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?

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.

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?

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

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. ;->

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 Al Gore for working to make the internet accessible to us common
citizens.

There are also 2.54 centimeters in one inch, exactly. Now you can measure
everything.

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

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

Inches are too hard to figure out. I know car wheels are measured in

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.

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.

Thanks, and good luck with the eighth-litre!

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

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

You've got a handle on it, Fred. But here is a suggestion for you based
on years of cylinder boring experience- jump TWO sizes. Your cylinder
may have had that .25 piston in it for a very long time and is probably
very worn around the exhaust port area. Another .25 mm of bore is only
.005" all the way around and there is a good chance the jug won't clean
up. I see this often and have finally got my steady customers (local
dealers) to always jump at least .020" on an overbore.. Don't worry
about using up your bores, the bike is over 20 years old and is at just
.25 over right now. Used cylinders are plentiful if you ever need one.

I never had to worry about it. I just hand the cylinder to the shop
that will do the bore and they tell me what I need. They have all
means of measuring everything. I had once to go two steps, but often,
one step is more than enough... Small bikes, like the ones we ride
around here, need rebuild often...

Here is a 1.5mm piston on eBay that will work for you at a price
very near dealer cost:

Z35595QQihZ017QQitemZ270094414060QQrdZ1QQsspagenameZWDVW

There may be other pistons on eBay that would work, maybe a .030" or a
.040" but if I were you I wouldn't hesitate at the .060" at that price.
Shouldn't cost any more to bore it the additional distance, if your
machinist says it will, find another machinist.
Jerry

Thanks Jerry, I appreciate the advice. I was thinking I'd go two
sizes larger if I mail ordered the piston, or take the machinist's
advice if I was allowing him to order the piston kit for me. You make
a good point about old cylinders like mine being fairly inexpensive
and somewhat plentiful. I think I've seen several go unsold on eBay in
the past (at $19.99).

The cheapest non-auction price I've found was $77 online, but the
shop I'm having do the bore job quoted me $79 (both Wiseco). So the
$56.00 Buy It Now price in the link you provided looks *very*
appealing. Maybe I'll search for a .030 or .040 on eBay before I pull
the trigger though. Anyway, thanks again and I'll let ya'll know how
things turn out.

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