Hi people, I'm looking for some guidance about pistons, rings and bores that I can't find in the many threads I've looked at on the subject. What a ton of great resources out there for this bike.
A friend & I pulled the jugs on his 79 650 Special to do the base gasket, cam chain guide, valve seals and a few other things. These bores and pistons are visually in great condition. I borrowed a mic and a Made in China bore gauge to see what was what.
Not that it matters, but both pistons are stamped 959 and the bores 010. Of course, my mic is metric and my bore gauge is imperial, but I am grateful for the loaners. 10mm up the the skirt, left piston measures 74.925mm and the right is 74.94mm.
Zeroing the bore gauge to these figures, (I am now switching to imperial) I have the following bore measures, in 0.001":
FB = front to back
SS = side to side
LEFT RIGHT
FB SS FB SS
T 4.75 4.5 4 3.75
M 5 4.25 4.25 3.5
B 3.75 3 3 2.75
Where spec is 2 to 2.2 thou over the piston.
Working backwards, the left bore is 75.001 to 75.052mm. The right is 75.001 to 75.048mm.
So much for the lengthy preamble, apologies. Possible actions:
a) new pistons, rings, and re-boring. $$$ is a problem.
b) hone and reassemble with new 75mm rings.
c) hone and reassemble with first OS rings with the gap filed.
In a perfect world, a). But I don't think I have the budget this season.
Can I get away with b) or c), and if so, which rings?
A friend & I pulled the jugs on his 79 650 Special to do the base gasket, cam chain guide, valve seals and a few other things. These bores and pistons are visually in great condition. I borrowed a mic and a Made in China bore gauge to see what was what.
Not that it matters, but both pistons are stamped 959 and the bores 010. Of course, my mic is metric and my bore gauge is imperial, but I am grateful for the loaners. 10mm up the the skirt, left piston measures 74.925mm and the right is 74.94mm.
Zeroing the bore gauge to these figures, (I am now switching to imperial) I have the following bore measures, in 0.001":
FB = front to back
SS = side to side
LEFT RIGHT
FB SS FB SS
T 4.75 4.5 4 3.75
M 5 4.25 4.25 3.5
B 3.75 3 3 2.75
Where spec is 2 to 2.2 thou over the piston.
Working backwards, the left bore is 75.001 to 75.052mm. The right is 75.001 to 75.048mm.
So much for the lengthy preamble, apologies. Possible actions:
a) new pistons, rings, and re-boring. $$$ is a problem.
b) hone and reassemble with new 75mm rings.
c) hone and reassemble with first OS rings with the gap filed.
In a perfect world, a). But I don't think I have the budget this season.
Can I get away with b) or c), and if so, which rings?
