Cylinder reboring

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Hi All.
My cylinder will necessitate reboring from + 0,20 to 0,30.
The +0,30 pistons are 73,787 mm.
Please, let me know the tollerance from liners and piston size.
Thank you .
Ciao
Piero
 
cast or forged?...surely the required information will be supplied with the pistons?
 
Piero, Most of the aftermarket piston suppliers don't make them in +30 although this was originally a factory oversize, these days common sizes are +20,40 and 60 for the 750s, therefore you will probably have to source some new old stock. Some earlier pistons had a weak area around the lower ring and could sometimes break in half there.
 
Hi,
I have a + 0,30 GPM pistons and rings nos new.
Now i have + 0,20.
Could i know, please, the tollerance from pistons and cylinder liners?
Cio
Piero
 
The piston clearances are set by the piston maker, only they know the alloy they used and the expansion expected. GPM is an Italian company so should be easier for you to ring them.
 
Great!.
Andover sent me the technical pecification:
"Inspection
Visually check the cylinder barrel for cracks and corrosion. Inspect the threads and studs to ensure they are not damaged or stripped.
Check the cylinder bore for scuffing and scoring. Bore wear is often evident in the form of a wear ridge at the top of the bore. This ridge marks the limit of piston ring travel.
Measurement
Measure the diameter of each cylinder bore, front to back, in two places:-
1) within 1” of the top, front to rear
2) near the bottom below the piston ring swept area, front to rear.
Measure the diameter of each piston:-
3) at right angles to the gudgeon pin axis just above the base of the skirt.
Assessment
If the bore is in acceptable condition, the difference between the two measurements (1 & 2, above) should not exceed 0.005” .
By comparing the measured piston diameter (3, above) with the bore diameter the clearances can be calculated and assessed to see if they are within specification.
Rectification
If the cylinder walls are badly scuffed or scored or if the piston -to-barrel clearance is excessive then the cylinder will need to be re-bored by an engine overhaul specialist. New oversize pistons will be re- quired.
N.B. It is important that the re-bore specialist is given both the pistons and cylinder barrel.
The pistons (even new ones) must be accurately measured to suit the re-bored cylinder. The correct piston to barrel clearance for GPM/Gandini pistons is 0.0040 to 0.0045 inches .
If the cylinders and pistons are within specification it may only be necessary to re-new the piston rings. In this case the bores should be honed to allow the new rings to bed in correctly and provide the best possible seal. This honing operation is normally left to an experienced motor engineer.
The correct piston ring end gap for GPM/Gandini pistons is 0.009 to 0.017 inches for all three rings.
Tools
Three good quality tools are required for the measuring operations described above:-
a. An external micrometer with a range of 2 to 3 inches to measure 750 pistons or a range of 3 to 4 inches to measure 850 pistons.
b. A telescope gauge with a range of 2 to 3.5 inches.
c. A set of feeler gauges."
Thank you.
Piero
 
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