1st over size piston & stock size rings

[badassjamie]

Going thru this basket case and found the pistons are .25, or 1st over.
I also have 2 new in the box stock bore ring kits.

Any chance using those or must I get the 1st size over rings as mandatory?

Thanks

 

[XSLeo]

It depends, Are the bores stock or first over? If stock you can use the stock rings with stock pistons. If first over you need the first over pistons and you will need first over rings.
The stock bore is about 75.010 to 72.016. At least in the engines I've torn down. The first over bore is 75.25. Rings have a certain amount of end gap. This is the gap between the ends of the rings when installed. the top two rings gap is .2 mm(.008 inch) minimum, .031(.8 mm) maximum, for the 70-77, the 78 up the maximum was changed to .040 (1.0 mm)
This applies to stock as well as oversizes. If you try using standard rings with the first over the gap will be too wide. Take the bore size times pi to find circumference of both sizes then subtract the standard from the oversize. Pi being 3.14.
75.010 x 3.14 = 235.5314, 75.25 x 3.14 = 236.285
236.285 - 235.5314 = .7536
So the end gap will increase by .7536
This would make the gap .7536 + .2 = .9523 mm to .7536 + .8 = 1.536 mm. The tension of the rings against the bore will be greatly reduced by a gap this large. Between the huge gap and very little pressure of the ring on the cylinders you will get tremendous blow by of compression prfessure past the rings.
What I might do is measure your bores to determine if stock pistons can be used or you need the first over. My guess is that they are there then you need the bore. If you need the bore you need the rings.
If you don't have the tools or skills needed to accurately measure the bore sizes take them to a machine shop. Most won't charge much. If they do the machining they may do it for free.
Leo

 

[badassjamie]

The bore I'm dealing with measures 74.50 on both cylinders so it looks like I have the stock cylinders with 1st over pistons in the case.

This was a basket case and the pistons in the lowend look like the've been used but are clean enough to suggest that they have been cleaned so I don't know that these pistons were the ones used in this cylinder.
There are new stock rings with the bike from Mikes in the package but the pistons are 0.25's.
I haven't measured the rings but I think it's safe to assume that I either re-bore to 1st over size or get stock pistons?
Suggestions?

 

[5twins]

You'd better measure that bore again. Stock bore is 75mm, you're a half MM under that. That's not possible unless maybe new undersized sleeves have been installed and need to be bored for whatever pistons are going in there.

 

[badassjamie]

It is 75mm, my measuring is off.

Is it better to keep the 1st over pistons, get new rings and bore my cylinders or go back to stock pistons in my stock cylinder thats in good shape and use the new 75mm ring kits I already have?

 

[5twins]

I don't know if one's better than the other, I'd go with what cost less to do. I don't think you'll save much going either way though. Rings are cheaper than pistons but the rings you need plus a bore job will probably be close to what new standard pistons cost.

 

[XSLeo]

On the stock pistons, when they cast them they are not all the same size. They sort them by size.They stamp the 3 digits after the decimal point into the piston crown. The sizes are all 74.xxx. So if a stock piston marked 956 it's size is 74.956.
They then match the pistons into sets by size. They then bore the cylinders to match the pistons. The standard clearance is .050 - .055 mm Most are bored right in the middle of this spec. So they would bore the cylinders to piston size plus the .050 - .055 clearance. Right in the middle would be .052, 74.956 + .052 = 75.008. The size is stamped on the sleeves below the cylinders just as the pistons are marked, the last three digits of the 75.008 are marked on the sleeve as 008.
The pistons are not all the same size, I have them that vary from 74.959 to 74.963. This makes the bores size vary from 75.010 to 75.016.
So as you can see your at 75 mm even is still a bit small.
I think what you need is more precise measuring tools or as mentioned have a machine shop measure them for you.
As it stands you can't really tell just what size your bores are. Without that you can't tell just what you need.
Leo