diff --git a/lib/bundler/gem_helper.rb b/lib/bundler/gem_helper.rb
index ca39f122e9b..e9ee03b8a2b 100644
--- a/lib/bundler/gem_helper.rb
+++ b/lib/bundler/gem_helper.rb
@@ -75,7 +75,8 @@ def install
 
     def build_gem
       file_name = nil
-      sh(%W[gem build -V #{spec_path}]) do
+      gem = ENV["BUNDLE_GEM"] ? ENV["BUNDLE_GEM"] : "gem"
+      sh(%W[#{gem} build -V #{spec_path}]) do
         file_name = File.basename(built_gem_path)
         SharedHelpers.filesystem_access(File.join(base, "pkg")) {|p| FileUtils.mkdir_p(p) }
         FileUtils.mv(built_gem_path, "pkg")
@@ -86,7 +87,8 @@ def build_gem
 
     def install_gem(built_gem_path = nil, local = false)
       built_gem_path ||= build_gem
-      cmd = %W[gem install #{built_gem_path}]
+      gem = ENV["BUNDLE_GEM"] ? ENV["BUNDLE_GEM"] : "gem"
+      cmd = %W[#{gem} install #{built_gem_path}]
       cmd << "--local" if local
       out, status = sh_with_status(cmd)
       unless status.success? && out[/Successfully installed/]
diff --git a/lib/bundler/shared_helpers.rb b/lib/bundler/shared_helpers.rb
index 6a0da52e1ed..02b3801c443 100644
--- a/lib/bundler/shared_helpers.rb
+++ b/lib/bundler/shared_helpers.rb
@@ -278,7 +278,7 @@ def search_up(*names)
           # avoid stepping above the tmp directory when testing
           gemspec = if ENV["BUNDLE_RUBY"] && ENV["BUNDLE_GEM"]
             # for Ruby Core
-            "lib/bundler.gemspec"
+            "lib/bundler/bundler.gemspec"
           else
             "bundler.gemspec"
           end
diff --git a/spec/commands/clean_spec.rb b/spec/commands/clean_spec.rb
index 2ed8b36a852..f9c76fdfd0e 100644
--- a/spec/commands/clean_spec.rb
+++ b/spec/commands/clean_spec.rb
@@ -339,7 +339,8 @@ def should_not_have_gems(*gems)
       gem "rack"
     G
 
-    sys_exec! "gem list"
+    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
+    sys_exec! "#{gem} list"
     expect(out).to include("rack (1.0.0)").and include("thin (1.0)")
   end
 
@@ -461,7 +462,8 @@ def should_not_have_gems(*gems)
     end
     bundle! :update, :all => bundle_update_requires_all?
 
-    sys_exec! "gem list"
+    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
+    sys_exec! "#{gem} list"
     expect(out).to include("foo (1.0.1, 1.0)")
   end
 
@@ -485,7 +487,8 @@ def should_not_have_gems(*gems)
     bundle "clean --force"
 
     expect(out).to include("Removing foo (1.0)")
-    sys_exec "gem list"
+    gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
+    sys_exec "#{gem} list"
     expect(out).not_to include("foo (1.0)")
     expect(out).to include("rack (1.0.0)")
   end
@@ -519,7 +522,8 @@ def should_not_have_gems(*gems)
       expect(err).to include(system_gem_path.to_s)
       expect(err).to include("grant write permissions")
 
-      sys_exec "gem list"
+      gem = ruby_core? ? ENV["BUNDLE_GEM"] : "gem"
+      sys_exec "#{gem} list"
       expect(out).to include("foo (1.0)")
       expect(out).to include("rack (1.0.0)")
     end
diff --git a/spec/quality_spec.rb b/spec/quality_spec.rb
index f800ceeebd6..cbeabb3b6e7 100644
--- a/spec/quality_spec.rb
+++ b/spec/quality_spec.rb
@@ -216,7 +216,16 @@ def check_for_specific_pronouns(filename)
   it "can still be built" do
     Dir.chdir(root) do
       begin
-        gem_command! :build, gemspec
+        if ruby_core?
+          spec = Gem::Specification.load(gemspec.to_s)
+          spec.bindir = "libexec"
+          File.open(root.join("bundler.gemspec").to_s, "w") {|f| f.write spec.to_ruby }
+          gem_command! :build, root.join("bundler.gemspec").to_s
+          FileUtils.rm(root.join("bundler.gemspec").to_s)
+        else
+          gem_command! :build, gemspec
+        end
+
         if Bundler.rubygems.provides?(">= 2.4")
           # there's no way around this warning
           last_command.stderr.sub!(/^YAML safe loading.*/, "")
@@ -227,8 +236,7 @@ def check_for_specific_pronouns(filename)
         end
       ensure
         # clean up the .gem generated
-        path_prefix = ruby_core? ? "lib/" : "./"
-        FileUtils.rm("#{path_prefix}bundler-#{Bundler::VERSION}.gem")
+        FileUtils.rm("bundler-#{Bundler::VERSION}.gem")
       end
     end
   end
diff --git a/spec/runtime/setup_spec.rb b/spec/runtime/setup_spec.rb
index b57d61c3a69..fe84fc53796 100644
--- a/spec/runtime/setup_spec.rb
+++ b/spec/runtime/setup_spec.rb
@@ -871,7 +871,7 @@ def clean_load_path(lp)
 
       FileUtils.ln_s(bundler_dir, File.join(gems_dir, "bundler-#{Bundler::VERSION}"))
 
-      gemspec_file = ruby_core? ? "#{bundler_dir}/lib/bundler.gemspec" : "#{bundler_dir}/bundler.gemspec"
+      gemspec_file = ruby_core? ? "#{bundler_dir}/lib/bundler/bundler.gemspec" : "#{bundler_dir}/bundler.gemspec"
       gemspec = File.read(gemspec_file).
                 sub("Bundler::VERSION", %("#{Bundler::VERSION}"))
       gemspec = gemspec.lines.reject {|line| line =~ %r{lib/bundler/version} }.join
diff --git a/spec/support/helpers.rb b/spec/support/helpers.rb
index 563af1eab3f..e9138728fc7 100644
--- a/spec/support/helpers.rb
+++ b/spec/support/helpers.rb
@@ -308,12 +308,16 @@ def install_gems(*gems)
       gem_repo = options.fetch(:gem_repo) { gem_repo1 }
       gems.each do |g|
         path = if g == :bundler
-          Dir.chdir(root) { gem_command! :build, gemspec.to_s }
-          bundler_path = if ruby_core?
-            root + "lib/bundler-#{Bundler::VERSION}.gem"
+          if ruby_core?
+            spec = Gem::Specification.load(gemspec.to_s)
+            spec.bindir = "libexec"
+            File.open(root.join("bundler.gemspec").to_s, "w") {|f| f.write spec.to_ruby }
+            Dir.chdir(root) { gem_command! :build, root.join("bundler.gemspec").to_s }
+            FileUtils.rm(root.join("bundler.gemspec"))
           else
-            root + "bundler-#{Bundler::VERSION}.gem"
+            Dir.chdir(root) { gem_command! :build, gemspec.to_s }
           end
+          bundler_path = root + "bundler-#{Bundler::VERSION}.gem"
         elsif g.to_s =~ %r{\A/.*\.gem\z}
           g
         else
diff --git a/spec/support/rubygems_ext.rb b/spec/support/rubygems_ext.rb
index c13ed35413f..478cd12eb5c 100644
--- a/spec/support/rubygems_ext.rb
+++ b/spec/support/rubygems_ext.rb
@@ -52,7 +52,8 @@ def self.install_gems(gems)
       no_reqs.map!(&:first)
       reqs.map! {|name, req| "'#{name}:#{req}'" }
       deps = reqs.concat(no_reqs).join(" ")
-      cmd = "#{Gem.ruby} -S gem install #{deps} --no-document --conservative"
+      gem = Spec::Path.ruby_core? ? ENV["BUNDLE_GEM"] : "#{Gem.ruby} -S gem"
+      cmd = "#{gem} install #{deps} --no-document --conservative"
       puts cmd
       system(cmd) || raise("Installing gems #{deps} for the tests to use failed!")
     end