diff --git a/test/-ext-/string/test_modify_expand.rb b/test/-ext-/string/test_modify_expand.rb
index b7f96426fc..9aa3b9a6ca 100644
--- a/test/-ext-/string/test_modify_expand.rb
+++ b/test/-ext-/string/test_modify_expand.rb
@@ -19,9 +19,8 @@ class Test_StringModifyExpand < Test::Unit::TestCase
     return if RbConfig::SIZEOF['size_t'] > RbConfig::SIZEOF['long']
     bug12390 = '[ruby-core:75592] [Bug #12390]'
     s = Bug::String.new
-    long_max = (1 << (8 * RbConfig::SIZEOF['long'] - 1)) - 1
     assert_raise(ArgumentError, bug12390) {
-      s.modify_expand!(long_max)
+      s.modify_expand!(RbConfig::LIMITS["LONG_MAX"])
     }
   end
 end
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index b4369ede5f..07fe350b46 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -199,9 +199,8 @@ class TestStringIO < Test::Unit::TestCase
   end
 
   def test_write_integer_overflow
-    long_max = (1 << (RbConfig::SIZEOF["long"] * 8 - 1)) - 1
     f = StringIO.new
-    f.pos = long_max
+    f.pos = RbConfig::LIMITS["LONG_MAX"]
     assert_raise(ArgumentError) {
       f.write("pos + len overflows")
     }
@@ -767,7 +766,7 @@ class TestStringIO < Test::Unit::TestCase
 
   def test_overflow
     skip if RbConfig::SIZEOF["void*"] > RbConfig::SIZEOF["long"]
-    limit = (1 << (RbConfig::SIZEOF["void*"]*8-1)) - 0x10
+    limit = RbConfig::LIMITS["INTPTR_MAX"] - 0x10
     assert_separately(%w[-rstringio], "#{<<-"begin;"}\n#{<<-"end;"}")
     begin;
       limit = #{limit}