diff --git a/test/functional/interface_rest.py b/test/functional/interface_rest.py
--- a/test/functional/interface_rest.py
+++ b/test/functional/interface_rest.py
@@ -21,6 +21,8 @@
     hex_str_to_bytes,
 )
 
+from test_framework.messages import BLOCK_HEADER_SIZE
+
 
 class ReqType(Enum):
     JSON = 1
@@ -234,20 +236,24 @@
         # Check binary format
         response = self.test_rest_request(
             "/block/{}".format(bb_hash), req_type=ReqType.BIN, ret_type=RetType.OBJ)
-        assert_greater_than(int(response.getheader('content-length')), 80)
+        assert_greater_than(int(response.getheader(
+            'content-length')), BLOCK_HEADER_SIZE)
         response_bytes = response.read()
 
         # Compare with block header
         response_header = self.test_rest_request(
             "/headers/1/{}".format(bb_hash), req_type=ReqType.BIN, ret_type=RetType.OBJ)
-        assert_equal(int(response_header.getheader('content-length')), 80)
+        assert_equal(int(response_header.getheader(
+            'content-length')), BLOCK_HEADER_SIZE)
         response_header_bytes = response_header.read()
-        assert_equal(response_bytes[0:80], response_header_bytes)
+        assert_equal(
+            response_bytes[0:BLOCK_HEADER_SIZE], response_header_bytes)
 
         # Check block hex format
         response_hex = self.test_rest_request(
             "/block/{}".format(bb_hash), req_type=ReqType.HEX, ret_type=RetType.OBJ)
-        assert_greater_than(int(response_hex.getheader('content-length')), 160)
+        assert_greater_than(int(response_hex.getheader(
+            'content-length')), BLOCK_HEADER_SIZE * 2)
         response_hex_bytes = response_hex.read().strip(b'\n')
         assert_equal(binascii.hexlify(response_bytes), response_hex_bytes)
 
@@ -255,10 +261,11 @@
         response_header_hex = self.test_rest_request(
             "/headers/1/{}".format(bb_hash), req_type=ReqType.HEX, ret_type=RetType.OBJ)
         assert_greater_than(
-            int(response_header_hex.getheader('content-length')), 160)
-        response_header_hex_bytes = response_header_hex.read(160)
+            int(response_header_hex.getheader('content-length')), BLOCK_HEADER_SIZE * 2)
+        response_header_hex_bytes = response_header_hex.read(
+            BLOCK_HEADER_SIZE * 2)
         assert_equal(binascii.hexlify(
-            response_bytes[:80]), response_header_hex_bytes)
+            response_bytes[:BLOCK_HEADER_SIZE]), response_header_hex_bytes)
 
         # Check json format
         block_json_obj = self.test_rest_request("/block/{}".format(bb_hash))
diff --git a/test/functional/mining_basic.py b/test/functional/mining_basic.py
--- a/test/functional/mining_basic.py
+++ b/test/functional/mining_basic.py
@@ -18,6 +18,7 @@
 from test_framework.messages import (
     CBlock,
     CBlockHeader,
+    BLOCK_HEADER_SIZE,
 )
 from test_framework.mininode import (
     P2PDataStore,
@@ -168,10 +169,9 @@
 
         self.log.info("getblocktemplate: Test bad tx count")
         # The tx count is immediately after the block header
-        TX_COUNT_OFFSET = 80
         bad_block_sn = bytearray(block.serialize())
-        assert_equal(bad_block_sn[TX_COUNT_OFFSET], 1)
-        bad_block_sn[TX_COUNT_OFFSET] += 1
+        assert_equal(bad_block_sn[BLOCK_HEADER_SIZE], 1)
+        bad_block_sn[BLOCK_HEADER_SIZE] += 1
         assert_raises_rpc_error(-22, "Block decode failed", node.getblocktemplate, {
                                 'data': bad_block_sn.hex(), 'mode': 'proposal'})
 
@@ -204,11 +204,11 @@
 
         self.log.info('submitheader tests')
         assert_raises_rpc_error(-22, 'Block header decode failed',
-                                lambda: node.submitheader(hexdata='xx' * 80))
+                                lambda: node.submitheader(hexdata='xx' * BLOCK_HEADER_SIZE))
         assert_raises_rpc_error(-22, 'Block header decode failed',
-                                lambda: node.submitheader(hexdata='ff' * 78))
+                                lambda: node.submitheader(hexdata='ff' * (BLOCK_HEADER_SIZE - 2)))
         assert_raises_rpc_error(-25, 'Must submit previous header',
-                                lambda: node.submitheader(hexdata='ff' * 80))
+                                lambda: node.submitheader(hexdata=super(CBlock, bad_block).serialize().hex()))
 
         block.nTime += 1
         block.solve()
diff --git a/test/functional/test_framework/messages.py b/test/functional/test_framework/messages.py
--- a/test/functional/test_framework/messages.py
+++ b/test/functional/test_framework/messages.py
@@ -28,7 +28,8 @@
 import time
 
 from test_framework.siphash import siphash256
-from test_framework.util import hex_str_to_bytes
+from test_framework.util import hex_str_to_bytes, assert_equal
+
 
 MIN_VERSION_SUPPORTED = 60001
 # past bip-31 for ping/pong
@@ -507,6 +508,10 @@
             self.nTime, self.nBits, self.nNonce)
 
 
+BLOCK_HEADER_SIZE = len(CBlockHeader().serialize())
+assert_equal(BLOCK_HEADER_SIZE, 80)
+
+
 class CBlock(CBlockHeader):
     __slots__ = ("vtx",)