Need help understanding cylinder to piston clearing - Snowmobile Forum: Your #1 Snowmobile Forum

guimond47 · 01-06-2013, 02:17 PM

Need help understanding cylinder to piston clearing
I have a 670 h.o and bought it blown up now, my cylinders are mesured at 79mm , im looking for spx or spi istins cause there priced fairly . Now stock bore is 78mm on these sleds so would 0.20 over pistons do the trick ? They seem to have a lil play in the cylinders wat over bore size do it need ? Thanks

minisrint521 · 01-06-2013, 06:21 PM

just did .020 over..78.50mm .004 piston to wall .014 ring end gap 149 psi both 200 miles 79mm is.040

guimond47 · 01-06-2013, 06:29 PM

Oh i see so 0.20 is .5mm? So if my cylinders mesure out to 79mm how many .** do i need over ?

minisrint521 · 01-06-2013, 06:45 PM

79mm bore needs .040 over pistons. . Pistons are .004 smaller then 79mm bore.Double check bore size...u want .004 .005 piston to wall clearance.

guimond47 · 01-07-2013, 08:09 PM

you were right i went to machine shop and that what they told me to. but then i relised my cylinder sleaves are too thin at .040 and it wouldnt last...... so i picked up used cylinders and they mesure 78.20mm roughly. i was told to get .010'' over. would that be correct?

guimond47 · 01-07-2013, 08:13 PM

who sells .010" over?

guimond47 · 01-07-2013, 08:26 PM

id like to get spx but i need to go up to .020 over , and thats 78.25mm i belive .. can i just hone my way up to 78.26mm .27mm ish. ??

dealer pistons are too expensive at .010 over..

guimond47 · 01-07-2013, 08:47 PM

with a stoner honer..

IRONDOG · 01-07-2013, 08:53 PM

.020" over would be 78.5 mm. It CAN be honed that far with the correct hone, it's not likely you have one capable of it.

guimond47 · 01-07-2013, 08:56 PM

really.. wow then it would be best to bite the bullet and try to find .010' over ehh.. i only have a lisle stoner honer

guimond47 · 01-07-2013, 08:58 PM

which brands out there carry 0.010'' over for the 670 h.o

minisrint521 · 01-07-2013, 10:40 PM

...factory sleeves should take .040 over!? Skidoo made a 78.15 mm piston-probably what was in those....dont think they make .010 over pistons

guimond47 · 01-08-2013, 07:37 AM

They should but it wouldnt last with time ...

IRONDOG · 01-08-2013, 08:49 AM

There is nothing wrong with boring 670 cylinders .040" over- they will not be prone to wearing out or "not lasting" any more than standard bore would.

guimond47 · 01-08-2013, 11:41 AM

They would be prone to over heating wouldnt they?

IRONDOG · 01-08-2013, 01:57 PM

Quote:

Originally Posted by guimond47

They would be prone to over heating wouldnt they?

No.
Why would you say that? Just curious.