In a message dated 4/8/06 7:05:10 PM Eastern Daylight Time, guy@genfiniti.com
writes:
<< 1. I ordered a replacement set of liners, piston and rings from Moss.
Specifically I ordered the 87mm liners, which came with Grant 86mm
rings. Is this correct? >>
The piston rings are sized to the bore, so a 87 mm ID liner should have a
87mm OD rated piston ring. Rings have a free measurement larger than the bore
so
measuring one really won't help.
Put a ring in the bore and measure the end gap. I you have 86mm rings in a
87 bore the end gap will be huge. 87mm * 3.1416 = 273.3185 circumference .
86 mm * 3.1416 = 270.1769 The difference between the two = 3.1416 mm
extra gap in addition to the end gap required for your motor.
<<It seems so to me. If the liners have an inside diameter slightly larger
than the piston outside diameter, then the rings take up the space, forming
the airtight seal between them. >>
That's the basics. There are some pressure dynamics involved and some oil
control action. Piston to bore clearance varies from motor to motor, A
piston
diameter of 3.800 inch ( 96.52 mm) would have a max clearance of .003 inch
( .0762 mm) This clearance is calculated on bore size ( larger bore more
clearance) , engine type ( air cooled has larger clearance) piston coatings
(coatings can run reduce clearance ) and piston type ( forged pistons grow more
with
heat so they need larger cold clearance )
<< Having never done this before, I feel motivated to get this right the
first time. I will place the rings on the piston, then place the piston
in the liner. The piston should travel in the liner with some friction,
but not too much. Can someone please point me to a source that
discusses the principle of this liner/piston/rings system? I need to
get an idea of how tight is too tight? >>
It really depends on liner finish and ring tension. A rough finish on the
bore will have a high friction fit. Some people use a fish scale to measure
piston ring drag when they are experimenting with ring tension. A race motor
will have less drag than a people motor since oil consumption is a lower
priority
especially during closed or light throttle. A people motor should have
piston ring tension in the high push with 2 fingers range. ( this is after the
piston clears the ring compressor tool and before the rod bearing hits the
crank.
)
I just put a Olds 307 motor together for my oval track car, it was honed by
the machine shop. The oil expander rings were modified by cutting 1/2 a zig
zag off and reforming the end. The 2 upper rings were stock production items.
piston pressure was in the mid 1 finger push range. With 6 of 8 pistons in the
motor I could still turn the crank by grabbing the crank snout.
As you may have guessed, oil rings are high friction items.
Harold
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