Compression, base gaskets, squish, valve clearance on little monsters.

[suzyj]

So nobody makes high compression pistons for my 695. They used to be available for the s2r 800, so I always had the option of swapping pistons and crank and turning my 695 into an 803cc fire breathing... Ummm... Monster. But they appear to have dried up as well, so back to the drawing board.

Brad has a cool note on playing with base gaskets on a 900 to increase compression, where he basically came to the conclusion that he could get better results by advancing the valve timing, which has  intake/piston clearance implications, making dropping out the base gasket problematic.

http://bradthebikeboy.blogspot.com.au/2012/05/setting-squish-and-cam-timing.html

Anyway I have been doing some math, and I'm starting to think that on the 695 there's more value from reducing the base gasket thickness than on a 900. The reasons are twofold - firstly the stroke is shorter (57.2mm vs 66mm), and the compression ratio starts bigger (10.5 vs 9.2) meaning the combustion chamber volume is much smaller. the end result is that, by my calculations (which might be stuffed up, as math was never my strong point), a 0.4mm decrease in gasket thickness (omitting the gasket, which is 0.4mm) increases compression ratio from 10.5 to 11.3, quite a big increase.

Of course I've not actually measured squish and valve clearances, either stock or with no base gasket, but now I'm intrigued.

Has anyone tried this sort of thing on the little monsters?

[Speeddog]

Omitting the base gasket will push your CR from 10.5 to 11.2.

The OEM CR may not actually *be* 10.5, but you'll get that ~0.7 increase anyway.

You may not be able to go without that 0.4mm base gasket, depending what the squish is now.
The generally accepted squish is 1.0 mm for Duc motors.
I suspect Brad would have an idea of how much less than that you could get away with.

As an alternative, or in addition, you could machine the head and the squish area in it.
Or just omit the gasket and set the squish by machining the squish area on the head.
That may require a bit of machining of the valve pockets in the pistons to restore clearance there.

The 695 seems to have pretty good cams, but it would be worth checking/setting them to the best number.

Brad's *done* a lot of this stuff, I'm working from the little I've done and the rest from what he's done.

[brad black]

I've not ever touched a 695, so have no specific experience.

see what clearance you have now.  generally on the 2v motors the piston outer crown taper is less than the head squish band taper, so when they get tight on squish it's right at the very outer edge.  so if you want to drop the cylinder the 0.4mm of the base gasket you may need to take 0.2mm or so off the very outer edge of the piston at the right angle.  maybe 3 vs 7 degrees or so.

but you'd need to check the piston to valve clearance first, to know what clearance you have and how much clearance you have to play with.  also, I tend to run the 800 cam at the spec, as they're fairly short duration, so that should mean there's a bit to play with.  if there's a heap of clearance then you could machine the bottom of the cylinder down and machine the piston as required to give it even more comp.

[MonsterHPD]

Just a small note out of exerience: if you drop the cylinder head by omitting the base gasket, also check the valve-to-piston clearance around the valve circumference. Dropping the cylhead will decrease the clearance here (if the piston has valve pockets; don't specifically know about the 695). 

[brad black]

they're not overly deep, probably due to the shorter duration

photo here: http://www.ebay.com/itm/DUCATI-MONSTER-695-2007-2008-PISTON-BRAND-NEW-/170933997885?pt=Motorcycles_Parts_Accessories&hash=item27cc75d13d&vxp=mtr