Numbers on top of 259 pistons

[59r2]

I started rebuilding my low milage 259(50k), Plastic guaged the rods and mains all right in specs , no ridge in bores ,so I sent the block for cam bearings ,boiling,honing.My question is while cleaning the pistons,on top I found numbers stamped in them ,and not all the numbers are the same no1-40,no2-40,no3-35,no-40,no5-20, no6-35,no7-40,no8-30. In the past numbers stamped in the top mean oversize but are in my experiance 10,20,30.etc. the 35 is throwing me into a fog of confusion. Also microming the pistons the stamping don't jive with the measurements.The 40 is smaller than the 30 by four 10thousands. Any adivice?
thanxs
JOE

1959 HARDTOP R2 clone
1960 conv
SDC member since 1972

[r1lark]

quote:Originally posted by 59r2

I started rebuilding my low milage 259(50k), Plastic guaged the rods and mains all right in specs , no ridge in bores ,so I sent the block for cam bearings ,boiling,honing.My question is while cleaning the pistons,on top I found numbers stamped in them ,and not all the numbers are the same no1-40,no2-40,no3-35,no-40,no5-20, no6-35,no7-40,no8-30. In the past numbers stamped in the top mean oversize but are in my experiance 10,20,30.etc. the 35 is throwing me into a fog of confusion. Also microming the pistons the stamping don't jive with the measurements.The 40 is smaller than the 30 by four 10thousands. Any adivice?
thanxs
JOE

1959 HARDTOP R2 clone
1960 conv
SDC member since 1972

Joe,

This is explained in the Shop Manual - these numbers represent the bore size in increments of 0.0005".

Your cylinder block should have been stamped with corresponding numbers. A bore size of 3.5625" would be stamped "25", and at the high end a bore size of 3.5645" would be stamped "45". According to the Shop Manual, 'Replacement pistons corresponding to these bore sizes are available on order under the part number for standard pistons. The size is marked on the top of each piston. They should be ordered by specifying the standard part number and cylinder bore oversize.'

This process was used for really low mileage engines that required piston replacement.

Paul
Winston-Salem, NC
Visit The NEW Studebaker Skytop Registry website at: www.studebakerskytop.com

[59r2]

Thank you ,I have left the rods on the pistons as to return them to the proper bores. I see the article in the shop man ,never read the piston replacement part,sorry

1959 HARDTOP R2 clone
1960 conv
SDC member since 1972