type 2 85.5mm dished pistons

[iam64185]

does anyone know of a difference in pin height between a flat top type 1 85.5mm piston (a type i believe is what this piston is called, not the b type for strokers) and the type 2 85.5mm dished bus piston? my dished pistons are cast mahles.

[Marc]

Pin height of the 211 pistons is the same (39.6mm) as the stock 311 flatttops....C.R. difference is solely due to the dish volume.

[iam64185]

ok, here's why i asked. i'm building a 1745, 85.5x76. many years ago (about 30 actually) i built this same engine. took my original 85.5's out of my 73 and milled the crown 3 mm. i didn't realize at the time that they made cylinder shims (no internet forums). built the motor with a deck of about 1 mm which is what i expected after doing some math. the crank throws the piston an extra 3.5mm out, so, 1.5mm(deck) - 3.5mm(crank) + 3mm(piston) = 1mm deck. it ran fantastic for about 6 months. one (only one) of the cylinders eventually cracked. ok, live and learn. i eventually picked up a 74 and built a 2016 with shims. did the same type of math and the deck worked out just right upon assembly. also discovered when pulling the 74 apart that vw made pistons with a dish as the 74's were dished. so when i decided to build this motor again i decided the way to go was dished pistons. extra crown material. i had them milled 1.5mm and included 1.5mm in cylinder shims. so, 1.5mm(orig deck) - 3.5mm(crank) +1.5mm(piston) +1.5mm(shim) = 1mm deck. imagine my surprise when i bolted this all together and had 2.2mm in deck height. my first thought was that the pistons were over machined. the original dish was 3mm deep. measured it before the machine work was done. the machined piston dish is 1.5mm deep. my machine shop didn't screw things up. this is all assembled on a brand new dual relief case that i got from scat. case came in a blue plastic bag with vw and audi logos on it. the crank and rods are the same ones that 30 years ago worked out correctly with the math. where did the 1.2mm in deck height error come from? near as i can tell, 2 places for error - case or pistons. you're telling me it's not the pistons.

[Marc]

...1.5mm(orig deck)...

But you assumed that, rather than actually measured it, right?

...2 places for error - case or pistons....

Actually 3, if the cylinder height is abnormal in this set.

I presume you're measuring piston deck with the cylinders snugged down against the case.

[iam64185]

ok, unfortunately for me but not for you, a few days ago i checked the piston ring install instructions and i have to redo the rings as i apparently installed the oil rings incorrectly. so i need to pull the cylinders off the pistons. when i do i will measure the cylinders and post the dimension.

[iam64185]

top end redone. 112.75mm was what i found for the cylinder install length. oh, and to answer the snug down question. yes, the assembly was measured tightly snugged.