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Stuck gudgeon pin in piston

The gudgeon pin in my BSA M20 (1943) piston has stuck in the small end bush which has caused the pin to wiggle in the piston holes.
The gudgeon holes in the piston is now not so tight, when the pin is in its right position in the piston, without the connenting rod, you could quite easily turn the pin around with your fingers. If I push the pin to where the circlip should be, the pin sits tight.
The piston and cylinder walls are very nice.
My question, is there an oversized gudgeon pin? Or somthing else I could do?
I would very much appreciate if you could help me with this.

email (option): anders.lundqvist.42@gmail.com

Re: Stuck gudgeon pin in piston

The next size up gudgeon pin that is fairly common is 20mm (.787") The standard pin is 3/4"(.750")...That would allow you to ream both the small end bush and the gudgeon pin bosses in the piston oversize to clean them up (if they are not already past that size)...You would also be able to get circlips...

However, it may also be necessary to re machine the circlip grooves to match the clips so you will need access to some good engineering facilities..20mm pins are used in quite a few modern bikes....Ian

email (option): ian@wright52.plus.com

Re: Stuck gudgeon pin in piston

Thanks Ian!
Are the Gideon pin supposed to be press fitt in the M20 piston?
I was thinking if I could use (the correct) Loctite to keep the pin from spinning. There is no real wear in the piston holes,the pin just "rotates" to easy.

Re: Stuck gudgeon pin in piston

The pin should be tight in the piston at the point of assembly...To fit the pin normally the piston is first heated to about the temperature of boiling water and the pin then slides in easily...Once the piston has cooled it will be tight again...When running the piston is subjected to a lot of heat and the resulting expansion of the piston results in the correct running clearance between the pin and the piston at which point the pin would be free again...

So really you can't start off with an oversize hole with the pin retained by Loctite...

The best solution would really be to fit a new small end bush and a new piston of the size you currently have...What oversize is the bore at the moment?...
When reaming or honing the small end bush it is important not to have it too tight...Under those conditions the bush/pin can/will overheat in an M20...The pin should be completely free in the rod and the piston should fall easily under its own weight from side to side...
At the same time the clearance should not be enough to feel anything other than a tiny amount of free play of the pin in the bush....Also ensure that if a standard bush is not fitted it has some oil grooves machined into it that line up with the hole drilled through it...The correct amount of access for oil into the bush is important...Ian

email (option): ian@wright52.plus.com

Re: Stuck gudgeon pin in piston

Thanks again Ian!
Yes a new piston seems to be the way to go.
Great information.

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