Scooterotica

Hi,
I’ve got a new MEC 175 crank. The big end does not look shimmed. There is too large a gap between the small end bearing and the sides of the piston. I don’t have shims on hand but I do have a wider small end bearing. Assuming it’s within tolerance of the sides of the piston, is that sufficient? I’m just trying to think this through. Wouldn’t the piston still float because the gap between the short arm of the connecting rod and the piston walls remains too large? Whereas a shim fills the gap of both the needle bearing and the small end of the conrod stopping the piston from floating around?



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Davidsquaredson wrote:Hi,
I’ve got a new MEC 175 crank. The big end does not look shimmed. There is too large a gap between the small end bearing and the sides of the piston. I don’t have shims on hand but I do have a wider small end bearing. Assuming it’s within tolerance of the sides of the piston, is that sufficient? I’m just trying to think this through. Wouldn’t the piston still float because the gap between the short arm of the connecting rod and the piston walls remains too large? Whereas a shim fills the gap of both the needle bearing and the small end of the conrod stopping the piston from floating around?



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I see now that I need shims. It’s only if I don’t need shims do I use the wider roller bearing

Thx
David


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I've always fitted the widest small end bearing that will fit. That said, I don't have an issue with proper shims.

A crankshaft will work without shims either side of the big end bearing, but not really preferable. I have seen overheated con rod sides as the result, so if the float can be practically controlled, I would opt for that. However, I would ensure that the oil slots in the conrod little end were sufficient, & even chamfer up to the centre line the piston bosses, just as OEM DT175 pistons were........