Can someone measure a late M20B25 pistons for me?

[16vCorey]

If anyone has a loose piston laying around, I need to know the distance from the flat spot in the picture to the wrist pin, and it has to be a short skirt piston.

[LJ851]

Can you narrow "flat spot" down ?

Which part of the piston?

[16vCorey]

Sure. Here's a pic of the area I'm talking about.

[Madhatter]

what are you using the measurement for?

[digger]

its gonna be 5mm

[16vCorey]

its gonna be 5mm

There's way more than 5mm of material there. I need a more precise measurement than that.

[16vCorey]

what are you using the measurement for?

I'm building a 2.8L stroker. The pistons I have are the early long skirt pistons, so I turned the skirts off of them. I've turned almost .4" off of the counterwieghts on the 2.8L crank, but it's still rubbing on the flat spot I circled, and I've heard that .3" should be enough. I need to see if the material is thicker there on my pistons than on short skirt pistons, because if it is, then I need short skirts. If it's not, I just need to cut more off of the crank. I don't want to cut too much off of the crank, because they get hard to balance if you do that.

[digger]

There's way more than 5mm of material there. I need a more precise measurement than that.

the one i had measured at 5mm 0.2" with verniers depth gauge from flat to the pin surface.......ill find another style if i have one lying around

if you have taken that much off the crank C/W and it rubs there something seems wrong. the one below the CW not touched but a 135mm rod.

11-12mm off the C/W and 5mm shorter rod should clear easily

http://www.r3vlimited.com/board/showpost.php?p=4008354&postcount=46

[16vCorey]

Yeah, the early i pistons have a weird raised spot that's the issue. I think some short skirt i pistons are all I need and it will clear with no problem.

[digger]

Yeah, the early i pistons have a weird raised spot that's the issue. I think some short skirt i pistons are all I need and it will clear with no problem.

I’ll check but i have early and old pistons KS and mahle and the only differences i noticed were the skirt. i also have a 9.7:1 b25 piston.

What is the radius of the C/W after you machined them? they start at around 73mm from memory which.

So doing the math’s with b28 crank, 130 mm rod

130 mm – 84/2 mm = 88 mm

So with a 73mm radius you have 88 mm – 73 mm = 15mm to fit the pin radius, throat thickness and some clearance

Stock pin is 22 mm (11mm radius) so with 5 mm throat the interference is

15-11-5= - 1 mm between the piston and C/W

So as a minimum you would need to take of about 5mm off the C/W to give 4mm clearance

[16vCorey]

5mm is probably right. Here's my measurements of an eta piston and an early i piston with the skirt shaved. There's a raised spot on the early piston that the late piston doesn't have, plus it looks like it's a bit thicker in general to begin with. So including the raised spot, it's they're about 8mm from the wrist pin and barely scrape. 5mm should clear just fine.