R975 Radial engines

[Bob Phillips]

So for now, we are locked down for covid 19and it gives me the opportunity to stay home and get a few things done. One thing I have done is start rooting through some old magazines I have collected and I want to share some of the information I found with the forum. From the publication "Warbirds International", May 2000. There is a very interesting article about what kind of oil to use in large, hard working radial engined aircraft used to fight California wild fires. The main point of the article is that the California Dept of Agriculture decided to change the type of oil used in large radials ( like 1500HP Wright 2800s) from single grade 120 weight aviation oils to Phillips 25w-60 multigrade olis ( sorry no family connection!) the maintenance team reported " a decrease of 35-50% in oil consumption and a noticeable decrease in both cylinder head and oil temperature" . They indicated the multigrade oil offered better protection of the master rod bearings than did single weight oil.
With respect to diagnostics on the engines .."the biggest problem indicator had been metal in the oil screen..usually from main bearing failure" Further.." the type of metal found will give a good idea of whats going on in the engine and whether it needs to be pulled for preventive maintenance...if we find aluminium..its either from a scuffing piston or the blower rubbing on the ..case...if its brass or bronze it's most likely from the bushing ..inside the nose case or ..accessory gears..But if its silver, it's either from the master rod bearing or from the cam bearing- the only two engine parts that have silver.." Very useful diagnostic information, though I am not sure that our 975s have any silver bearings in them!
There is a final comment about the fuel used. " We used to use 115-145 octane fuel...(high lead) now, 100 octane low lead aviation fuel..this effectively drops horse power from 1525 to down to 1475.." but mechanics compensate somewhat for this fuel change by changing the spark advance from 25 to 20 degrees..". They claim the toll of damaged engines is typically about 3 per season, but there were no failures the season they started to use multigrade fuels. So fuel and oil quality clearly matter!

[Bob Phillips]

Here are some inspiring photos for radial engine fans ! From the book " Making Waves... Navy Women of WW2" Looks like P&W engines.

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[Bob Phillips]

Just a follow up on the discussion of suitable fuel grades for the 975. I have exchanged emails with retired engine builder and instructor Tom Henderson from down under. Tom owns and operates a beautifully restored C1 that he demonstrates at shows and events from time to time. He has been kind enough to respond in detail to a variety of newbie questions about fuels, oils, compression ratios and ring types and I really appreciate his patience! Tom tells me he is running 92 octane no alcohol fuel in his engine. He thinks this is a good compromise between lower alcohol infused fuels and higher aircraft oriented gas. While the dangers of using too low a grade of fuel in a higher compression engine are well known, there may also be consequences for running too high a grade as well. This may include elevated chamber temps and burnt valves for little gain in performance or economy. He also runs a semi synthetic 20-40 diesel oil which has better anti corrosion additives and detergents to try and offset corrosion in an engine that only runs occasionally. Cylinder temps (#1)! are about 400 degrees which are well within the acceptable range.
Another interesting thing Tom pointed out is that an aircraft engine runs a cam where there is higher cam lift and more overlap of valve opening, a cam which favors higher speed operation and which idles poorly, hence limiting their use in a land vehicle engine. Hmm, I wonder if I could get some -46 cams reground to C1-C4 specs? In this day and age it is unlikely that any of the 975 engines would get the abuse and hours of a vehicle actually involved in conflict. Hopefully with care and routine maintenance, a radial engine should run, for shows and events, for many years without major problems.

[Bob Phillips]

Here are the comparisons of a -46 cam on the left and a C1 cam on the right.You can clearly see the difference in the lobes. The closest lobes are for the intake valves and not surprisingly, the -46 has a lot more air/gas valve inlet time!
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[David Herbert]

Bob,
I know that C4 and C1 had different superchargers to each other but I thought that the cams were different too ?

David

[Bob Phillips]

Good question about the cams, David. I had a quick run through the manuals and here is what I found. The M18 (G163) book lists 3 cams;
-G104-7006836 (CO201932) being standard for C4
-G104-0100293 (CO200742) (C66800) bolted for the C1
( this C1 being replaced/superceded by)
-G104-1531497 (CO242347) rivetted for C1
so it seems to suggest that there were both C1 and C4 cams...BUT
-my interchange book lists cam 201932 as interchangable in both C1 and C4 engines. Then check out the photos below. I just got these today and they show the damaged engine with sheared off bolts (that restarted this thread a couple of weeks ago) going back together with NOS bolts. Look closely and you can see the number on the cam of this C4 engine. It says 202347 which is supposed to be a C1 cam.(This is the cam in my early Sexton book with C1)
Finally, I reread the " Ordnance Development of the Wright-Continental R975 Radial Engine", from the Sherman Site. It makes reference to changing the aircraft cam for ground use. " To obtain the highly variable speeds required in tank operations, the engineers changed the intake and exhaust cams to decrease the valve overlap, to obtain higher torque at lower RPM and to improve idling characteristics...". But there is no mention through the rest of the report (including where changes made to upgrade C1 engines to C4s are listed) about a different cam. the report says that 85 percent of the parts between a C1 and C4 are interchangable.

So, without a 201932 C4 cam to examine, I would conclude that the cams are interchangable between the C1 and C4.

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[Bob Phillips]

As I am stuck at home during the continuing covid days I thought I would write abit about cylinders (mostly C1) and an interesting historical story I uncovered during my research.
Much earlier in these pages I included descriptions and photos of both C1 and C4 cylinders. I also compared them to the -46 cylinder off the helicopter engine. I have been rooting through my trailers looking to put together a few sets of cylinders and I thought I would give you a look at what I found. First, most cylinders are available in either steel bores (usually new black cylinders ) or chrome cylinders ( reconditioned with chrome bore to bring them back out to standard dimensions).They are usually painted primer red. These cylinders are choke bored meaning they have a taper of about.007 being bigger at the bore opening.They are very similar to the original aircraft type cylinders made by the Wright Corporation in the prewar years, but probably don't have the nitride hardening of earlier Wright cylinders.. Both the black and red cylinders have spark plug coolers and cast bosses beside the rocker boxes. The aviation cylinders (photos 3&4) are easily identified by the large size of the spark plug coolers and by the fact that the cast bosses along side the rocker boxes are drilled and threaded to accommodate baffles.
While some 7 cylinder (760) Wright aircraft engines apparently did not use any spark plug coolers, most did. They were a repair item and could be replaced on C1 engines. Note that C4 engines did not use them.
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C4 cylinders were designed to significantly increase cooling. The increased fin area amounted to 50% on the head and 100% more for the barrel. The barrel no longer was machined with fins but instead an aluminum "muff" was slid over the steel barrel and this design was used on many other large radial aircraft engines of the time. At the bottom of the next post are photos of a steel barrel of the C1 style with machined cooling fins, and a much larger radial cylinder from a bomber, with aluminium muff on the cylinder base.