Another cr250 question

[Gerry MacNutt]

Hi, on 2002-2004 CR250 could someone please tell me in stock OEM state with the one stock OEM base gasket what the piston the deck height is measured from side edge of piston and what that cc volume is for the volume below the deck.

Thanks

[Ian Harrison]

Hi Gerry

With a 1.5mm spacer plate and 2 x Honda standard (0.5mm) base gaskets (2.5mm total spacing) we are about 2.2mm below the deck.

So knock 2mm off that (spacer plate plus 1 x gasket) = 0.7mm below the deck in standard format.

Volume in that depth below the deck = 3.47 (cc's/mm) x 0.7 = 2.43cc

We always measure piston recession with a digital vernier depth gauge with it's foot spanning the cylinder face and the probe in the centre of the piston.

We also measure the squish clearance by mounting a dial gauge in the head insert and zeroing it against a piston resting against the insert (zero squish clearance), then reading the difference when installed on the engine (actual squish clearance). This is the only really accurate way to do it!

Likewise the volume included in the squish clearance is 3.47 x measured squish clearance in mm.

All the above assumes you are using a flat top Honda piston (std 2002-2004 spec).

If your using a head gaket (rather than 'O'-Rings) remember to add-in the volume and clearance for that, about 0.5mm compressed from memory. 3.47 x 0.5mm = 1.735cc

I'll measure on a brand new stock motor at work tommorrow just to check if there is a bit of variance due to gasket crush, etc.

We ignore the tiny volume between the piston and bore between the top ring and the top of the piston. As it's always there and we always ignore it, so it has no significance.

Incidentally, if you are measuring installed volume when using a toroidal head insert with a burrette, you can't do it when the head is installed on the engine as there will always be a small volume (perhaps 0.5cc) of air trapped above the edge of the plug hole. You have to measure the volume of the insert uspide down on the bench and then add the measued volume to the calculated clearance volume to give you the absolute installed volume.
With a "pent-roof" type insert, you can just measure on the engine with the piston grease sealed to the bore at TDC.

At Viper we always talk about combustion volume in installed cc's including the plug-hole.

Long-winded answer to a simple question!!

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Gerry MacNutt]

Thanks you very much, once again you provided exactly what I needed
Thanks!