Need Piston Help

[udubec]

I have recently been restoring a 1990 Vmax 1200 and decided to replace the pistons from the original size to 0.25mm oversized. I also purchased the oversized rings with the aim to improve the overall performance back to ‘new’. As I went through the process of taking measurements I found the piston to be a different size at the top than at the bottom for both the old and the new pistons. Both by 0.55mm.

Is this right?

What should I do about the reboring of the cylinder? Should it be straight or is it supposed to be tapered? I do not want to rebore and find that I have messed something up. Ultimately what size should the cylinder be rebored to for the new pistons?

Please forgive me if I am asking rudimentary questions, but after researching I cannot find anything and as a newbie I really need your help.

Thank you in advance!!!

 

[sdt354]

That is normal piston shape. Remember the rings are not installed when you measured it and wouldn't fit, with room for expansion if the lands were the same size as the skirt. Plus friction is taken into account with piston design. You wouldn't want the top of the piston touching the cylinder walls. A piston also isn't perfectly round and will be largest perpendicular to the wrist pin. The shop manual will have bore and piston clearance specs to answer your vmax specific questions. Cylinders are bored with the hope of perfection, though not practically achieved, then honed to final size. As with anything there are specs to keep within. https://dl.dropboxusercontent.com/u/55030067/VMX12%20Service%20Manual.pdf will have a lot of answers for you. Damon's video would be a great help also. Sean Morley does a lot of this kind of work and is a great source of information and parts too.
Steve-o

 

[one2dmax]

Steve - exact information there and no extra comment is needed from me!

 

[udubec]

Thanks Steve and Sean! Huge help!