////Piston ring gap - R3 / engine rebuild////

Like .009" according to the Stude sheets.  Those were ForgedTrue's. 
Venolia recommends around .004 to .007" for their pistons depending on
type AFAIR (without checking their reference), ROSS anywhere from .004"
to .010"+ depending on type of piston.  As always you have to go with
the spec provided by the piston manufacturer for the intended service.

My situation is that I'm trying to assess a set of original ForgedTrue's in a
stock bore.  There is .001 of taper in the bore (all on the thrust side) and
the piston skirts show a very large wear pattern and all of them check out
exactly at .010 clearance.

The engine never had any hint of excessive piston noise.


.010" clearance sounds fine, an extra .001 won't hurt, especially if the
blower has high output pulleys.  AFAIR those pistons, like most
forgings, were not tin plated so a large wear pattern is not
surprising.  I would expect them to rattle a bit when cold tho.  .001"
taper is pretty minimal.  You can get that if they were kinda sloppy in
the original honing.  

ForgedTrue is still around.  AFAIR the original company (quite highly
respected for their pistons) was sold when the owner retired (he was
actually the originator of the piston forging process during WWII, I
forget his name).  The company went thru a period of decline and poor
products in the 70's and 80's.  It has since been brought (probably for
the name mostly) by Bill Miller Engineering (BME), which has mostly been
known for rods.  AFAiR they are now located in Nevada and only deal in
the popular applications, I don't think they do custom pistons like Ross
or Venolia, so you may be out of luck trying to get another set of
ForgedTrue's for a STude.




Found my Paxton R3 spec sheet.

The pistons were

Alloy:  2018T6 tin plated
Weight:  R3: 16.64 oz., R4: 19.82 oz.
Clearance:  .009" below oil ring

and the pins

Offset 1/16" towards thrust side,
Length 3 1/16
Dia .875
Weight 5.44 oz.
Clearance in piston .0005 to .0007"

The real question is what was the shape of the skirts?

Cylinder wall clearance defined at a lesser piston width at the oil ring is not
too meaningful.  The real question was what was the clearance at the max piston
skirt width.  My guess is that this max width was perhaps .003" more than that
just below the oil ring.  This would result in a .006" clearance that could be
measured with a feeler gage.  I'm measuring .010 where I would expect this
.006" 



Clearance at maximum piston diameter below oil ring.  I hadn't noticed
they were tin plated.  Not to worry, any current brand will tell you
exactly how they want the clearance measured.


Next decision is to get standards and use seasoned bore of .010 OS.

I am concerned about where the new top ring position will be relative to the
ridge in the STD approach.  I would not want the new ring running up on the
ridge reamed out surface.

FYI, but for .010 OS, Total Seal said they can only show metric 1.5, 1.5, 3/32.
 Since 1.5mm is slightly thinner than 1/16, this is even tricker.



You will have to hone the bores so the ridge should not be a problem. 
In any case, send a sample of your current piston to Venolia or Ross,
tell them your exact bore, and they will build the clearance into the
piston.  The can put the top ring in the same place as the current
piston.  Again not a problem, just work with your piston maker.  Oh, and
don't forget honing is different depending on whether you are using
chrome or moly rings.  The chrome rings are what I have found over the
counter in Perfect Circle and Sealed Power.

The piston maker will probably want to know if you are using a hone
plate.