Piston Question (I may have royally F'd up)!

[76static]

So I'm doing a top end rebuild and I ordered a set of oversized pistons. I got them in and took them along with my jug to get bored. I got them and the jug back today and while looking at the new piston and an old one side by side I notice that pin size and location is different. "oh shit these are for fist gen XS!" I ordered the 70-73 pistons and I have a 76. I'm sure I can ship them back and exchange them but the cylinders are already bored. Is there any difference in the bore on the early model XS?

 

[nj1639]

A bore is a bore is a bore-- IF the diameter of the proper oversized piston is the same as the one they bored the jugs for then I'd swap, run it, see what happens and chalk it up to experience.

 

[76static]

A bore is a bore is a bore.

I get that, that's why I'm asking if there is any difference in the bore on 70-73 from 74-82. If it's just the pin size/placement, then I know I'm alright I just wanted to know diameter.

 

[burns]

I get that, that's why I'm asking if there is any difference in the bore on 70-73 from 74-82. If it's just the pin size/placement, then I know I'm alright I just wanted to know diameter.

My Haynes manuel tells me that they are all the 650's have a 75 mm bore. Which makes since since they all displace 653cc and all have the same stroke.

Because there has historically been a fair amount of variance in the actual diameter of individual pistons the tradition is to measure each piston and bore to match.

I understand that some car guys don't do that any more because the specs on modern pistons can be trusted to be on the money - at least for the common pistons (e.g. SBC)

I don't know if that is the case with XS650 pistons in general or those of any particular manufacturer.

If it were me I'd have the pistons that I was putting in mic'ed and the liners bored to match.

Of course I would not be putting a stock bore piston in anyway.

 

[76static]

Okay cool. I plan to take the new/correct ones to my guy that did the bore and have him measure the new ones and make sure they are still within spec. I figured they were the same size I just couldn't find anywhere online saying that the 70-73 pistons were 75mm.