Decking block for budget stroker

[Nohawkscott]

I’ve heard all the basic guidance for the needed machine work to the m20 block for this build but I want to ask the pros here before my machinist starts removing material.

This build has been done many times before but here are the details:
M20B25 Block
M20B27 Crank
M20B27 Rods
M20B25 Pistons .050” over
885 Cylinder Head .008” removed
Stock M20B25 Cam

My current question is regarding the amount to deck the block. I’ve heard that 2mm is the norm but the machinist is being thrown off by the funky profile of the pistons. I’ve attached some pictures he’s sent me today of the piston position at TDC, he’s not wanting to screw up the piston to head clearance. What would the recommended decking amount be? I feel like the .008” removed from the head should be deducted from the 2mm (.0787)
I'm not looking for the max CR here at all, just a good, reliable engine..

Also I seem to remember seeing a picture on one of the 5,000 threads I’ve read that showed a 325i piston at TDC with a machinists ruler laid across it, showing how it sits proud of the block but I can’t find that picture anywhere now..

What do you guys think?

[digger]

The nominal value is 2mm but,

The more accurate method (tbh the correct way) is to use is get a spare head gasket (same thickness as you plan to use) or some shims the same thickness, the head in question (w/o rockers/cam installed ideally) and machine the block such to set the squish (clearance between piston and head) to 0.040" +/- say 0.005" at the big chamfer area around the perimeter checking using clay, or solder etc. The chamfer area on piston mates to the chamfer area on the 885 head

areas on head (corresponding area on piston should be obvious)

be sure to check piston to valve clearance to

[Nohawkscott]

Thanks for the guidance digger, you’re a legend sir.

Im taking my head over to the machine shop in the morning, along with a head gasket, the old head bolts and some play doh. The head is assembled though and I really don’t want to take it apart again. Is there a particular reason why it would be better if the head was disassembled?

[digger]

Thanks for the guidance digger, you’re a legend sir.

Im taking my head over to the machine shop in the morning, along with a head gasket, the old head bolts and some play doh. The head is assembled though and I really don’t want to take it apart again. Is there a particular reason why it would be better if the head was disassembled?

just dont rotate the engine fully without a timing belt for obvious reasons. you could put at TDC #1 (and #6) and use clay on those and then put head on without rotating the crank

[TobyB]

set the squish to 0.040" +/- say 0.005"

I was going to say that you are braver than I am,
but then realized you measured in inches, not mm.
1.0 mm seems about the minimum I can stomach, too.

I WOULD suggest using a used head gasket, unless you're
planning to torque it to spec for the measurement-
they do compress a bit, and some more than others...

t
chicken, and not scared to admit it.

[digger]

I was going to say that you are braver than I am,
but then realized you measured in inches, not mm.
1.0 mm seems about the minimum I can stomach, too.

I WOULD suggest using a used head gasket, unless you're
planning to torque it to spec for the measurement-
they do compress a bit, and some more than others...

t
chicken, and not scared to admit it.

i do know some that say even 0.025" is ideal (thats with forgies to) but to me it is not worth the risk depending how you measure and account for rock, what parts are used, rpm etc and i am not sure there is a reward.

You could definitely do less than 0.040" on a stock low rpm engine with a cast piston and i would if it was for some competitive thing