diff --git a/kmod/src/btree.c b/kmod/src/btree.c
index eee9c547..a7b0f130 100644
--- a/kmod/src/btree.c
+++ b/kmod/src/btree.c
@@ -781,6 +781,7 @@ retry:
 		bt->free_end = cpu_to_le16(SCOUTFS_BLOCK_SIZE);
 	}
 
+	bt->hdr.magic = cpu_to_le32(SCOUTFS_BLOCK_MAGIC_BTREE);
 	bt->hdr.blkno = cpu_to_le64(blkno);
 	bt->hdr.seq = cpu_to_le64(seq);
 	if (ref) {
@@ -1646,7 +1647,6 @@ int scoutfs_btree_write_dirty(struct super_block *sb)
 	/* checksum everything to reduce time between io submission merging */
 	for_each_dirty_bh(bti, bh, tmp) {
 		bt = (void *)bh->b_data;
-		bt->hdr._pad = 0;
 		bt->hdr.crc = scoutfs_block_calc_crc(&bt->hdr);
 	}
 
diff --git a/kmod/src/format.h b/kmod/src/format.h
index a8ebb465..e2bfecfa 100644
--- a/kmod/src/format.h
+++ b/kmod/src/format.h
@@ -3,8 +3,10 @@
 
 /* statfs(2) f_type */
 #define SCOUTFS_SUPER_MAGIC	0x554f4353		/* "SCOU" */
-/* super block id */
-#define SCOUTFS_SUPER_ID	0x2e736674756f6373ULL	/* "scoutfs." */
+
+/* block header magic values, chosen at random */
+#define SCOUTFS_BLOCK_MAGIC_SUPER	0x103c428b
+#define SCOUTFS_BLOCK_MAGIC_BTREE	0xe597f96d
 
 /*
  * The super block and btree blocks are fixed 4k.
@@ -67,12 +69,12 @@ struct scoutfs_inet_addr {
 
 /*
  * This header is stored at the start of btree blocks and the super
- * block for verification.  The crc is calculated by zeroing the crc and
- * padding so the buffer is large and aligned.
+ * block for verification.  The crc field is not included in the
+ * calculation of the crc.
  */
 struct scoutfs_block_header {
 	__le32 crc;
-	__le32 _pad;
+	__le32 magic;
 	__le64 fsid;
 	__le64 seq;
 	__le64 blkno;
diff --git a/kmod/src/super.c b/kmod/src/super.c
index 2e9e6875..8149abd1 100644
--- a/kmod/src/super.c
+++ b/kmod/src/super.c
@@ -191,7 +191,7 @@ int scoutfs_write_dirty_super(struct super_block *sb)
 
 	super = page_address(page);
 	memcpy(super, &sbi->super, sizeof(*super));
-	super->hdr._pad = 0;
+	super->hdr.magic = cpu_to_le32(SCOUTFS_BLOCK_MAGIC_SUPER);
 	super->hdr.crc = scoutfs_block_calc_crc(&super->hdr);
 
 	ret = scoutfs_bio_write(sb, &page, le64_to_cpu(super->hdr.blkno), 1);
@@ -226,9 +226,9 @@ int scoutfs_read_super(struct super_block *sb,
 
 	super = scoutfs_page_block_address(&page, 0);
 
-	if (super->id != cpu_to_le64(SCOUTFS_SUPER_ID)) {
-		scoutfs_err(sb, "super block has invalid id %llx",
-			    le64_to_cpu(super->id));
+	if (super->hdr.magic != cpu_to_le32(SCOUTFS_BLOCK_MAGIC_SUPER)) {
+		scoutfs_err(sb, "super block has invalid magic value 0x%08x",
+			    le32_to_cpu(super->hdr.magic));
 		ret = -EINVAL;
 		goto out;
 	}