Piston Bore Size

cpotts454 · Posted: February 12, 2009, 11:18 pm Post subject: Piston Bore Size

Hey guys and gals.

Stupid question. I've got the manual here in front of me but for the life of me I can't figure out what size rings I need to get. The piston has .953 on the bell. Has my jugs been bored, and to what size over?

xsjohn · Posted: February 12, 2009, 11:37 pm Post subject: Re: Piston Bore Size

75 MM is standard bore.........think when the factory bored them they mark the barrels.........target being 75mm but marked if not exact..?...not sure what the numbers mean on the barrels in reference to 75mm...was explained to me but I forgot....think I remember they had 3 different standard sizes when fitted at the factory.......maybe someone else remembers.......me I am still on standard.....not bad for 67 thou....I always smooth the edges of the rings.....and dimple the skirts for oil retention......

1st over is + .25mm 75.25mm
2nd over is + .50mm
3rd over is + .75mm
4th over is + 1mm

Retiredgentleman · Posted: February 13, 2009, 12:24 am Post subject: Re: Piston Bore Size

The .953 on the piston means 74.953 mm, which is 2.9509". That would be a stock piston, not bored out.

cpotts454 · Posted: February 13, 2009, 2:44 am Post subject: Re: Piston Bore Size

You know, xsjohn... retiredgentlemen, you guys are the best. If and when I get a little extra money, I owe you guys a dinner. Thanks for the info.

Retiredgentleman · Posted: February 13, 2009, 2:58 am Post subject: Re: Piston Bore Size

Hey John, how do you do the skirt dimpling?

xsjohn · Posted: February 13, 2009, 9:46 am Post subject: Re: Piston Bore Size

RG....(Photoshop Rendition) .Started doing this to the 2 strokes years ago and just continued...seemed to make my RT1 pistons last for a long long time......talked to some piston guys and none of them said it would hurt anything.......rationale being some oil retention on the skirt......the oil comes out the larger holes below the oil rings to lube the skirt.....some being trapped in the dimples keeping more onthe skirt.........small drill like a 1/8" on a drill press just going in a bit....and smoothing lightly when finished with some very fine paper to remove any ridges........

When builders balance these engines they also shave somewhere inside the piston like on the pin boss the heavier piston one to match the lighter one if there is a difference.....gram scale.......did mine............also removed the sharp edges from the 2 compression rings with some 280.........

xsjohn

jayel · Posted: February 13, 2009, 10:17 am Post subject: Re: Piston Bore Size

check this out RG

www.energetics.com/rec...adeghi.pdf

xsjohn · Posted: February 13, 2009, 10:25 am Post subject: Re: Piston Bore Size

So now they are doing it with lazers....go figure......the ole piston knurling probably did some good too............67 thou on the original standard pistons.....will change them next time down......"maybe"......did my first one in 1971.....royfisk says he drilled holes all the way through on 2 strokes in the 4 corners where there is no ports.....the oil would be shoved through the holes then on the down stroke.....

For all the guys with huge main jets.....good luck there.......will wash out the bore .........no good friction proofing will help that..........guess everyone has to learn that one the hard way....I have......usually only one size up on the MJ is required........catch 22 with these is that they seem to run better the more fuel you dump on them.............

xsjohn
8-1 now

Retiredgentleman · Posted: February 13, 2009, 1:55 pm Post subject: Re: Piston Bore Size

Very interesting link, jayel. John, not sure if that's within my skill level. No access to a gram scale. It would make the pistons last longer.

xsjohn · Posted: February 13, 2009, 2:00 pm Post subject: Re: Piston Bore Size

Gram scale would be for balancing the pistons to each other not dimpling them...same amount of dimples on front and back of the skirt on both pistons wouldn't effect balance.........harbor freight has a decent gram scale for about nothing.....good for weighing dope too.... Laughing

xsjohn

jayel · Posted: February 13, 2009, 2:07 pm Post subject: Re: Piston Bore Size

yeah and when you get busted for drug paraphernalia you can say

"I wuz jes' weighing muh pistons ossifer"

xsjohn · Posted: February 13, 2009, 2:15 pm Post subject: Re: Piston Bore Size

Well then you would be measuring some pistons in prison...... Laughing

xsjohn

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