stu_d909

hi lads

building a cosworth na at the stage of working out the comp ratio using this site as a guide http://chainganger.co.uk/Page2/Page2.html now i am using accralite 92mm pistons high comp ones as shown on burton power which say it gives an 11.8:1 CR but on accralites site it says 12:1 so thought i best check it now using the site i mentioned earlier i put it in excel to make the calculator work and it came out with 15.03:1 which is way to high i need to be about 11:1 so i gotta skim the pistons down to lower the comp ratio has anyone else built one of these engines and had to machine the pistons or have they fitted em off the shelf and went with it????

cheers for any help

stu

nixon_2wd

Measure the combustion chamber volume after you mocked up a piston. Measure the cylinder head chamber aswell, and add the thickness of the gasket you are going to use.

stu_d909

done that going by the site i put the piston 3mm down from deck height to top of the dome on the piston bit o plastic n grease filled it up it took 26ml to fill. head is 47cc n i put in a gasket thickness of 1.5mm to save skimming the pistons but looks like i will have to take a couple of mm off them was just wondering if anyone has got away with firing in pistons off the shelf n that was that