Riddle me this: Pistons

 
    Okay, so my old piston is stamped 0.25 on top. It's an ART brand
piston. The factory bore on my RM is 70mm. Yesterday I got out my
calipers and measured the current bore. Unfortunately, my calipers
aren't digital and millimeter markings are every one mm, so getting a
precise measurement is difficult. But... it appears to me that the
cylinder is currently about 70.25 millimeters. Does that sound right?

   It's definitely not 70 and a half yet, and it's nowhere close to 71
millimeters, so I assume the 0.25 refers to one-quarter of
millimeter, yes?

   Okay, assuming that's correct, would the first oversize Wiseco
piston (0.5/.020), resulting in a 70.5 bore, be my next overbore? And
the ".020" refers to "inches" over, right? If this is all correct then
I think I'm starting to figure this out. And I'm happy because it
appears that my cylinder is currently on the 1st overbore, which means
that I can go five more times, according to the Wiseco piston chart
(or four more times after I have it bored this time). Of course that's
assuming I don't grenade the thing at some point and score the walls
so badly that it takes a couple of overbores to clean it up.

   Anyway, can some of you experts let me know whether my thinking
here is correct or not?


Fred Bradford - CrashTestDummy
f.j.bradfordREMOVE@verizon.net

 wrote:

yes, it does. Difficult to measure milimeters? Once I tried to measure
a screw head with my caliper, that was not a metrical screw, I needed
to know what wrench I needed. I gave up. Waaaaaay too hard to figure
out that many divisions in 1/128 increments.


Yup, 0.25 is a quarter milimeter. It goes .25, .50, .75, 1.00 and some
brands (PRO-X is the one I remember) have 1.25 up to 2.00



Inches are too hard to figure out. I know car wheels are measured in
inches and I know a 13 inch wheel is not as cool as a 17 inch, but my
inch knowledge ends here. And I know I like 21 inch up front and 18 or
19 rear.


Anyway... Good luck... You gave me inspiration to ressurect my 94
RM125.

-- Tiago

 On 28 Feb 2007 11:15:21 -0800, "Tiago Rocha"


   Excellent, thanks Tiago! I think my initial confusion came about
due to the difference between Japenese measurements (ie - mm), and
(American-made) Wiseco measurements (ie - inches).

   The Wiseco catalog, for example, lists the pistons applicable for
my RM as:

STD
.020
.030
.040
.060
.080

   None of which jive with the "0.25" stamped atop my current piston
(or the ".25, .50, .75, 1.00" you mention above). However, the Wiseco
chart also shows the corresponding bore result (in millimeters) using
the above oversizes:

70
70.5
70.75
71
71.5
72

   Since I've determined that my current bore is 70.25 mm, I'm
assuming that the Wiseco .020 (that results in a 70.5 mm bore) would
be my next step.


   Easy to measure whole millimeters, difficult to measure fractional
millimeters when the caliper isn't setup to measure fractions of a
millimeter. ;->


   Only if large wheels impress you. My Dakota's rollin' on 16s, but
only because that's what came on it. I have no desire to put 20s on
it, no matter how kewl the punks around here may think they look. ;-p


   I prefer an 18 rear for the kind riding I do, but I could probably
live with either.  


   Thanks, and good luck with the eighth-litre!


Fred Bradford - CrashTestDummy
f.j.bradfordREMOVE@verizon.net

 Dummy wrote:

What kind of caliper isn't setup to measure fractions? You don't know how
to read a vernier caliper, do you?

 

   Ha, you're exactly right! I just studied an online tutorial and
figured it out though. I wasn't even aware that you look for
*additional* marks that line up (in addition to the whole number).
Thank you.


Fred Bradford - CrashTestDummy
f.j.bradfordREMOVE@verizon.net