diff --git a/ethcore/res/ethereum/kovan.json b/ethcore/res/ethereum/kovan.json
index 330b00c58..78a82ccca 100644
--- a/ethcore/res/ethereum/kovan.json
+++ b/ethcore/res/ethereum/kovan.json
@@ -23,7 +23,8 @@
 
             "0x00a0a24b9f0e5ec7aa4c7389b8302fd0123194de"
           ]
-        }
+        },
+        "validateScoreTransition": 1000000
       }
     }
   },
@@ -31,7 +32,7 @@
     "maximumExtraDataSize": "0x20",
     "minGasLimit": "0x1388",
     "networkID" : "0x2A",
-    "validateReceipts" : false
+    "validateReceiptsTransition" : 1000000
   },
   "genesis": {
     "seal": {
diff --git a/ethcore/src/client/client.rs b/ethcore/src/client/client.rs
index 978f7a99c..12f92177c 100644
--- a/ethcore/src/client/client.rs
+++ b/ethcore/src/client/client.rs
@@ -393,7 +393,7 @@ impl Client {
 			})?;
 
 			// Final Verification
-			if let Err(e) = self.verifier.verify_block_final(header, locked_block.block().header(), self.engine().params().validate_receipts) {
+			if let Err(e) = self.verifier.verify_block_final(header, locked_block.block().header(), self.engine().params().validate_receipts_transition) {
 				warn!(target: "client", "Stage 4 block verification failed for #{} ({})\nError: {:?}", header.number(), header.hash(), e);
 				return Err(());
 			}
diff --git a/ethcore/src/engines/authority_round.rs b/ethcore/src/engines/authority_round.rs
index 4f823fa8d..aeef176a9 100644
--- a/ethcore/src/engines/authority_round.rs
+++ b/ethcore/src/engines/authority_round.rs
@@ -53,6 +53,8 @@ pub struct AuthorityRoundParams {
 	pub start_step: Option<u64>,
 	/// Valid validators.
 	pub validators: ethjson::spec::ValidatorSet,
+	/// Chain score validation transition block.
+	pub validate_score_transition: u64,
 }
 
 impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
@@ -64,6 +66,7 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 			block_reward: p.block_reward.map_or_else(U256::zero, Into::into),
 			registrar: p.registrar.map_or_else(Address::new, Into::into),
 			start_step: p.start_step.map(Into::into),
+			validate_score_transition: p.validate_score_transition.map_or(0, Into::into),
 		}
 	}
 }
@@ -85,6 +88,7 @@ pub struct AuthorityRound {
 	validators: Box<ValidatorSet>,
 	/// Is this Engine just for testing (prevents step calibration).
 	calibrate_step: bool,
+	validate_score_transition: u64,
 }
 
 fn header_step(header: &Header) -> Result<usize, ::rlp::DecoderError> {
@@ -125,6 +129,7 @@ impl AuthorityRound {
 				signer: Default::default(),
 				validators: new_validator_set(our_params.validators),
 				calibrate_step: our_params.start_step.is_none(),
+				validate_score_transition: our_params.validate_score_transition,
 			});
 		// Do not initialize timeouts for tests.
 		if should_timeout {
@@ -295,13 +300,17 @@ impl Engine for AuthorityRound {
 			Err(From::from(BlockError::InvalidSealArity(
 				Mismatch { expected: self.seal_fields(), found: header.seal().len() }
 			)))
+		} else if header.number() >= self.validate_score_transition && *header.difficulty() >= U256::from(U128::max_value()) {
+			Err(From::from(BlockError::DifficultyOutOfBounds(
+				OutOfBounds { min: None, max: Some(U256::from(U128::max_value())), found: *header.difficulty() }
+			)))
 		} else {
 			Ok(())
 		}
 	}
 
 	fn verify_block_unordered(&self, _header: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
-				Ok(())
+		Ok(())
 	}
 
 	/// Do the validator and gas limit validation.
@@ -327,7 +336,8 @@ impl Engine for AuthorityRound {
 		}
 
 		// Check if parent is from a previous step.
-		if step == header_step(parent)? {
+		let parent_step = header_step(parent)?;
+		if step == parent_step {
 			trace!(target: "engine", "Multiple blocks proposed for step {}.", step);
 			self.validators.report_malicious(header.author());
 			Err(EngineError::DoubleVote(header.author().clone()))?;
diff --git a/ethcore/src/spec/spec.rs b/ethcore/src/spec/spec.rs
index 455d0745f..9080f2929 100644
--- a/ethcore/src/spec/spec.rs
+++ b/ethcore/src/spec/spec.rs
@@ -56,7 +56,7 @@ pub struct CommonParams {
 	/// Number of first block where EIP-98 rules begin.
 	pub eip98_transition: BlockNumber,
 	/// Validate block receipts root.
-	pub validate_receipts: bool,
+	pub validate_receipts_transition: u64,
 }
 
 impl From<ethjson::spec::Params> for CommonParams {
@@ -70,7 +70,7 @@ impl From<ethjson::spec::Params> for CommonParams {
 			min_gas_limit: p.min_gas_limit.into(),
 			fork_block: if let (Some(n), Some(h)) = (p.fork_block, p.fork_hash) { Some((n.into(), h.into())) } else { None },
 			eip98_transition: p.eip98_transition.map_or(0, Into::into),
-			validate_receipts: p.validate_receipts.unwrap_or(true),
+			validate_receipts_transition: p.validate_receipts_transition.map_or(0, Into::into),
 		}
 	}
 }
diff --git a/ethcore/src/verification/canon_verifier.rs b/ethcore/src/verification/canon_verifier.rs
index 67d7e0e85..188d94a10 100644
--- a/ethcore/src/verification/canon_verifier.rs
+++ b/ethcore/src/verification/canon_verifier.rs
@@ -31,7 +31,7 @@ impl Verifier for CanonVerifier {
 		verification::verify_block_family(header, bytes, engine, bc)
 	}
 
-	fn verify_block_final(&self, expected: &Header, got: &Header, receipts: bool) -> Result<(), Error> {
+	fn verify_block_final(&self, expected: &Header, got: &Header, receipts: u64) -> Result<(), Error> {
 		verification::verify_block_final(expected, got, receipts)
 	}
 }
diff --git a/ethcore/src/verification/noop_verifier.rs b/ethcore/src/verification/noop_verifier.rs
index b60dbed2e..0512c8208 100644
--- a/ethcore/src/verification/noop_verifier.rs
+++ b/ethcore/src/verification/noop_verifier.rs
@@ -31,7 +31,7 @@ impl Verifier for NoopVerifier {
 		Ok(())
 	}
 
-	fn verify_block_final(&self, _expected: &Header, _got: &Header, _receipts: bool) -> Result<(), Error> {
+	fn verify_block_final(&self, _expected: &Header, _got: &Header, _receipts: u64) -> Result<(), Error> {
 		Ok(())
 	}
 }
diff --git a/ethcore/src/verification/verification.rs b/ethcore/src/verification/verification.rs
index ab21ef6c2..2e913a305 100644
--- a/ethcore/src/verification/verification.rs
+++ b/ethcore/src/verification/verification.rs
@@ -178,7 +178,7 @@ pub fn verify_block_family(header: &Header, bytes: &[u8], engine: &Engine, bc: &
 }
 
 /// Phase 4 verification. Check block information against transaction enactment results,
-pub fn verify_block_final(expected: &Header, got: &Header, check_receipts: bool) -> Result<(), Error> {
+pub fn verify_block_final(expected: &Header, got: &Header, check_receipts: u64) -> Result<(), Error> {
 	if expected.gas_used() != got.gas_used() {
 		return Err(From::from(BlockError::InvalidGasUsed(Mismatch { expected: expected.gas_used().clone(), found: got.gas_used().clone() })))
 	}
@@ -188,7 +188,7 @@ pub fn verify_block_final(expected: &Header, got: &Header, check_receipts: bool)
 	if expected.state_root() != got.state_root() {
 		return Err(From::from(BlockError::InvalidStateRoot(Mismatch { expected: expected.state_root().clone(), found: got.state_root().clone() })))
 	}
-	if check_receipts && expected.receipts_root() != got.receipts_root() {
+	if got.number() >= check_receipts && expected.receipts_root() != got.receipts_root() {
 		return Err(From::from(BlockError::InvalidReceiptsRoot(Mismatch { expected: expected.receipts_root().clone(), found: got.receipts_root().clone() })))
 	}
 	Ok(())
diff --git a/ethcore/src/verification/verifier.rs b/ethcore/src/verification/verifier.rs
index 9f0173003..08ec8c25a 100644
--- a/ethcore/src/verification/verifier.rs
+++ b/ethcore/src/verification/verifier.rs
@@ -26,5 +26,5 @@ pub trait Verifier: Send + Sync {
 	/// Verify a block relative to its parent and uncles.
 	fn verify_block_family(&self, header: &Header, bytes: &[u8], engine: &Engine, bc: &BlockProvider) -> Result<(), Error>;
 	/// Do a final verification check for an enacted header vs its expected counterpart.
-	fn verify_block_final(&self, expected: &Header, got: &Header, receipts: bool) -> Result<(), Error>;
+	fn verify_block_final(&self, expected: &Header, got: &Header, receipts: u64) -> Result<(), Error>;
 }
diff --git a/json/src/spec/authority_round.rs b/json/src/spec/authority_round.rs
index 35f6298ae..1c01d81fe 100644
--- a/json/src/spec/authority_round.rs
+++ b/json/src/spec/authority_round.rs
@@ -40,6 +40,9 @@ pub struct AuthorityRoundParams {
 	/// To be used for testing only.
 	#[serde(rename="startStep")]
 	pub start_step: Option<Uint>,
+	/// Block at which score validation should start.
+	#[serde(rename="validateScoreTransition")]
+	pub validate_score_transition: Option<Uint>,
 }
 
 /// Authority engine deserialization.
diff --git a/json/src/spec/params.rs b/json/src/spec/params.rs
index 1b2622316..aa2c15da4 100644
--- a/json/src/spec/params.rs
+++ b/json/src/spec/params.rs
@@ -54,8 +54,8 @@ pub struct Params {
 	#[serde(rename="eip98Transition")]
 	pub eip98_transition: Option<Uint>,
 	/// See `CommonParams` docs.
-	#[serde(rename="validateReceipts")]
-	pub validate_receipts: Option<bool>,
+	#[serde(rename="validateReceiptsTransition")]
+	pub validate_receipts_transition: Option<Uint>,
 }
 
 #[cfg(test)]