R975 Radial engines

[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!

[Keith Webb]

Interesting stories and information, thanks for posting this Bob.

I've spent a fair bit of time around aircraft radials such as the R1850, there's nothing like the "sound of round".

[drcowie]

Hi Bob - do you know of manuals specifically relevant to the R975-EC3 ?

[Lang]

Des,

Give me a call 0439677279. I think I might have a manual but will take a while after the flood cleanup ( we missed by inches but everything is on tables and benches)

[Bob Phillips]

Just an update- drcowie! sorry I do not have specific mauals for the early 975 EC 3 but I don't think its very much different than the later manuals. There are 2 manuals on the G503 forum to download, one for the engine and one for accessories. I should also add that an aircraft manual for the R975 E series engines would also be very similar and are availble from a number of commercial suppliers ( Brian Asbury has a tank version also). My aircraft books identify the E series engines as having been first made in the mid 1930s, One version the 975 E 1 was rated at 365 hp @ 2100 rpm with supercharger impellor ratio of 7.8 :1 fuel of 73 octane and comp ratio of 6.1. The last model the commercial E-3 version was upgraded to 420 hp @ 2200 rpm, comp ratio 6.3 blower ratio increased to 10.15:1 and using 80 octane fuel.I believe this is the version put into tank use, with modifications.
Please note that Wright Whirlwind aircraft books are generally not as detailed as the American TM books.

[Hanno Spoelstra]

Originally Posted by Bob Phillips

Continental manufactured the R975 under licence from Wright and made many thousands of engines- far more than were ever made as aircraft engnes. 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?

Bob,

Thanks for posting this interesting info. I love the sound of radial engines! As you say they are not particularly well suited for use in tanks, but at the time aircraft engines were the only engines with the power output needed to propel AFVs.

Early M3 Medium tanks and Ram Cruiser tanks were fitted with the Continental R975-EC2, a licence produced Wright Whirlwind. My copy of Data Book: Tank Type Vehicles of Canadian Manufacture dated January 1944, states: "The R975-EC2 Series Engine requires 91 Octane Aviation fuel, while the R975-C1 operates on 80 octane motor fuel (the standard fuel used by the mechanized forces of the United States Army). This is accomplished by changing the compression ratio of 6.3:1 in the R975-EC2 to 5.7:1 in the R975-C1, by a design change in pistons and changes in spark advance and in the carburetor main metering jet."
It lists for the R975-EC2:
- rated horsepower: 400 at 2400 rpm
- torque: max. 890 ft.lbs. at 1800 rpm
The Data Book refers to: Hand Book Continental R975-EC2 Ordnance Engine. Continental Motors Corporation, Detroit, Mich., April 1941.

Regards,
Hanno

[Bob Phillips]

Thanks Hanno for confirming that early R975s had high compression pistons for use with higher octane fuels. Twenty years ago when I was actively buying R975 parts one item that every big dealer had was quantities of the modified pistons ( # 202050 I think) these were the downgrades to 5.7 compression ratio for use in the early C1 engines. Unlike aircraft pistons ( and later tank pistons) these downgrades were externally very similar in dimension to the aircraft piston and achieved the lower compression ratio by creating a concave centre in the piston hence a lower compression ratio. Unlike the other pistons it was of cast ( not forged) construction and did not have the heavy reinforcing grid/baffles in the underside of the cylinder head. I remember being told by one of the " old timers " that they were " no dam good " because the cast pistons tended to crack early in their life span and were soon replaced by another 5.7 ratio piston in which the piston pin was moved slighly closer to the top of the piston, lowering the comp ratio but also being of forged manufacture with reinforcing grids inside. There were also apparently oversize pistons available ( I have a cast one in .010 os, and several rings sets in .020) but I never have seen or heard much about them aside from the few examples I have.

[drcowie]

Thanks Bob for more info. How do I access those manuals to download - can't see anything on the Forum ? also could you help with contact details for Brian Asbury?

[BCA]

Hi Des,
Your R975 Base Shop Data Manual went out to you by airmail yesterday. Perhaps there is another person by my name you'd like to contact!?!
.......... Brian

[Bob Phillips]

I wanted to revive this thread to address the issue of using lower cost 975-46 engines as a parts source for tank engines. I have had the good luck to aquire a couple of these high horsepower helicopter engines and will share what I have learned about them. Please include your comments and experiences with these engines. There are still very nice -46 engines available for reasonable prices. These are helicopter engines, some of which can produce up to 550 hp ( take off) using high octane aviation fuel (100/130 or 115/145). These are the ultimate refinement of the R975 engine and were designed and built by Continental after WW2.



The initial impression you get is that this is a much more heavily built engine. The crankcase is heavier, the rear crankcase is simpler and heavier than a C1 or C4 and the magnetoes are larger, the carb huge! In front a larger oil sump, a longer crankshaft (as per all aircraft engines and cylinders that are bigger than C1 but smaller than C4s. The thin web of a C1 or C4 crankcase is easily damaged either by rod damage (piston fails striking sides) or when bearing ejecta is smashed between the case and rotating crankshaft counterweights. The -46 case is solid where cylinders mount and much sturdier.
Inside there are several other differences. Pistons in C1 & C4 engines are 5.7 comp ratio (early ones cast later forged) while aircraft and the -46 are 6.3 comp ratio ( though higher ratios were used in prewar aircraft engines). All tank engines use "square" comp rings. The -46 and some aircraft variants use tapered keystone compression rings. The tapered rings are widely used on diesel engines and are currently used on the standard Continental 1790 diesel tank engine. A variety of oil scraper rings can be used. Note that all the forged pistons have reinforced ridges in the back, only the 202050 cast has none.

[Bob Phillips]

All early aircraft engines and C1 tank engines use a 6 7/8 inch impeller with 12 vanes. I do not have a C4 impeller but the manual shows what appears to be a larger diameter impeller with 16 vanes. Can anyone provide outside dimensions? The -46 impeller is 8 1/8 inches diameter and has 16 vanes. It also has holes through to the back.
Valves in the -46 and C4 have larger diameter stems than C1, and are not interchangable. I believe the -46 exh valve has an even larger diam stem than a C4.
Crankshafts, of course much longer in aircraft applications than tank engines but the C4 and -46 use a single nut and sleeve to hold the two front bearings in versus the earlier two nut method of the C1. Could you cut down a -46 to work in a C4?? Not sure.

[Bob Phillips]

Cylinders- I was prompted to provide this information by an internet photo of a C4 engine that appears to have been rebuilt using -46 cylinders. This seems like a great idea given the scarcity and high prices of C4 cylinders. Hopefully one of the forum readers will recognize the picture and provide details on this rebuild. I assume it will require custom baffles around the cylinders.

[Hanno Spoelstra]

Hello Bob,

Over a year ago I came across pictures of a Continental R-975-46A radial engine for sale for not too silly money. I too wondered about the interchangeability of parts, or even a complete engine swap into a M4 Medium tank or related AFV.

Thanks for offering some insight into the detail differences.

Hanno

PS: Here's what I found about about it:

Continental R-975-46A radial: from aero, to tank, to aero engine

Aircraft variant of the Continental R-975. This is the -46A version as used post-WW2 in the Piasecki PV-18 (Navy HUP-3 Retriever, Army H-25A Army Mule) helicopter. It was offered for sale on the Antique Aircraft Parts Exchange FB group, and shared here as it may be of interest to Sherman tank restorers. While originally a Wright design, during WW2 Continental built more R-975's under licence for tanks than Wright did for aircraft (> 53,000 vs. >7,000). After the war, Wright showed no interest in continuing to manufacture the R-975. Continental then introduced its own R-975 version for aircraft, the R9-A. It was basically similar to other R-975 engines, and its compression ratio and supercharger gear ratio were unchanged from the R-975E-3, other improvements in the R9-A allowed it to achieve 525 hp (391 kW) for takeoff, surpassing any Wright version. A military version, the R-975-46, could reach 550 hp (410 kW), and was used in Piasecki's HUP Retriever and H-25 Army Mule helicopters. Continental's production of R-975 engines continued into the 1950s.



Power to the HUP-1, HUP-2 and HUP-3 helicopters is supplied by Continental R-975-42, R-975-46 or R-975-46A air-cooled, nine cylinder radial engines. These were based on the Wright-designed tank engine.

The engine is located within the fuselage aft of the fuel cell and firewall. It simultaneously drives two three-bladed rotors through drive shafts and reduction transmissions. The rotor blades of the HUP-2 are of either wood or metal (later versions) construction.


Continental R-975-42 as used in the Piasecki PV-18 (Navy HUP-2, later UH-25 Retriever)

[David Herbert]

There were a number of prototype and very low volume vehicles built with R975 engines. The most obvious being the original M8 High speed tractor (R975D4) and some of the very high mobility 8 x 8 trucks by Corbitt (T33 and T33E1)(R975C4). The fan in Bob's post #12 is a design that I have not seen before, with the blades braced to each other and the exhaust looks as if it has the outlets very low down compared to WW2 applications that had them right at the top or half way down each side.

One of the significant improvements with C4 cylinders was that the cooling fins were made much bigger with bigger gaps between them as the very fine fins on the EC2/C1 and presumably the -46 engines clog up with dust in tank applications.

David

[drcowie]

Fitting in a Stuart M3

[Bob Phillips]

Thank you gentlemen for your input and comments!
The idea of using a -46 in a tank is tempting but I think it has fundamental dimension issues because the crankshaft is significantly longer. Then the cooling shroud would have to be redesigned to the longer length, unless... you machined the front crankshaft shorter. I think it could be done ( will post c/s pics later). The engine in post #12 is probably intended for an automatic transmission vehicle (M18 ) and has no clutch. The cooling fan around the front shaft is for the oil coolers. I had one of these engines and (sadly) I let it slip away.
Here are some more photos the C1éC4 carb ( stromberg NA9RG) versus the much larger Stromberg QD 9A1 of the -46. Also magnetoes , two types used on the -46, the SF9RN which looks very similar to the C1 and C4 mag (VAG9DFA), and another type SF9RN-8 which was an automatic retarding mag.

[Bob Phillips]

On the bottom mag picture the C1 C4 mag is on the left and has the small rotating aluminium cap. The -46 was used in some crop duster aircraft applications and I understand they used a smaller carb and just covered up the two barrel intake with a plate with a single hole.
Inside link rods are interchangable but the knuckle pins and master rods are not. The diameter of the end of the knuckle pins is different.
The -46 is an impressive engine and the culmination of decades of refinement on the 975 engine. The 1950s was the era of overkill in radial engines with mega engines with as many as 28 cylinders arranged in four rows were built. What a mechanical marvel but also a mechanics nighmare!

[Bob Phillips]

P & W 3460, 28 cyls 3500HP

[Bob Phillips]

Piasecki helicopters;


you tube video, hopefully will work

www.youtube.com/watch?v=5gTyZ7grjeA

[Perry Kitson]

Here are a couple photo's of a new C4 supercharger impeller. Outside diameter is 8.875", 16 vanes, .750" 6 tooth spline.

In my opinion, the bane of the radial is the seperation of the two angular contact bearings on the supercharger impeller shaft. Angular contact bearings are meant to be used in pairs, as they are on the impeller shaft, BUT they are designed to be installed back to back to give the bearings the proper preload. Seperating the bearings on the impeller shaft necessitates very precise shimming of the housings so the bearings run neither too loose or too tight, as they run up to very high speeds.

[Bob Phillips]

Perry thanks very much for the impeller info! Looks a whole lot like the -46 impeller, does anyone know the significance of the holes through to the back?
As I mentioned earlier in this post tghere were weak points in the 975 engines, the impeller shaft/ bearings were certainly one. A second was the master rod/ crank bearing which often failed due to abuse or possible oil starvation. Any other known issues?
I also need to thank Perry for the opportunity to photograph his C4 cylinders next to a -46 cylinder. Please note -46 cylinder does not have an exhaust elbow attached.

[David Herbert]

I would suggest that the holes in the impeller are to more balance the pressure on each side of it as without them it would draw itself towards the side with the vanes on it (as it sucks air from that side) causing high axial load on its bearings. A certain amount of recirculation will happen but it may be that that was judged to be the lesser evil.

In my experience the primary cause of failure of these engines (in preservation) is that people let them idle at too low a speed and the master rod to crank bearing fails due to oil starvation as most of the lubrication pressure of this bearing is from centrifugal force within the crank itself and that is negligable below 800rpm. They will idle nicely at 500rpm so people think that that is better - wrong !

David

[Malcolm Towrie]

I looked after about 1500 pumps in a nuke plant and I agree with Perry. Angular contact bearing pairs must be shimmed or matched so that the axially unloaded bearing (not the loaded) is still preloaded enough to ensure the balls roll and don't skid. Skidding causes nasty damage to happen. Modern Vortec blowers which spin up to thousands of rpm are supported by tiny angular contact bearing pairs and your best bet for a good rebuild is to send them back to Vortec, IMO. It's that tricky to set them up.

And I agree with David. The holes in the impeller back shroud are to reduce axial load on the shaft bearings. This is common practice in pump design. The holes bleed the discharge pressure seen on the back side of the impeller back to the suction side. This reduces the thrust load towards the suction side.

Malcolm

[Jesse Browning]

I learned from a pump manufacturer that the holes are to relieve the vacuume pressure, not to protect the bearings, but so that sewage (or oil in engine application) doesn’t get sucked past the seal.

[Perry Kitson]

David,

Good point on the low idle oil pressure issue. I was once told by a pilot with decades of experience flying radial engined aircraft, that after a proper warm up, RPM was kept no lower than 1000 RPM for that reason.

[David Herbert]

Thanks Perry,
It also says it in the operating section of the manuals for the various radial engined Sherman variants and of course private owners are really good at reading manuals and doing what they are told.....

David

[rob love]

So if a guy needs to overhaul one of these, is it doable, or does it need to be sent somewhere that specializes in the radials? The boss keeps talking about wanting to get the Sexton going. I's hard at this point to even know what the problems with the engine are....something initially caused it to be parked, and about 6 years back a volunteer tore out the magneto and all the ignition wiring. We just got back the magneto (untouched) after about 5 years of it sitting at the local air museum.

Any recommendations for rebuilders? We had company in Winnipeg (standard aero) who used to do all the Dakota engines a few decades back, until the air force retired them in 1989. Or are the C1 radials too oddball for a regular commercial rebuilder?

I think I saw in the US a company that said they do them for around the $30K mark. They warned that they were often fixing other rebuilder's mistakes.

[Matthew Noonan]

Speaking of R975 mods

https://imgur.com/a/cnnMRnk

Are these towers? with the repositioned exhaust?

[David Herbert]

There are other photos of Rams with this same modification. The engine is completely standard but the exhaust is routed out through the engine decks instead of the standard arrangement as built with two mufflers side by side just bellow the engine deck discharging through fishtails that are hidden behind the rear upper hull and discharge downwards.

The other photos that I have seen were gun tower conversions for towing 17pdrs and I had assumed that the relocation was to make hooking up easier but the photos Matthew has found appear to be ordinary Kangaroos so it may be related to the fact that all these Rams are equipped for the fitting of wading trunking. However the standard wading trunking did not require the relocation of the exhaust and was very similar to the radial engine Sherman design which never had the exhaust relocated.

David

[Bob Phillips]

Hi Rob,
I think your question about overhauling a radial might best be directed to Jesse as I recall he tackled the overhaul himself. Would welcome your comments Jesse. On a related note I think that with care and patience the overhaul might be much easier to do using the TM manuals as they are very detailed compared to aircraft OH books. There are a number of engine companies that specialize in radial overhauls. The price quoted (30K US) is not out of line. Last year I talked to a large company overhauling a pair of C4s, they needed a lot of work (including salvaging badly smashed up cylinders) and the offhand comments about cost put it much higher than the 30k number.