I think I must have done something wrong when I installed the piston. I put the engine back on and when I looked in the exhaust port and pedalled the piston doesn't move. I have to turn the flywheel by hand to get the piston to go up and down, and when it reaches TDC the flywheel locks and the piston stops. Then I have to turn it the other way to get the piston to go back down. Please help!!!!!

RE: help!!!!!

hmmm. mabey, this is just a sugestion but u got the wrong piston? mabey u got one with a longer stroke then the normal one.thats my 2 cents. anyone else wanna take a whack at this? REE?!


RE: help!!!!!

Just pop it back apart.....takes no time at all...(you don't have to take the motor out do you?)...... it might be the wrong piston.... hopefully you didn't throw the old one out yet........ if not set them right next to each other on a bench to "match them up"... particularly the wrist pin to top dimension.

A common cause is that you didn't locate the ring gap properly at the locator pin in the ring groove.... which means it's jammed in the cylinder pretty tight..... but the only destroyed part might be the ring.........sometimes its hard to locate the ring ends correctly while also trying to get the cylinder back on.

RE: help!!!!!

You may be right Fred, the new rings weren't made exactly like the old ones. You know how most rings are kind of cut out at the ends so you don't put them on upside down ? Well, the new ones didn't have this, so I assumed it didn't matter which way they went on. I doubt it's the wrong piston because I quoted the letter on the cylinder when I ordered it. And I do have to take the engine off again, which is kinda frustrating. Thanks guys.

RE: help!!!!!

Reeperette /

I would have to concur with Fred, as that being the most likely cause.


RE: help!!!!!

Can I use the rings from the old piston on the new piston ?

RE: help!!!!!

Probably .... but you are losing half the advantage of having a new piston and rings.

A new ring should only be $5 or $10 and then you will have good compression.

As far as using the old one.... just make sure it's not too tight in the groove... and has the same style ends as the ring that came with the new piston.

Make sure the piston is in the right way (not backwards) arrow pointing towards exhaust.

Make sure the rings are correct side up (any letter on ring goes toward spark plug)

Make sure the ring gap is at the locater pin.

Make sure the wrist pin circlips are FULLY seated in their groove.

RE: help!!!!!

Reeperette /

There's this gadget called a compression sleeve that is supposed to make this gig a bit easier, ask yer mechanically inclined friends, or the shop you bought the parts from, if they can get you one for this.

It's worth a try.


RE: help!!!!!

Arrow pointing towards the exhaust ? I didn't see an arrow. Where would it be ? I found that when I oiled the piston it moved up and down easier, but is stiff when it reaches TDC, and then moves down pretty easily again. When I had the engine back on the bike I had to pedal pretty hard to get the flywheel to move, but it wasn't smooth. I'll take it apart again and check for a letter on the rings.


Stiff at TDC?... with the spark plug IN?.. that could just be compression.. stiff with the plug OUT means trouble... with the plug out the flywheel should rotate easily both directions....(if you oiled it during ass'y)

The arrow on the Piston should be right on the top of the points toward the exhaust.

Not all pistons have the arrow..... bout 95% do though

and I'd say 99% of all pistons have a front and back.... you need to get that right.

Not all rings have a letter on their top.... but MOST do, and if they don't you need to look at the ends closely to see if they are symmetrical.

RE: pistons

Fred, I found that I put the piston on backwards. Could this be causing the stiffness ?

RE: pistons

yeah that is most likely the reason

RE: help!!!!!

The Moped WareHouse of New Jer /

Here is the Low down on motobecane pistons. prior to July of 1975 motobecane pistons came with what is known as a standard lock relief. This is what you find on tomos and most puch engines. These rings must be installed right side up. In july of 1975 motobecane switched to what is known as an internal lock relief. These give a better seal and can be installed either side up. Of corse the rings need to be matched to that type of piston so they are not interchangable. The new style supersceeds the older style and is a dirrect replacement. Original equiptment mb pistons will not have an arrow on them and you must install it with the ring openings facing the exhaust posrt. On most other engines you will find an arrow and that arrow will always point toward the exhaust port. Make sure you have it well oiled and make sure you torque the cylinder head bolts to 9 ft lbs. Also check these tolerences as generally you will have the most wear at the top of piston travel and it should be loosest at this point rather than tight. piston skirt clearance .005 to .010 is ideal. With a max allowable of .050

ring end gap .10 is ideal. With a max allowable of .40

ring side groove .03 to .06 ideal with a max allowable of .15

Note the factory was still installing the older type pistons on models up to June of 1976.


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