Is it true that a piston is't exactly round My cylinder has been honed and the measurement at the front is 0.010 mm (0.004") The measurement at the gudgeonpin side 0.025 mm (0.10"). My old piston gave the same difference.



John

email (option): john.kater@chello.nl

Yes John. The pistons skirts are ground to an oval shape. The measurement for skirt clearance should be taken from the widest part fore and aft. Ron

email (option): ronpier@talk21.com

Thanks Ron,I am reassured. But why is the piston oval shaped
Is that for lubricaton purpose.

John

email (option): john.kater@chello.nl

As with slipper type racing pistons with almost nothing at the sides at all, its evident that the front & rear areas are the important areas as far as force is concerned.

Add that to the theory that the gudgeon pin & immediate area around it is the part that gets the hottest & thicker material & is therefore subject to greater expansion those areas (the sides) are given more clearance.
Another benefit is that friction is reduced at the sides.

Someone will no doubt offer a more technical explanation but you get the idea.

Hi John..Pistons are actually quite a complex shape..The diameter from the bottom ring upwards is usually smaller than the measurement across the thrust faces (fore and aft as Ron described). Also the piston is smaller along the gudgeon pin axis (side to side). This 'side relief' and the reduction around the ring lands (when used) is to reduce the amount of friction generated by piston contact with the bore....Ian

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