61mm Bore To Piston Rule Of Thumb

There are some odd sayings out there, and this is one of them. Rule of Thumb. Something about the King's Thumb being the rule of measurement???

Anyway, I' have a used zx550 Engine that came with a 615cc kit(61mm pistons) in it(XC Rings, and no one can locate a set, not even Wiseco, Hastings...), and I've lightly hand-drill-honed the cylinders, and have measured the bores. 12 and 6, then 3 and 9 o'clock top, middle and bottom. Only 5 of the 25 measurements weren't 2.402", and these were 2.401" and 2.403". This engine only had 5000 miles on the odometer, but from the saftey wiring, it's obviously been tracked then sat more than 10 years. Hand honing came out very clean and before I took the measurements listed above.

What's the rule of thumb for an oversize aftermarket piston to cylinder clearance? I know I'll need to check the new rings when they arrive, and set the gap. I'm not boring the cylinders, just cleaning them up for new pistons and rings, since the XC Pistons and Rings in 61mm are a thing of the past.

Any feedback would help.

If you are using new pistons, go by the manuf recomended clearance, some vary a bit.
also they should have a sopt to measure the piston, but most of the time its in the center of the skirt, the bottom will be a bit less in size.

Thanks. The Factory Clearance was what I was going to go by with the stock pistons, and stock bore, but even with the 3mm overbore and pistons?

I think what bskz650 means by factory recomended clearance is Wiesco's , or J&E's clearance #'s.
I just had Fast BY Gast bore my cyl for J&E's . the clearance J&E recomends is .0025".
I take it that u could not find rings for the old pistons?

I can't help myself. A guy at work told me that the "Rule of Thumb" pertained to beating your wife back in medival times. The instrument used for said beating could not exceed the width of your thumb. (This is third party information and should not be construed as fact) Ride safe, Steve

if they are wiseco's with that small of a cylinder they usally run .0015"-.002" piston to wall clearance what are your piston measurements and what did you measure the cylinder with.

mark1122 wrote:

I take it that u could not find rings for the old pistons?

No I've been this route on these pistons and rings. They are the thicker rings, and I'd need new 61mm Pistons, and Pins, to go with my new XA Rings. I have a new set of XA rings because I ordered them before inspecting the rings there now. :blink: I can't measure the piston to cyinder wall clearence without the set of pistons.


timebomb33 wrote:

if they are wiseco's with that small of a cylinder they usally run .0015"-.002" piston to wall clearance what are your piston measurements and what did you measure the cylinder with.

Thanks for the information. I measured the cylinders with a telescoping gauge, then 2-3" micrometer, converting inches to mm afterwards. I have a dial bore gauge that rolls down the bore, and that needle didn't budge much on both checks. 12 and 6, then 3 and 9. I haven't measured the piston pins that came out of the XC pistons. Wouldn't I need new pins with new pistons? There were very hard to extract from the piston.

Thanks guys.

Forged pistons need more clearance than cast ones. they expand more with heat. allways go by the clearance the manufacturer of the piston you will be using reccommends, and measure the piston in the location they specify, as pistons are tapered not a perfect cylinder shape. someone has rings for those pistons...check a few performance shops. or maybe has a custom set made if its not too much

Corey, if u buy new pistons they will come with new pins and rings. The pins should not be hard to remove.u must have a burr in the pin hole or something.
If u call a shop that sells pistons,like MTC, J&E or even APE, they can tell u what bore size u need. This will help u determin if u need a stock sized piston, or a rebore for bigger size piston .

corey how where you measuring the bores with a bore gauge if you did not have the pistons you have to set the bore gauge up to piston size in order to use that tool properly.

3 tools used in measuring the bore. I know the bore of a 61mm oversize piston, so I used the telescoping gauge that the bore would be in between.

2-3" micrometer to measure the bore once I've removed the tool from the bore,

and then I have a dial bore gauge to measure taper from top to bottom for any variance.


I was just checking the cylinder bore for wear after lightly honing it by hand, to see if there was any deep grooves. I would need the pistons that are going to be used to get the correct piston to bore clearance measurement.