The engine assembly sequence, like the fuselage interior, is poorly set forth in the instructions. It was necessary to remove the locating ridge for the firewall from the cowls and thin down the inside of the depression for the exhaust pipe before the engines could slide into the cowl. The firewall was too far forward and not perpendicular to the thrust line. In the end the engines ended up in the right place, but I would suggest a better way. When assembling the engines, do not attach the firewall to the engine during engine assembly. Do not install the exhaust stubs. Replace the accessory section that fits into the back of the engine with plastic or brass tubing that fits in the hole in the firewall. The accessory section is not molded well enough for the accuracy of fit needed. When the engines are complete and painted, assemble the cowl, nose bowl, and oil cooler duct. Next put the engines in the cowl as far forward as they will go and slip the firewall into the back of the cowl and over the tube that replaced the accessory section and glue it up against the locating ridge in the cowl. Installing the propeller will help in positioning the engines in the center of the cowl and at the proper distance from the front of the cowl. Tack glue the tube to the firewall and check the engine alignment as the glue sets.