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I'm going to have the cylinder of my 2004 TT600RE motorcycle rebored (because it's damaged, not to increase power).

The manual says the piston size is 95mm and the compression ratio is 8.5 : 1.

I'm considering ebaying a 96mm Wiseco piston kit for this engine to save money, but it says the compression ratio is 11.5 : 1.

Does boring 1mm off the cylinder alone increase the compression ratio by that much? Am I looking at the wrong piston model?

Are there pistons that will give a similar compression ratio to the original? I'll be traveling in places with bad fuel (Africa) so this seems important. What are other considerations?

Edit: on the Wiseco website it lists two seemingly identical 96mm pistons for this bike, one with 11.5:1 compression and one with "STD" compression. However, both are 96mm and have a 84mm stroke. One is $184 and the other $196. Have no idea what accounts for the compression ratio difference, and assume from the answers that it must be a typo. Any other thoughts would be much appreciated.

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    There is one thing which I assumed in the answer that might explain the difference in CR even though the bore and stroke are the same. Do the piston heads/crowns look the same, or do they have different surface geometries? – Zaid Aug 14 '15 at 13:51
  • The Wiseco pistons appear to have three cylindrical cutouts on the piston head (judging from the eBay pictures); and I do not believe the OEM pistons do. Could that be it? – bevanb Aug 14 '15 at 14:15
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    It's difficult to tell without comparing the two side by side, but CR will increase if you're adding material to the piston head and go down if material is removed (I think they're called "fly cuts"). If the surface of the stock piston is smooth, it's probably a non-interference design (valves and piston head never occupy the same physical space) and the Wiseco piston has to make the cuts into the head to allow for sufficient clearances. I believe this would explain it. A picture of the stock piston would confirm this. – Zaid Aug 14 '15 at 14:23
  • Looks like there is way more material on the high comp piston than the stock piston. Stock Compression overbore piston ebay.co.uk/itm/… compared to high compression ebay.co.uk/itm/… – Mauro Aug 14 '15 at 14:40
  • Ah, that clears it up. There is one STD compression piston, and one 11.5 : 1 piston. – bevanb Aug 15 '15 at 9:39
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If the height of the new piston above the rod connection centerline is greater than for the old piston, that will increase the compression ratio quickly. For the assumption made by Zaid on the stroke, a height change of 3.2 mm will produce that new compression.

If that is the case (taller piston) you would also need to wonder about valve clearance.

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  • The number matches my calculation as well. Valid point on the valve clearance as well - just because the engine was non-interference in stock guise doesn't mean it would be after modifying the stroke. – Zaid Aug 13 '15 at 13:12
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There is no way that a 1 mm increase in bore diameter will change compression ratio that drastically unless the stroke is changed as well.

My rough calculations indicate that the change in bore will result in a new compression ratio of 8.66:1¹. Even though you're not after power per se, the higher CR should translate to more torque. A rebored piston will also increase the engine's displacement by around 13 cc.

As the eBay item doesn't look like a 'stroker' kit (no connecting rod included), so it looks like item is either incorrectly labeled or is intended for a different engine.

My suggestion would be to get in touch with the vendor to clarify this concern before considering its purchase.

¹ - Assuming bore = 95 mm, stroke = 84 mm

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  • Thanks. I've emailed the company. It seems like it must be a typo or there's something I'm not understanding. – bevanb Aug 14 '15 at 13:47
  • hastingsmfg.com/ServiceTips/piston.htm The top section A could be taller which would change the compression ratio without changing the stroke (i think) – Mauro Aug 14 '15 at 13:52

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