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I have a GP125 in my workshop which has a GP crank with no shims.

I have an OE piston which has space for gudgeon-pin shims.

Surely I need something to centralise the conrod at one end of the other.

Sounds like the bog standard later specification format enabling Innocenti to recomend 50:1 oil.

If the conrod float would otherwise be excessive, then shim the piston, as you suspected.

Although I rarely need them, I've never had an OEM shim fail, but be careful of pattern cr@p from what other's have experienced.

I've just used shims that I bought from Scooter Restorations on mine.
I assume these are decent enough quality?

Tequipment wrote:I've just used shims that I bought from Scooter Restorations on mine.
I assume these are decent enough quality?

You'd hope so, but disastrous if not up to standard.

I've always kept any that have been in an OEM engine strip down, though I have acquired some in piston kits. TBH I have never experienced failure either. For some reason, I have sets of three different thicknesses. That said, I hope that I never have to use them, but best kept "just in case"

Things to look for are that they are a ground finish, which is one indicator of a good hard steel being utilised in their manufacture, & rubbing a file across the surface. If the file skids, then they are hard & most likely durable. Their purpose is to act as a thrust bearing surface for the small end faces & also the needle roller bearing.