Knocking from fresh rebuild...

[TMali]

Hello,

I just had a weisco piston and twenty over bore. Reason for new pistion was the stock one started to have the skirt break off, and had some slop in it. Bearings looked to be in very good condition and eveything went back together very snug. I did a proper break in using lucas 2cycle oil mixed at 32 to 1. Everything seemed to be going smoothly, then about 2 hour after break in i started to get a sloppy knock come from the top end. And im 99% sure it is comeing from the top end.

Here i have some pictures of the cyclinder and a crappy video of it running (atleast you can here it run in the video) Im just not liking the sound. Im just no liking the sound of this knock.

Please let me know what you guys think. I just spent alot of money fixing this up and i dont wanna break anything more then it may possibly be. I just dont think the cyclinder wore that much that quick cause i can still see the hone marks

Once again, sorry for the crappy video. My junky phone is all i have





Please let me know what you guys think

Thanks

 

[UncagedShadow]

That sounds good in my opinion. These little air cooled motors are really noisey at low rpm and when not moving to fast. Mine sounded like that after my rebuild. Only note I have is that the cylinder looks like it already has some scuffing, unless that is a shadow cast by something. Am i right to assume that is scuffing? You can see it alot better. Take a micrometer and mike the cylinder, and report back the measurement.

 

[Blaaster]

I would be lifting that jug off, that scuffing is not normal, something is amiss.

 

[jlsparky7]

Some pistons are not perfectly round yet somehow make it through quality control (if they even have it).

Did you seat the piston rings over the holding pins on the piston?

 

[Larry's Shee]

This was a fresh bore? What was bore/piston clearance? What was ring end gap? How much time total is on this?

 

[Blaner]

i DO NOT like how its got thet up and down knock at tdc, did you reuse the wrist pin roller bearing? i take it the new piston came with a new pin.

what is the cyl/bore clearance?

was there end play on the conrod? ie: could you wiggle it up and down at TDC without the piston on?

That scuffing is not normal, did you line up the rings with the locators correctly?

 

[TMali]

That sounds good in my opinion. These little air cooled motors are really noisey at low rpm and when not moving to fast. Mine sounded like that after my rebuild. Only note I have is that the cylinder looks like it already has some scuffing, unless that is a shadow cast by something. Am i right to assume that is scuffing? You can see it alot better. Take a micrometer and mike the cylinder, and report back the measurement.

No, there is some scuffing in like 4 spots

 

[TMali]

Some pistons are not perfectly round yet somehow make it through quality control (if they even have it).

Did you seat the piston rings over the holding pins on the piston?

yes, the rings are correctly seated over the holding pins... or it would of been really hard to get the rings and piston into the sleeve lol

 

[Blaner]

clearance??......

 

[TMali]

This was a fresh bore? What was bore/piston clearance? What was ring end gap? How much time total is on this?

Complete freash bore and piston. Had a cylinder shop do everything besides dissasemble and reasemble. Are you asking what the bore/piston gap was or what it is now? When putting it in the gap was really tight, now it has that little bit of play. And the ring end gap was nice and tight... not sure what it is now. And after rebuild and break in period it has about 2 hours on it

 

[TMali]

i DO NOT like how its got thet up and down knock at tdc, did you reuse the wrist pin roller bearing? i take it the new piston came with a new pin.

what is the cyl/bore clearance?

was there end play on the conrod? ie: could you wiggle it up and down at TDC without the piston on?

That scuffing is not normal, did you line up the rings with the locators correctly?

Slow down, haha im getting back to everyone... i just dont know how to multi quote, or if you even can.

A new wrist pin was used but the roller bearing was still very tight not alot of play and and all bearings rollers rolled fine, so that was reused. There was no play in the crank shaft bearing. And im really sorry but i dont know what TDC means

 

[Blaner]

you need to measure what the difference in diameter is between the size of the piston and the diameter of the bore. if you used a wiseco forged piston, the gap should be about 0.07mm. that means the cylinder bore must be 0.07mm bigger than the piston. i would not go smaller than 0.05mm.

if this gap is too small, your piston will expand and sieze in the bore, early signs of this are the scuffing you are seeing.

you need to measure the clearance and talk to the shop who bored it, ask them what the clearance was they did it at. remember when you give your cylinder in, you must always give them the piston too for this reason.

Another measurement you MUST do is the ring gap, place a ring in the bore and square it up using the piston to place is square in the bore, use a feeler gauge to measure the ring end gap, it should be about 0.020mm if im not mistaken

these measurements are CRITICAL! get them wrong and your motor will blow soon!

 

[Blaner]

TDC = Top Dead Center, its when the piston is at the very top of the stroke as in your video.

the bottom line is that something is worn or else you wouldnt have that up and down movement as seen in the vid when your press on it....that is very bad.

You need to take off the barrel and measure it properly, get a machine shop to do it for you if possible, then measure your piston about 10mm from the bottom of the skirt. work out the difference.

also check your wrist pin cir-clips, make sure they are in their groove properly.

while everything is off, get the con rod at TDC, and test it to see if there is any up and down play. side to side wiggle is normal but there must not be any up and down play on the rod. Id also suggest you use a new top roller bearing.