G2 piston skirt clearance

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go old
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Location: Cambridgeshire UK

G2 piston skirt clearance

Post by go old »

Hi guys
I am renovating a matchless G2 1961 and have come across a problem fitting an oversize piston bought from amc
Bore of my cylinder is +0.060 making a nominal bore of 2.810 inch. New piston is 71.5mm or 2.815, and of course bottom of piston skirt will not enter the bore so I will have to have bore opened out. Engine reconditioner is asking what clearance I require (thought they would know this)
Looking at piston and bore tolerances in manual, I have calculated that clearance on top of skirt should be 0.007 to 0.009 thou. Could anyone confirm or advise me on these figures, have a few days before the machining will start so any expert advise would be appreciated
With thanks
Graham
g80csp11
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Re: G2 piston skirt clearance

Post by g80csp11 »

nominal clearance would be .0015 per inch of bore , so .0045"
They should measure the piston at bottom of skirt
hope this helps

if you are worried err on just over this size is .0045 to .005" clearance
go old
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Re: G2 piston skirt clearance

Post by go old »

Thanks for reply G80csp11
Little bit concerned about measuring at bottom of skirt
According to manual piston skirt measurement should be taken at top of skirt at right angles to gudgeon pin. Think this is because skirt at bottom is thin and easily distorted. Do you have personal experience of your method. The reconditioner measured at top of skirt before asking what clearance I wanted. Piston enters bore at top of skirt but being tapered fouled at bottom.
g80csp11
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Joined: Mon Jan 01, 1990 12:00 am
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Re: G2 piston skirt clearance

Post by g80csp11 »

have a look at the Hepolite catalogue it explains everything !
your machinist should follow these guidelines for air cooled pistons ( 0.0015 per inch of bore size measured at bottom of skirt ) or(0.0025 per inch at top of skirt)

it is more normal to use bottom of skirt value and set clearance accordingly

Unless you have a very odd piston design the largest diameter will be close to the bottom of the skirt , actually very slightly above bottom . it should be measured/verified by the machinist

first time ive see a matchless manual measure top of piston skirt , most odd ! just means the profile is biased towards top of piston skirt , not of the piston

is your piston a modern replacement ?

http://archives.jampot.dk/technical/Pis ... 4-1963.pdf
go old
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Location: Cambridgeshire UK

Re: G2 piston skirt clearance

Post by go old »

Method of measuring top of piston skirt is stated in small manual for matchless G2 model 250cc ohv issued by MMC page 4 Data. New piston was purchased from jam pot club amc spares dept as standard +0.060 size but stated on box it was 71.5 dia. A direct conversion to imperial means it is 5thou oversize for a standard +0.060 bore. Surprised this wasn’t mentioned in description. Largest diameter is at the bottom of skirt, piston fouled 2/3 of way into bore.
Will see if I can find hepolite info and print off to take to reconditioner. Many thanks for help
go old
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Location: Cambridgeshire UK

Re: G2 piston skirt clearance

Post by go old »

The 0.0025/inch clearance measurement at top of skirt bears out with tolerances given in large manual which was total of 0.007 to 0.009 thou on a 2.8 inch dia piston
Many thanks for help
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