diff --git a/crates/fj-kernel/src/algorithms/intersect/face_face.rs b/crates/fj-kernel/src/algorithms/intersect/face_face.rs
index 9207a208e..6c3fb9c77 100644
--- a/crates/fj-kernel/src/algorithms/intersect/face_face.rs
+++ b/crates/fj-kernel/src/algorithms/intersect/face_face.rs
@@ -1,4 +1,5 @@
 use fj_interop::ext::ArrayExt;
+use iter_fixed::IntoIteratorFixed;
 
 use crate::{
     objects::{Curve, Face, Objects},
@@ -41,8 +42,10 @@ impl FaceFaceIntersection {
 
         let curve_face_intersections = intersection_curves
             .each_ref_ext()
-            .zip_ext(faces)
-            .map(|(curve, face)| CurveFaceIntersection::compute(curve, face));
+            .into_iter_fixed()
+            .zip(faces)
+            .map(|(curve, face)| CurveFaceIntersection::compute(curve, face))
+            .collect::<[_; 2]>();
 
         let intersection_intervals = {
             let [a, b] = curve_face_intersections;
diff --git a/crates/fj-kernel/src/algorithms/sweep/edge.rs b/crates/fj-kernel/src/algorithms/sweep/edge.rs
index e2d98a175..09ade40ac 100644
--- a/crates/fj-kernel/src/algorithms/sweep/edge.rs
+++ b/crates/fj-kernel/src/algorithms/sweep/edge.rs
@@ -1,5 +1,6 @@
 use fj_interop::{ext::ArrayExt, mesh::Color};
 use fj_math::{Line, Scalar, Vector};
+use iter_fixed::IntoIteratorFixed;
 
 use crate::{
     algorithms::{reverse::Reverse, transform::TransformObject},
@@ -65,7 +66,9 @@ impl Sweep for (Handle<HalfEdge>, Color) {
 
                 vertices
                     .each_ref_ext()
-                    .zip_ext(points_surface)
+                    .into_iter_fixed()
+                    .zip(points_surface)
+                    .collect::<[_; 2]>()
                     .try_map_ext(
                     |(vertex, point_surface)| -> Result<_, ValidationError> {
                         let surface_vertex = objects.surface_vertices.insert(
@@ -135,7 +138,9 @@ impl Sweep for (Handle<HalfEdge>, Color) {
 
             let vertices = bottom_vertices
                 .each_ref_ext()
-                .zip_ext(surface_vertices)
+                .into_iter_fixed()
+                .zip(surface_vertices)
+                .collect::<[_; 2]>()
                 .try_map_ext(|(vertex, surface_form)| {
                     objects.vertices.insert(Vertex::new(
                         vertex.position(),
diff --git a/crates/fj-kernel/src/builder/edge.rs b/crates/fj-kernel/src/builder/edge.rs
index 674183e11..928583ca0 100644
--- a/crates/fj-kernel/src/builder/edge.rs
+++ b/crates/fj-kernel/src/builder/edge.rs
@@ -1,5 +1,5 @@
-use fj_interop::ext::ArrayExt;
 use fj_math::{Point, Scalar};
+use iter_fixed::IntoIteratorFixed;
 
 use crate::{
     insert::Insert,
@@ -187,17 +187,21 @@ impl HalfEdgeBuilder for PartialHalfEdge {
                     .unwrap_or([None, None])
             };
 
-            vertices.zip_ext(global_forms).map(|(vertex, global_form)| {
-                vertex.update_partial(|vertex| {
-                    vertex.clone().with_surface_form(
-                        vertex.surface_form().update_partial(
-                            |surface_vertex| {
-                                surface_vertex.with_global_form(global_form)
-                            },
-                        ),
-                    )
+            vertices
+                .into_iter_fixed()
+                .zip(global_forms)
+                .collect::<[_; 2]>()
+                .map(|(vertex, global_form)| {
+                    vertex.update_partial(|vertex| {
+                        vertex.clone().with_surface_form(
+                            vertex.surface_form().update_partial(
+                                |surface_vertex| {
+                                    surface_vertex.with_global_form(global_form)
+                                },
+                            ),
+                        )
+                    })
                 })
-            })
         };
 
         self.with_curve(curve).with_vertices([back, front])
diff --git a/crates/fj-kernel/src/builder/shell.rs b/crates/fj-kernel/src/builder/shell.rs
index 34534a022..bee310e32 100644
--- a/crates/fj-kernel/src/builder/shell.rs
+++ b/crates/fj-kernel/src/builder/shell.rs
@@ -2,6 +2,7 @@ use std::array;
 
 use fj_interop::ext::{ArrayExt, SliceExt};
 use fj_math::Scalar;
+use iter_fixed::IntoIteratorFixed;
 
 use crate::{
     algorithms::transform::TransformObject,
@@ -235,8 +236,11 @@ impl<'a> ShellBuilder<'a> {
                 let mut edges = top_edges.iter();
                 let half_edges = array::from_fn(|_| edges.next().unwrap());
 
-                let [a, b, c, d] =
-                    points.zip_ext(half_edges).map(|(point, edge)| {
+                let [a, b, c, d] = points
+                    .into_iter_fixed()
+                    .zip(half_edges)
+                    .collect::<[_; 4]>()
+                    .map(|(point, edge)| {
                         let vertex = edge.back();
 
                         SurfaceVertex::partial()
@@ -263,7 +267,9 @@ impl<'a> ShellBuilder<'a> {
                 let vertices = edge
                     .vertices()
                     .each_ref_ext()
-                    .zip_ext(surface_vertices.clone())
+                    .into_iter_fixed()
+                    .zip(surface_vertices.clone())
+                    .collect::<[_; 2]>()
                     .map(|(vertex, surface_form)| {
                         Vertex::partial()
                             .with_position(Some(vertex.position()))
diff --git a/crates/fj-kernel/src/partial/util.rs b/crates/fj-kernel/src/partial/util.rs
index e0605ee1e..a0c11dc7c 100644
--- a/crates/fj-kernel/src/partial/util.rs
+++ b/crates/fj-kernel/src/partial/util.rs
@@ -1,4 +1,4 @@
-use fj_interop::ext::ArrayExt;
+use iter_fixed::IntoIteratorFixed;
 
 use super::{HasPartial, MaybePartial};
 
@@ -22,5 +22,8 @@ pub fn merge_arrays<T: HasPartial>(
     a: [MaybePartial<T>; 2],
     b: [MaybePartial<T>; 2],
 ) -> [MaybePartial<T>; 2] {
-    a.zip_ext(b).map(|(a, b)| a.merge_with(b))
+    a.into_iter_fixed()
+        .zip(b)
+        .collect::<[_; 2]>()
+        .map(|(a, b)| a.merge_with(b))
 }