From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Cylinder head cracking issues and a comparison of design solutions from Wright and Pratt & Whitney.
| Identifier | ExFiles\Box 27\3\ Scan217 | |
| Date | 15th May 1939 guessed | |
| 11. Trouble has been experienced through cracking of heads in Service, however, and four failures per week are admitted. A new stiffened head has been developed which can be run continuously at 225 lbs/sq.in. B.M.E.P. Mr.Beall stated that at 45" Mercury boost occasional detonation on peak pressures of 2000 lbs/sq.in. were experienced. On the existing cylinder head 1200 lbs/sq.in was a maximum safe pressure. Wrights have a cast cylinder with fins 3" deep 5½ to the inch. They feel that such a head would be practicable in production and well worth while from a cooling point of view as higher outputs are obtained. The foundry manager, however, told me that scrap is now 20%, due he said, to being pushed to get a higher output. The Wright cylinder flange is very thin, about 3/8" thick, but is now provided with a big fillet on its upper side and all studs are fitted with spherical washers locating in spherical recesses in the flange. The washers are about 5/32" thick, and the radius is struck at 45° for the steel crankcase and 30° for the duralumin crankcase. Palnuts are no longer used as they were inclined to score the threads and cause picking when the nuts were taken off. A special thin castle nut is now fitted, being drilled instead of slotted, and locked by a single wire going through 4 nuts. Set bolts are used instead of studs on the latest type of steel crankcase. Wrights use 16 studs per cylinder in groups of 4, i.e. not equally spaced. Cylinder barrel fins are 6 to the inch. Pratt & Whitney also say that more finning will pay, and have cast up to 2¼" deep, while Mr Hobbs thinks that fins up to 3½" deep will be worth while . The fin area of the Hornet cylinders head is 2200 sq. inches. The Pratt & Whitney cylinder flanges are only about ¼" thick with 16 equally spaced studs. They also are using spherical washers but have retained palnuts. Cylinder studs have a coarse thread in the case and a fine thread outside. They are not ground. Pratt & Whitney grind their cylinder barrels but do not hone them as they have found this to have | ||