Hello all I am new to this site and im looking for some good info from yous guys here, I have bought a 87 yamaha DT200 engine was stuck so i took it apart an found the piston skirt was cracked off on the bottom no other damage was done so i ordered a new piston & rings gaskets an put it all back together and it worked great for two months then one day it quits again take it all apart an same the again piston skirt broke no scratches on the cyl wall does anyone have any idea what may be causing this ? I had the cyl micrometered it is not out of round.
piston skirt failure dt 200
- Thread starter Da yammie
- Start date
Generally, piston skirt failures are uncommon The most prominent reason for piston skirt failure is cylinder to piston clearance too great. The piston is allowed to "slap" which puts a LOT of stress on the skirt and eventually forms a crack which allows that part of the skirt to break off.
The second reason is usually manunfacturing related. Internal stresses in the piston form a crack that cannot be seen. Since you've had two pistons do the same thing, I think we can rule that out in this case.
The third reason is because of some misalignment which is creating abnormal "thrust" on the piston. If the crankshaft is dragging in any position, it can slow the piston down in the same spot every time eventually "slapping" it until it breaks. Check the rotation of the crankshaft with the cylinder and piston off and note ANY abnormalities. Check to make sure that the rod isn't twisting and roll it over to feel anywhere it's dragging.
The second reason is usually manunfacturing related. Internal stresses in the piston form a crack that cannot be seen. Since you've had two pistons do the same thing, I think we can rule that out in this case.
The third reason is because of some misalignment which is creating abnormal "thrust" on the piston. If the crankshaft is dragging in any position, it can slow the piston down in the same spot every time eventually "slapping" it until it breaks. Check the rotation of the crankshaft with the cylinder and piston off and note ANY abnormalities. Check to make sure that the rod isn't twisting and roll it over to feel anywhere it's dragging.
There does seem to be one spot where it is a bit sticky ,when i rotate the engine and the rod is at BDC it seems to have a very faint stickyness only in this one spot though i thought this might just be the pull from the magneto .I will remove the magneto an try again an also have the cyl bore micrometered for size the last piston i used was a 66mm.
Find the source of that drag, it's not supposed to be there!
Have the bore measured by a machine shop and if it needs boring, take it to the next size.
Blaner
Your Friendly South African Ambassador
you dont get a BBK fo the DT as far as I know?
my money is one piston/cyl clearance. you must get that sorted out, beautiful bike you have their
my money is one piston/cyl clearance. you must get that sorted out, beautiful bike you have their
