diff --git a/fr32/fr32.go b/fr32/fr32.go
index cb2dcaed6..06579dd0d 100644
--- a/fr32/fr32.go
+++ b/fr32/fr32.go
@@ -10,7 +10,7 @@ import (
 
 var mtTresh = 32 << 20
 
-func mt(in, out []byte, padLen int, op func(in, out []byte)) {
+func mt(in, out []byte, padLen int, op func(unpadded, padded []byte)) {
 	threads := padLen / mtTresh
 	if threads > runtime.NumCPU() {
 		threads = 1 << (32 - bits.LeadingZeros32(uint32(runtime.NumCPU())))
@@ -35,6 +35,15 @@ func mt(in, out []byte, padLen int, op func(in, out []byte)) {
 
 // Assumes len(in)%127==0 and len(out)%128==0
 func Pad(in, out []byte) {
+	if len(out) > mtTresh {
+		mt(in, out, len(out), pad)
+		return
+	}
+
+	pad(in, out)
+}
+
+func pad(in, out []byte) {
 	if len(out) > mtTresh {
 		mt(in, out, len(out), Pad)
 		return
@@ -79,13 +88,19 @@ func Pad(in, out []byte) {
 	}
 }
 
+
+
 // Assumes len(in)%128==0 and len(out)%127==0
 func Unpad(in []byte, out []byte) {
-	if len(out) > mtTresh {
-		mt(in, out, len(in), Unpad)
+	if len(in) > mtTresh {
+		mt(out, in, len(in), unpad)
 		return
 	}
 
+	unpad(out, in)
+}
+
+func unpad(out, in []byte) {
 	chunks := len(in) / 128
 	for chunk := 0; chunk < chunks; chunk++ {
 		inOffNext := chunk*128 + 1
diff --git a/fr32/fr32_ffi_cmp_test.go b/fr32/fr32_ffi_cmp_test.go
new file mode 100644
index 000000000..24bb24d49
--- /dev/null
+++ b/fr32/fr32_ffi_cmp_test.go
@@ -0,0 +1,66 @@
+package fr32
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	ffi "github.com/filecoin-project/filecoin-ffi"
+	"github.com/filecoin-project/specs-actors/actors/abi"
+
+	"github.com/filecoin-project/sector-storage/ffiwrapper"
+)
+
+func TestWriteTwoPcs(t *testing.T) {
+	tf, _ := ioutil.TempFile("/tmp/", "scrb-")
+
+	paddedSize := abi.PaddedPieceSize(16 << 20)
+	n := 2
+
+	var rawBytes []byte
+
+	for i := 0; i < n; i++ {
+		buf := bytes.Repeat([]byte{0xab * byte(i)}, int(paddedSize.Unpadded()))
+		rawBytes = append(rawBytes, buf...)
+
+		rf, w, _ := ffiwrapper.ToReadableFile(bytes.NewReader(buf), int64(len(buf)))
+
+		_, _, _, err := ffi.WriteWithAlignment(abi.RegisteredProof_StackedDRG32GiBSeal, rf, abi.UnpaddedPieceSize(len(buf)), tf, nil)
+		if err != nil {
+			panic(err)
+		}
+		if err := w(); err != nil {
+			panic(err)
+		}
+	}
+
+	if _, err := tf.Seek(io.SeekStart, 0); err != nil {
+		panic(err)
+	}
+
+	ffiBytes, err := ioutil.ReadAll(tf)
+	if err != nil {
+		panic(err)
+	}
+
+	if err := tf.Close(); err != nil {
+		panic(err)
+	}
+
+	if err := os.Remove(tf.Name()); err != nil {
+		panic(err)
+	}
+
+	outBytes := make([]byte, int(paddedSize) * n)
+	Pad(rawBytes, outBytes)
+	require.Equal(t, ffiBytes, outBytes)
+
+	unpadBytes := make([]byte, int(paddedSize.Unpadded()) * n)
+	Unpad(ffiBytes, unpadBytes)
+	require.Equal(t, rawBytes, unpadBytes)
+}
+