Casa 185 piston edge (deck?) to Exhaust port height

[Pompeydave]

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? 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.

[mickyb]

I believe that the piston should be level with or just above the bottom of he exhaust port at BDC not below as this can cause a heat build up at that point on the piston. No doubt someone who knows better will correct me if I'm wrong

[Wack]

I would have thought you'll be going the wrong way if the piston is below the exhaust and transfer ports as the timings are longer.

[Pompeydave]

Ok, many thanks. looks like 1mm above the bottom of the port is the way to go.

[Scooterlam]

what case are you using, indian, Italian Spanish?
what squish (piston to head) if any, measured around the edge with out head gasket.
Alternatively what's the measurement from the bottom of the exhaust to the top of the barrel and is the head recessed.

[Pompeydave]

Apologies, should have mentioned the country of origin and squish- Italian GP125 casing with a squish of 1.1mm.

[Scooterlam]

With or with out base gasket?

[Pompeydave]

With base gasket. No head gasket- looks like it doesn't use one as the head is recessed to fit more squarely on the barrel.

[Scooterlam]

1.1mm squish is at the lower end of safe but I,ve run this before with no problems.
My thoughts are that the piston should be flush with bottom of the exhaust port at Bdc. the theory is that the hot gases escaping will hit the wall of the barrel and cause excessive heat. But If my understanding of the 2 stroke is correct by the time the piston is that far down, the exhaust port will effectively be closed or being feed by a fresh charge that has already passed thru the cylinder and is being forced back into the cylinder by the reverse pulse from the chamber.
a rather long winded way of saying, If it were mine I would fit and run.

[Pompeydave]

Yep, that's my instinct too. I'll run it as it is and report back. Thanks for the advice .