rest went pretty smoothly. Anyways, now that there's a new piston, should I do anything to break it in ? Thanks for all the help!

RE: piston replacement

Yeah.. just don't take it right up to top speed right away and leave it there.

Fire it up and ride it slowly (10 to 15 mph) for 10 minutes... then stop and let it cool for 30 minutes.

Then do that again

Then ride it at 15 to 20 mph for 10 or 15 minutes.... then stop and let it cool for 30 min.

Then do it again...... (that should be about 15 miles total now)

Now it should be ready to go top speed...... but don't keep it at top speed for a half hour straight..... just say 5 minutes at a time.

Once you get say 40 miles on it taking it easy... it should be ready for anything... flog it.

I t wouldn't hurt to run a little extra oil in the first tank of gas.... maybe 1 ounce more.

RE: piston replacement

Thanks for the advice. When I rotate the crankshaft, I notice that the piston moves up and down kind of stiffly and is a bit difficult. Does this loosen up ?

RE: piston replacement

Congratulations, Moby!

When you installed the piston, did you oil the piston and rings? This is very important as this will be the initial lubrication the engine will receive when you fire it off the first time. After this the fuel/oil mix takes care of it.

If you didn't oil those parts, you can squirt some oil on the piston in the cylinder and turn the crank over slowly to spread the oil around.


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