3.1 Stroker Questions

[E30arsen]

So I'm in the middle of a 3.1 stroker and the machine shop called me today and said the pistons are sticking passed the deck of the block by 0.066". I was wondering if that's what happens because of the longer stroke or is something else wrong?

For reference I'm running vac 11:1 compression ratio pistons with m20 connecting rods and s52 crank.

[downforce22]

So I'm in the middle of a 3.1 stroker and the machine shop called me today and said the pistons are sticking passed the deck of the block by 0.066". I was wondering if that's what happens because of the longer stroke or is something else wrong?

For reference I'm running vac 11:1 compression ratio pistons with m20 connecting rods and s52 crank.

If you are building a 3.1l stroker you need custom pistons (Which you have) and the crank sprocket spacer/ oil seal. It also needs a block overbore and possibly the bottom of the cylinder bored clearances to clear the rods.

Stock rods are 135 mm
the s52 crank has a 89.6 mm throw
overbore + 1 or + 2 mm to get over 3.0 L of displacement, 86 mm bore is about the biggest you can go.

In order for the combo to work you need custom pistons which you seem to have gotten from VAC. I would verify the order of the pistons because it sounds like they were not built to spec, or the specs they used were wrong for this combo.

Depending on the design of the piston, the crown could stick above the block (like the stock 'i' pistons) which is why most 3.1l strokers can't use a stock headgasket, the pistons come out of the bock a bit and might hit the headgasket.

.066" is 1.68 mm which is way more than a correctly designed pistons should come out of the block, atleast if they are measuring from the side of the piston and not the center.

Looks like if these are your pistons they come out of the bore a bit, maybe the 'piano top' style pistons is what they are measuring. I would verify where the difference is and call Vac with the order number to get some dimensions and have them verified with the machine shop.

https://store.vacmotorsports.com/cp-carrillo-bmw-m20-forged-piston-set-for-896mm-stroker-crank-p2921.aspx

https://store.vacmotorsports.com/cp-carrillo-forged-piston-set-bmw-m20-b25-stroked-p3724.aspx

[E30arsen]

If you are building a 3.1l stroker you need custom pistons (Which you have) and the crank sprocket spacer/ oil seal. It also needs a block overbore and possibly the bottom of the cylinder bored clearances to clear the rods.

Stock rods are 135 mm
the s52 crank has a 89.6 mm throw
overbore + 1 or + 2 mm to get over 3.0 L of displacement, 86 mm bore is about the biggest you can go.

In order for the combo to work you need custom pistons which you seem to have gotten from VAC. I would verify the order of the pistons because it sounds like they were not built to spec, or the specs they used were wrong for this combo.

Depending on the design of the piston, the crown could stick above the block (like the stock 'i' pistons) which is why most 3.1l strokers can't use a stock headgasket, the pistons come out of the bock a bit and might hit the headgasket.

.066" is 1.68 mm which is way more than a correctly designed pistons should come out of the block, atleast if they are measuring from the side of the piston and not the center.

Looks like if these are your pistons they come out of the bore a bit, maybe the 'piano top' style pistons is what they are measuring. I would verify where the difference is and call Vac with the order number to get some dimensions and have them verified with the machine shop.

https://store.vacmotorsports.com/cp-carrillo-bmw-m20-forged-piston-set-for-896mm-stroker-crank-p2921.aspx

https://store.vacmotorsports.com/cp-...ked-p3724.aspx

I bought these pistons, crank, and oil seal spacer from the forum. He never used them. The pistons are 86 mm and for an 89.6 mm stroke.

Would it make a difference using s52 connecting rods?

Because the guy I got them off of were going to use high strength s52 rods but I couldn't afford them, so I decided to reuse mine.

I'll try calling vac tomorrow because they're already closed right now.

[digger]

What part of the piston sticks above the deck of the block ? I.e. Is it at The edge or is it the dome closer to the middle.

Find out the compression height CH of the Pistons from vac.

Ask the machine shop for the deck height.

The s52 rods are the same Length as m20b25 rods

[E30arsen]

So I think it's the edge of the piston.

The compression height is 1.128"/28.65 mm

So I called CP Pistons and they said the block was probably decked. Because the amount it's sticking out by is way too much.

The person I was talking with said that my best bet would be to run a thicker head gasket.

So my next question is how thick should I get? Because the stock thickness is 0.070", so should I just get a 0.120" or a 0.140"?

[E30-325iS]

The pistons are for a 86 mm stroke crank.
(86/2) +135 +28.65 = 206.65

[ForcedFirebird]

So my next question is how thick should I get? Because the stock thickness is 0.070", so should I just get a 0.120" or a 0.140"?

Place the pistons as close to .045" (1.14mm) from the head as you can, at TDC. Since we don't know how much your head is shaved, the head gasket advice will vary - plus you will want to leave room for head rebuilds in the future, safe to say, leave .005" (.13mm) extra for future rebuilds.

.120" sounds like the ticket to me, but best to measure.

[E30arsen]

I was think that these pistons might be for a different crank. I figured it was matched for each other because the guy I bought it off of was super knolageable.

Okay, I'll talk with my machine shop and get all the measurements verified before buying the head gasket.

Thanks everybody for the help.

[digger]

yeah just needs custom MLS, like FF said whatever gives something 0.040-0.050 clearance is a good area to be in

[E30arsen]

Alright, so .120" will work perfectly. Thanks for the help guys.
Also a follow up question, what weight of oil should I run?


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