Ariel Owners Motorcycle Club

Hi
I had reason to remove the piston recently on my w/ng. Had to heat the piston to remove the gudgeon pin.
After putting it back together, again with heat, the piston was not as free as it had been previously, so I took it out apart and tried with the pin round the other way; it was only marginally better.

I read somewhere that the piston should flop about using its own weight?

Mine did not do that and now having reassembled and run the engine, I am having a worry. Its not super tight, just tight enough to stop it flopping about. I am wondering if it is has worn slightly oval and that is the reason?

Should I run it as is, or should I really take it apart (again) and find the route cause?
What are the risks in leaving it alone to bed in?

Cheers

Steve W

hi steve i would take it apart the gudgeon pin is to tight in the small end and it can make the gudgeon pin turn in the piston when the engine gets hot it should move under its own weight like you say

The pin should move freely in the small end and be tight in the piston. If it is too tight it can make the engine hard to kick over, however your engine is not that bad. The danger is that when the engine warms up the piston will expand and if the small end is tight the piston will pivot on the pin instead of the pin pivoting in the bush. It will not run in unless you don't have any oil in there! Likely cause is that you caught the edge of the bush when removing or fitting the pin and that it just requires de-burring.

Roger Gwynn wrote: Wed Dec 27, 2023 11:46 amThe pin should move freely in the small end and be tight in the piston.

I agree. If the pin is difficult to rotate by hand in the small end bush, the clearance is smaller than it should be, and if feels like it can rock even a small amount, it is bigger than it should be . Quantitatively, modern recommendations are that the clearance between the pin and bush be in the range 0.0008"–0.0012". Such a clearance will make the pin feel like there is no clearance, but will allow it to easily turn.

Thank you all for the replies, sadly it a strip down job again!
Can almost do it with my eyes closed now