Scooterotica

Hi all, just building what I hope is going to be a grunty little Casa 185 engine. For maximum torque should the edge of the piston crown (is that the deck?) be below the bottom of the exhaust port by 1mm or above it? I'm sure I read '1mm below' somewhere, but can't find the info. Any tips appreciated please!

Going to be running it with a PWK 28mm and MB BGM Clubman, and Electrix stator with Kytronic adv/retarder.

Preferably level or just below

It's open to debate. Many tuners recommend that the piston crown descend no lower than the bottom edge of the exhaust port. Any lower and in theory exhaust gas flow would be disrupted at the abrupt edge of the exhaust port. The theory is also that a piston crown that is slightly above the bottom port edge will flow gases better than one below the port edge.

I'm struggling with this one. The theory seems to indicate that raising the exhaust port (or lowering the piston, whichever way you want to look at it) gives greater power higher up the rev range at the expense of low down torque- something I'm keen to avoid as the Casa kit is all about torque.

So is it better to run with the piston edge level with the bottom of the exhaust port, or with the edge slightly above? Mine is about 1mm above currently.

Deck height is where the top edge of the piston is in relation to the top face of barrel at tdc as far as i know. I would try to get the piston as level as i could with bottom of exhaust port at bdc with use of a packer. Though this might throw your squish out depending what crank/rod your using.

Yep, re squish, I'm at 1.1mm as is, so I'm loath to fit anything more than a 0.5mm packer, and ditch the fibre base gasket.

bazza3004 wrote:Deck height is where the top edge of the piston is in relation to the top face of barrel at tdc as far as i know. I would try to get the piston as level as i could with bottom of exhaust port at bdc with use of a packer. Though this might throw your squish out depending what crank/rod your using.

it was explaind to me by the late terry frankland as the measurement from the crank center to the gasket face were the barrel fits on the casing mouth ?

Level is supposed to be ideal, too high and the heat builds up at the crown of the piston and can lead to erosion there, too low and the timing's too long. A small bit either way won't make much difference on a low level tune, but check your timings anyway.

shocky wrote:

bazza3004 wrote:Deck height is where the top edge of the piston is in relation to the top face of barrel at tdc as far as i know. I would try to get the piston as level as i could with bottom of exhaust port at bdc with use of a packer. Though this might throw your squish out depending what crank/rod your using.

it was explaind to me by the late terry frankland as the measurement from the crank center to the gasket face were the barrel fits on the casing mouth ?

That would actually make sense shocky re the deck height of the casing. Although when setting top ends tuners have stated to me deck heights relating to piston to barrel face. Not saying this is correct , just what ive been told. Also had barrels back from tuners with( for example ) +1mm deck height wrote on top of barrel. Cheers. Baz.

Ok, many thanks. I'm going to stick with the 1mm above the bottom of the port, and see how it goes. I'll report back with findings...