diff --git a/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2021-09-01-preview/virtualMachines.json b/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2021-09-01-preview/virtualMachines.json
index cdc2c64b17ae..508fc6d882b1 100644
--- a/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2021-09-01-preview/virtualMachines.json
+++ b/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2021-09-01-preview/virtualMachines.json
@@ -1208,7 +1208,8 @@
           "type": "object",
           "properties": {
             "vmSize": {
-              "type": "string"
+              "type": "string",
+              "default": "Default"
             },
             "processors": {
               "type": "integer",
@@ -1383,13 +1384,15 @@
           "type": "object",
           "properties": {
             "enableTPM": {
-              "type": "boolean"
+              "type": "boolean",
+              "default": false
             },
             "uefiSettings": {
               "type": "object",
               "properties": {
                 "secureBootEnabled": {
                   "type": "boolean",
+                  "default": false,
                   "description": "Specifies whether secure boot should be enabled on the virtual machine."
                 }
               }
@@ -1420,7 +1423,11 @@
               "properties": {
                 "name": {
                   "type": "string",
-                  "description": "Name - Name of the image"
+                  "description": "Name - Name of the image",
+                  "x-ms-mutability": [
+                    "create",
+                    "read"
+                  ]
                 }
               },
               "description": "Which Image to use for the virtual machine"
diff --git a/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2022-12-15-preview/virtualMachines.json b/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2022-12-15-preview/virtualMachines.json
index a1421591550d..0bb88fd764c8 100644
--- a/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2022-12-15-preview/virtualMachines.json
+++ b/specification/azurestackhci/resource-manager/Microsoft.AzureStackHCI/preview/2022-12-15-preview/virtualMachines.json
@@ -1414,13 +1414,15 @@
           "type": "object",
           "properties": {
             "enableTPM": {
-              "type": "boolean"
+              "type": "boolean",
+              "default": false
             },
             "uefiSettings": {
               "type": "object",
               "properties": {
                 "secureBootEnabled": {
                   "type": "boolean",
+                  "default": false,
                   "description": "Specifies whether secure boot should be enabled on the virtual machine."
                 }
               }