Gudgeon Pin oversize

According to the manual, play should be between half a thou and 1.2 thou. There is no small end bushing in a 650SS, so I don't believe that your conrods left the factory undersize. Maybe oval which could be misinterpreted as an interference fit. I think that reaming the conrods is a bad idea as it has to be done perfectly square.But most likely you got oversized gudgeon pins.

Please let us know about outcome.

As it happens many years ago I ordered some fork bushes from RGM to go with the new stanchions I'd had from Andover Norton. The bushes wouldn't even start to go over the stanchions. When I used the micrometer on the stanchions they were perfect - clearly the bushes were undersize. I did have some difficulty getting RGM to accept a return. I did order bushes from AN and they were a perfect fit.

You may be ok using the old pins, if they’re compatible with the new circlip type and position.

A close sliding fit in the piston is satisfactory.

... go back to RGM as the piston & pin clearly aren't to original tolerances. If the pin is as free in the piston as it sounds it will tend to rotate there rather than in the little end......

In a warm engine, the pin is free to rotate in the piston anyway.

There are no such thing as standard 650ss pistons in the market place. The gudgeon pin you have is for the 600 pistons which used rods with a bushed small end, later rods with no bush use a different size gudgeon pin. Some owners get away with it as the wear in the small end will allow the larger pin to fit, but not always. 

This seems to be a common problem with new pistons that seem to be machined to imperial sizes via a metric conversion! I had the same problem with the Gandini and JPM pistons I bought - In the end  I used an adjustable reamer from Axminster tools on my ES2 it’s been fine ever since. If you are Near East Devon you are welcome to borrow mine!

Since pins typically come slack with hot water (well under 100C effectively) they must be very loose at piston working temperature (200C perhaps?)

Had the same problem with the JP pistons I bought for my lightweight.  It only took a few moments on my honing machine to sort out. 

Do you have an engine reconditioner nearby that could do the job for you? 

Where are you based? If near me (north Wilts/ south Oxon) I could sort it for you, if I have a suitable size mandrel.

John, Is it really a good idea to hone the con rods to suit pistons/gudgeon pins that appear to be oversize? If you later have issues with the pistons and need to replace them, and the replacement pins are the correct size, you will have a bigger job getting the small ends bushed, or the con rods replaced.

Wouldn’t it be better to source pistons of the correct dimensions and return those you have now? That’s what I would do anyway.  AN sell 650 pistons - have you asked what their gudgeon pin diameter is?

Andrew, If the JP pin is exactly to specified size (mine were), then they will fit the rod if it is near or on the upper limit of the Norton specified size then the pins will fit.  On the bottom limit then no chance of a fit.  Therefore altering the con rod to fit is not quite as bad you suggest.

I tried to find a pair of NOS pistons for my Navigator, same diameter pins, no luck so settled for JP, might it be easier to find NOS pistons for the 650?  I did consider using pistons from a Triumph, but ended up using (modified) JP pistons.

... use grinding paste as any abrasive will leave a residue however carefully cleaned out.

Not sure what oversize gudgeon pin means, the 600 used a different size gudgeon pin than the 650ss as the small end is a different diameter, very very close but different. In a worn 650 small end you will most probably find the 600 pin fits, however, it is not unknown for the pistons to cross seize in the bore. I will dig out the dimension that the 650ss small end should be so that it can sized correctly. Where the pin is to tight the piston will seize showing marks across adjacent to either side of the gudgeon pin hole on both sides of the piston. 

The pin in any engine must be either a sliding fit in the small end or in the piston depending on what part is held tight or how it has been designed to work with the con rod. There should be no play in the pin in the part in which it slides - especially important in an engine that has been honed. 

I would ring RGM rather than email them as they never seem to read their emails. Even then I have been told they will sort a problem out straight away only to ring a week later when nothing had appeared to be told they had forgot all about it! They are very good at getting orders out though but not sorting problems.

The pin is supposed to float in the piston once warmed up . Its also supposed to be free in the small end of the rod. My 99 has run for 30 years with 99 pins in 650 pistons , I seem to remember having to spend some time polishing the pins to fit the pistons.