[Bob Phillips]

I just wanted to add some information about R975 radial engines as there is much interest in them, some really nice restored ones to admire in forum pictures and apparently lots of mis information about them out there.

The Continental R975 C1 radail engine is a descendant of the earlier Wright Whirlwind engine that propelled Charles Lindberg across the Atlantic in the late 1920s. I believe these early engines were called the J5 version and the nine cylinder Whirlwind was a 788 cubic inch engine of 220 hp. By 1930 a new engine was designed and called the J6 series. Three sizes were offered a five cylinder (540c.i + 150 hp) a seven cylinder ( 760 cu in +225 hp) and a nine cylinder ( 975 cu in + 300 hp) By WW2 the nine cylinder was producing about 400 hp, due to increased compression ratios, supercharger blower speeds, fuel quality etc. Continental manufactured the R975 under licence from Wright and made many thousands of engines- far more than were ever made as aircraft engines. The earliest tank versions were called R975-EC3 I believe, and rumour has it they used high octane (100) aviation gas - but none of my manuals confirm this. Hanno do you know?

In tanks the compression ratio was reduced from 6.1 and 6.3 as used in aircraft to 5.7 to allow use of 70 (?) octane fuel. This is just one of several significant differences between tank and aircraft engines They are similar and share some components but are not the same. While an impressive design, it was probably not a great engine to put in a tank. The aluminum crankcase is fragile having hollow webs around the cylinder pads and the master rod - crankshaft bearing is a common failure when the driver dumps the clutch. When this happens shards of bearing material are extruded from the bearing and smashed into the crankcase as the counter weigh spins with very tight clearance between the case. The C4 is a redesigned engine but has similar characteristics, though it is designed for more HP and greater cooling. In the 1950s Continental designed a helicopter engine of similar displacement but it was 550-600 HP, a very heavily constructed crankcase, much more carburetion and bigger valve stems, more cooling fins etc etc.

Any R975 that has sat around for some time - even an arsenal overhaul- needs to be torn down before running. I have had several aquaintances who suffered massive engine failure when grease, rust, dirt or crap on the supercharger blower bearings seized while running and broke the supercharger shaft ramming the impeller into the case and sending a pile of fractured aluminium into the engines cylinders.

I know a guy who was rebuilding an airplane engine using tank parts to produce an engine kicking out 500 plus HP - he combined parts from a -46 engine with his Wrighjt (J6) aircraft engine. I know it can be done but I would be concerned about having a good margin of safety!

By the way, the aircraft version of the 975 was used in the Yale trainer ( Ernie Simmons sale) Thanks!