wrist pin needle bearings

Jason Luther /

the bianchi (morini mo-1) make a terrible vibrating rattle that definatly comes from the cylinder area. its frequency is proportional to the engine speed, and the only thing i can come up with are the needle bearings that surround the wrist pin. they are still intact, but have a tiny bit of play. the wrist pin very easily slides in and out of both the needle bearings and the con-rod, and the piston, plus the pin is about 1/2 mm shorter than the diameter of the piston, so potentially it could slide back and forth between the circlips. so i guess my question is, does this sound normal? i dont want to just start buying parts to stop the rattling. -jason

Re: wrist pin needle bearings


Most needle bearing small-ends have some perceptible play. But if the piston pin can shake up & down in the bearing, it's bad. 0.5mm of lateral play is nothing to worry about; the running engine will not make noise in that area.

Are you sure the sound is not piston slap? Have you measured the clearance?

Also, rythmic popping noises can be due to a bad head gasket, compression release valve, spark plug seal etc.

Re: wrist pin needle bearings

sounds pretty normal.. but you might still check for too much total bearing slop.

remove the head, fasten the cylinder back down against the crankcase. Turn the piston over and just beyond TDC. Smack the piston crown with the heel of your hand. If you hear a loud click, something's got way too much clearance.

otherwise piston slap could be it.. Measure the widest part of the piston skirt (just above it's bottom) and the cylinder bore. Cylinder should be a straight bore.. piston shape is a tapered oval.

clearance should be only about 0.002 inch or so if in a 'new' condition.

It's kinda hard to isolate noises.. i dunno how you could tell it's the piston and not the flywheel or something.

Re: wrist pin needle bearings

Jason Luther /

i have checked the piston slap possibility and i am well within tolerances. i pretty much put my ear right on the head, and checked the flywheel for rubbing and stuff. its a loud rattle, not a growl or other noise associated with bearings (ball) going bad. ill price out new roller bearing and see. thanks for the advice.

Re: wrist pin needle bearings

heh.. this thread reminds me of another one a few months ago.. i bet you recall..

i don't know if it's possible to detect bad main bearings by jiggling the crankshaft ends up and down, but if i felt any play at all, i'd go a little deeper.

Re: wrist pin needle bearings

Probably the magneto. don-ohio

Re: wrist pin needle bearings

See Ya Moped Army /

Make sure the piston is not hitting the spark plug. I recently had this problem with an MO1 motor and had to shim the spark plug to keep the piston from hitting it.

Re: wrist pin needle bearings


Now what the heck do you think that was about? Base / head gasket too thin? Modern spark plug slightly different than what the engine designer expected?

What a weird situation.

Re: wrist pin needle bearings

Jason Luther /

main bearings and i have issues. hmmm the spark plug hitting the piston could hold some water. the previous owner i think tried to mill the head down with a chain saw or something, so i had to clean it up with the 'ole plate glass and sandpaper technique (worked great by-the-way). using the 'eye gauge' i reckoned there was enough room, perhaps not. the crank itself is as tight as a, well, use your imagination. the only play that seems excessive (but thats so subjective) is the needle bearing, wrist-pin set up. i tear the engine apart every other day, this is really starting to drive me crazy! i need a stethescope i think

Re: wrist pin needle bearings


Detecting bad main bearings on a simple one-cylinder crank is easy.

Remove the cylinder, head and piston. Set the motor at TDC. Push the conrod sideways, along the axis of the crankshaft until it stops. Then, grasp the crankcase (or studs) in both hands, and bear against the small end of the conrod with your thumbs.

As thumb pressure is applied & released, you will feel a 'clunk' in the bottom end, if there is play in the main bearing(s). Do this again from the opposite side, to check the other main bearing.

If you can feel just the slightest movement, then that is probably OK - all used ball bearings have some play - that's how they work. But if it's clunk-clunk, they are bad.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account