diff --git a/blockchain/blocks/Cargo.toml b/blockchain/blocks/Cargo.toml
index 1e84f8480385..ce4386b0e5c6 100644
--- a/blockchain/blocks/Cargo.toml
+++ b/blockchain/blocks/Cargo.toml
@@ -14,3 +14,4 @@ multihash = "0.9.3"
 derive_builder = "0.9"
 serde = { version = "1.0", features = ["derive"] }
 encoding = { path = "../../encoding" }
+num-bigint =  { git = "https://github.com/austinabell/num-bigint", rev = "f7084a9ed5a2b08d9bfb67790cb4ce9212193f31" }
diff --git a/blockchain/blocks/src/header.rs b/blockchain/blocks/src/header.rs
index 92d9a56a88eb..aa37d4cc62e1 100644
--- a/blockchain/blocks/src/header.rs
+++ b/blockchain/blocks/src/header.rs
@@ -9,6 +9,7 @@ use crypto::Signature;
 use derive_builder::Builder;
 use encoding::{Cbor, Error as EncodingError};
 use multihash::Hash;
+use num_bigint::BigUint;
 use serde::{Deserialize, Serialize};
 use std::fmt;
 
@@ -20,13 +21,14 @@ use std::fmt;
 /// use address::Address;
 /// use cid::{Cid, Codec, Prefix, Version};
 /// use clock::ChainEpoch;
+/// use num_bigint::BigUint;
 /// use crypto::Signature;
 ///
 /// BlockHeader::builder()
 ///     .miner_address(Address::new_id(0).unwrap()) // optional
 ///     .bls_aggregate(Signature::new_bls(vec![])) // optional
 ///     .parents(TipSetKeys::default()) // optional
-///     .weight(0) // optional
+///     .weight(BigUint::from(0u8)) // optional
 ///     .epoch(ChainEpoch::default()) // optional
 ///     .messages(TxMeta::default()) // optional
 ///     .message_receipts(Cid::default()) // optional
@@ -48,7 +50,7 @@ pub struct BlockHeader {
 
     /// weight is the aggregate chain weight of the parent set
     #[builder(default)]
-    weight: u64,
+    weight: BigUint,
 
     /// epoch is the period in which a new block is generated.
     /// There may be multiple rounds in an epoch
@@ -136,8 +138,8 @@ impl BlockHeader {
         &self.parents
     }
     /// Getter for BlockHeader weight
-    pub fn weight(&self) -> u64 {
-        self.weight
+    pub fn weight(&self) -> &BigUint {
+        &self.weight
     }
     /// Getter for BlockHeader epoch
     pub fn epoch(&self) -> &ChainEpoch {
diff --git a/blockchain/blocks/src/tipset.rs b/blockchain/blocks/src/tipset.rs
index b0edd9bc0827..2d8608924645 100644
--- a/blockchain/blocks/src/tipset.rs
+++ b/blockchain/blocks/src/tipset.rs
@@ -7,12 +7,17 @@
 use super::{Block, BlockHeader, Error, Ticket};
 use cid::Cid;
 use clock::ChainEpoch;
-use serde::{Deserialize, Serialize};
+use encoding::{
+    de::{self, Deserializer},
+    ser::{self, Serializer},
+};
+use num_bigint::BigUint;
+use serde::Deserialize;
 
 /// A set of CIDs forming a unique key for a TipSet.
 /// Equal keys will have equivalent iteration order, but note that the CIDs are *not* maintained in
 /// the same order as the canonical iteration order of blocks in a tipset (which is by ticket)
-#[derive(Clone, Debug, PartialEq, Eq, Hash, Default, Serialize, Deserialize)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Default)]
 pub struct TipSetKeys {
     pub cids: Vec<Cid>,
 }
@@ -35,6 +40,26 @@ impl TipSetKeys {
     }
 }
 
+impl ser::Serialize for TipSetKeys {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let value = self.cids.clone();
+        value.serialize(serializer)
+    }
+}
+
+impl<'de> de::Deserialize<'de> for TipSetKeys {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let cids: Vec<Cid> = Deserialize::deserialize(deserializer)?;
+        Ok(TipSetKeys { cids })
+    }
+}
+
 /// An immutable set of blocks at the same height with the same parent set.
 /// Blocks in a tipset are canonically ordered by ticket size.
 #[derive(Clone, PartialEq, Debug, Default)]
@@ -158,8 +183,8 @@ impl Tipset {
         &self.blocks[0].parents()
     }
     /// Returns the tipset's calculated weight
-    pub fn weight(&self) -> u64 {
-        self.blocks[0].weight()
+    pub fn weight(&self) -> &BigUint {
+        &self.blocks[0].weight()
     }
     /// Returns the tipset's epoch
     pub fn tip_epoch(&self) -> &ChainEpoch {
@@ -198,6 +223,7 @@ mod tests {
     use address::Address;
     use cid::Cid;
     use crypto::VRFResult;
+    use num_bigint::BigUint;
 
     const WEIGHT: u64 = 1;
     const CACHED_BYTES: [u8; 1] = [0];
@@ -228,7 +254,7 @@ mod tests {
             .ticket(Ticket {
                 vrfproof: VRFResult::new(ticket_p),
             })
-            .weight(WEIGHT)
+            .weight(BigUint::from(WEIGHT))
             .cached_cid(cid)
             .build()
             .unwrap();
@@ -302,7 +328,7 @@ mod tests {
     #[test]
     fn weight_test() {
         let tipset = setup();
-        assert_eq!(tipset.weight(), WEIGHT);
+        assert_eq!(tipset.weight(), &BigUint::from(WEIGHT));
     }
 
     #[test]
diff --git a/blockchain/sync_manager/Cargo.toml b/blockchain/sync_manager/Cargo.toml
index e5918364568b..c45d63b9e3ee 100644
--- a/blockchain/sync_manager/Cargo.toml
+++ b/blockchain/sync_manager/Cargo.toml
@@ -11,4 +11,5 @@ libp2p =  { git = "https://github.com/SigP/rust-libp2p", rev = "776d13ef04635896
 chain = { path ="../chain" }
 
 [dev-dependencies]
-cid = { package = "ferret_cid", path = "../../ipld/cid" }
\ No newline at end of file
+cid = { package = "ferret_cid", path = "../../ipld/cid" }
+num-bigint =  { git = "https://github.com/austinabell/num-bigint", rev = "f7084a9ed5a2b08d9bfb67790cb4ce9212193f31" }
diff --git a/blockchain/sync_manager/src/bucket.rs b/blockchain/sync_manager/src/bucket.rs
index eaf94a8abbdb..78058f8b1adb 100644
--- a/blockchain/sync_manager/src/bucket.rs
+++ b/blockchain/sync_manager/src/bucket.rs
@@ -74,10 +74,11 @@ mod tests {
     use super::*;
     use blocks::BlockHeader;
     use cid::Cid;
+    use num_bigint::BigUint;
 
     fn create_header(weight: u64, parent_bz: &[u8], cached_bytes: &[u8]) -> BlockHeader {
         let header = BlockHeader::builder()
-            .weight(weight)
+            .weight(BigUint::from(weight))
             .cached_bytes(cached_bytes.to_vec())
             .cached_cid(Cid::from_bytes_default(parent_bz).unwrap())
             .build()
@@ -97,12 +98,18 @@ mod tests {
 
         // Test the comparison of tipsets
         let bucket = SyncBucket::new(vec![&l_tip, &h_tip]);
-        assert_eq!(bucket.heaviest_tipset().unwrap().weight(), 3);
+        assert_eq!(
+            bucket.heaviest_tipset().unwrap().weight(),
+            &BigUint::from(3u8)
+        );
         assert_eq!(bucket.tips.len(), 2);
 
         // assert bucket with just one tipset still resolves
         let bucket = SyncBucket::new(vec![&l_tip]);
-        assert_eq!(bucket.heaviest_tipset().unwrap().weight(), 1);
+        assert_eq!(
+            bucket.heaviest_tipset().unwrap().weight(),
+            &BigUint::from(1u8)
+        );
     }
 
     #[test]
diff --git a/blockchain/sync_manager/src/sync.rs b/blockchain/sync_manager/src/sync.rs
index cea91ccf04a4..ccdad2fd40b0 100644
--- a/blockchain/sync_manager/src/sync.rs
+++ b/blockchain/sync_manager/src/sync.rs
@@ -42,9 +42,10 @@ impl<'a> Syncer<'a> {
         // TODO send pubsub message indicating incoming blocks
         // TODO Add peer to blocksync
 
-        // compare targetweight to heaviest weight stored; ignore otherwise
+        // compare target_weight to heaviest weight stored; ignore otherwise
         let best_weight = self.chain_store.get_heaviest_tipset().blocks()[0].weight();
-        let target_weight = fts.tipset()?.blocks()[0].weight();
+        let target_weight = fts.blocks()[0].to_header().weight();
+
         if !target_weight.lt(&best_weight) {
             // Store incoming block header
             self.chain_store.persist_headers(&fts.tipset()?).ok();
diff --git a/blockchain/sync_manager/tests/manager_test.rs b/blockchain/sync_manager/tests/manager_test.rs
index bedcf1ab38f0..57d941322960 100644
--- a/blockchain/sync_manager/tests/manager_test.rs
+++ b/blockchain/sync_manager/tests/manager_test.rs
@@ -3,11 +3,12 @@
 
 use blocks::{BlockHeader, Tipset};
 use cid::Cid;
+use num_bigint::BigUint;
 use sync_manager::SyncManager;
 
 fn create_header(weight: u64, parent_bz: &[u8], cached_bytes: &[u8]) -> BlockHeader {
     let header = BlockHeader::builder()
-        .weight(weight)
+        .weight(BigUint::from(weight))
         .cached_bytes(cached_bytes.to_vec())
         .cached_cid(Cid::from_bytes_default(parent_bz).unwrap())
         .build()