Rb25 piston to deck clearance.

Hi i have fitted wossner pistons and a set of rods to my rb25det. I have measured the piston to deck clearance in two possible locations. One gets me -0.59mm the other -2.75 which seems way too much. Ive attached photos if someone could advise me which one to go off. Its not included in the data that came with pistons.

Hey buddy the measurements are normally taken at the front and back of the piston inline with the crankshaft centerline. So in your situation that would be on the lower ridge of the piston (directly in front of the arrow) but first zero the dial to the deck surface adjacent to that location. If the tip on your dial is to big to fit on that spot then there is an alternate method of taking it at the side of the piston (at the edge), rock the piston fully to one side then the other and the total between both positions you divide by two.

There are two schools of thought on measuring piston deck height. One school is that you measure on the edges of the piston as you've done in the first photo and rock the piston to each extreme of travel and average the results. I don't agree with this because the amount of rock you'll get at room temp is nothing like what the piston will experience at operating temp so the numbers are misleading. I prefer the technique where the deck height is measured in line with the wrist pin as Mahlon has referred to. Check out our webinar on the topic here: https://www.hpacademy.com/previous-webinars/154-measuring-deck-clearance/