2 Stroke Help

[Yamaha Tri-Moto]

Hello
I recently rebuilt the whole top end of my YTM 175 2 stroke I installed a new wiseco piston and had the cylinder boerd out. It only has about 4 hours of riding time on it. There is a loud rattle comming from my cylinder now, could it just be that the rings are to small for the cylinder? If so would i be able to just replace the rings to help tighten it up? Thaks in advance.

[Wickedfinger]

Something is definitely very wrong - take it back apart now before you cause serious damage. The issue should become very apparent.

[Yamaha Tri-Moto]

so would you guys suggest rebuilding the whole top end again or just replacing the rings to get rid of the rattle?

[jakey]

give it a look and see what it is before you go buying parts

[Yamaha Tri-Moto]

i already took it apart tonight, everything looks ok but there is a small dent or indent on the right side of the piston should i file that down then see what happens? maby its hitting the crank or something

[MTS]

i already took it apart tonight, everything looks ok but there is a small dent or indent on the right side of the piston should i file that down then see what happens? maby its hitting the crank or something
indent on the bottem of the skirt or on the side of the piston, like a gouge?

[Yamaha Tri-Moto]

yeah its like a gouge or a scrath on the side of the piston also stock compression is supposed to be at 128 but my machine has been bored out and its only at 90psi

[MTS]

sounds like you didint install your rings properly, the end gaps have to be up agist the little pin on the ring gland., if you can get a pic it would be better...how deep is the scar, also is it in the cylinderwall too?, as for 90 psi, that tells you right there something is wrong, rings pooched, or it was incorrectly bored.

[Yamaha Tri-Moto]

well i know the rings are lined up with the pins ok cause i just made sure of that. there is minor scuffing on the cylinder wall from the gouge on the cylinder. If i put new rings on it do * think they may tighten everything up?

[MTS]

where your circlips installd correctly, as far as new rings, you may need a new piston and a hone, depends how deep the gouge in the piston is, if you can stick your fingernail in your screwed, you did use wiseco rings with the wiseco piston right? did you install the rings properly, with the numbers facing up? also did you set the ring end gap? what is the clearnce between the piston and the cylinder? (use a feeler guage)...?

[Yamaha Tri-Moto]

i will have to check the clearence tomorrow but what do * mean set the ring end gap?? As far as the gouge in the pistion it is very very small i dont think its much to worry about.

[MTS]

the end gap on the rings, when sit in the cylinder should be .003 thou on the outer and the inner usally says in the thing you get with the piston in the box, or call up wiseco for it...should be about .002-.004 thou between the piston and cylinder without rings, something was not set properly, im guessing ring end gap...ask the machine shop what they set the bore to piston clearnces and ring end gap at (if they set the ring end gap)....

[Yamaha Tri-Moto]

alright thnx man looks like i got some work to do lol

[TimSr]

Who did your bore job? Yammies like to piston slap when the bore is too big for the piston. Did you go through a reputable boring place that specializes in motorcycle and ATV stuff, or did you give it to some discount corner garage that "does it real cheap". First thing Id suspect is your bore job, and no, bigger rings will not tighten it up. It needs to be bore to the proper size for a specific piston.

[TeamGeek6]

...what Tim said, plus it has to be honed and chamfered. Chamfer means sanding or filing sharp edges off the ports that will gouge a piston or ring.

Take the head off and crank it to TDC, then rotate the crank back and forth about 50 degrees each way of TDC, look for piston movement front and back, may be slapping.
The rod coming up tends to push the piston sideways and thats a source of slap.

Hopefully the shop bothered to use a micrometer on the bore before fitting a piston!!