Sexton R975-C1 a mess

David Herbert · #1 21-01-14, 11:46

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Originally Posted by motto

That thing must have made an awful clattering noise when it breathed its last. Talk about a catsarsetrophy.
It would be very interesting to know why it failed.

Actually they make a great deal of noise when they are running well ! This is partly because the exhaust manifolds are thin sheet metal and leak at the joints so the exhaust noise is very loud, also there is a balance weight free to float on the crank which rattles at low revs anyway. The reasion that so many radials fail in the hands of inexperienced operators is that they will idle smoothly at 500rpm and this makes driving them in close spaces much nicer.

However there is a reasion that the book tells you to never idle below 800rpm and even then not for long. That is that the oil pressure at the big end is mostly generated by centrifugal force in the drilling to that bearing from the main bearing. At low revs: not enough oil, bearing dies, lots of clearance, bearing rattles, con rods break, scrap engine. This engine is derived from an aircraft engine which spends its whole life at maximum revs, so it is not a problem. In tanks it was not a problem either as a driver abusing his engine could be put on a charge and had recieved proper training anyway. However most restored tanks are driven by people who have not read the book and in any case think they know better! It is NOT a car engine !!

There is another common failure that just stops the engine. That is that all the accessories on the back of the engine are driven from one gear that is screwed onto the end of the crankshaft by a number of socket head screws. These sometimes decide to all shear at once and the engine stops. It is a huge amount of work to fix but usually only requires replacement of the screws.

The parts are out there but people are asking a lot of money these days, good luck with the rebuild.

David

LRDG · #2 22-01-14, 02:05

Interesting info, David. Thanks.

I see the manual specifies minimum 30 psi at 800 rpm immediately after a cold start. That's pretty low for a cold oil pressure. With hot oil I could see that dropping to 10 psi, and even lower at 500 rpm. That's not much to keep the big ends floating on the pins, especially if the engine is loaded.

Hard to say which domino fell first, but one guess based on the badly worn master rod big end is the master rod brg clearance increased to the point the end leakage starved the articulated rod bearings and they started seizing on the pins. The rods that broke all broke by tearing the big end apart, which looks very thin-walled and weak to me. But I'm not used to engines with 5.7:1 CR and 2400 rpm redline.

The crank feels looser than it should in the rear main bearing so another possibility is that it wore out and starved all the rod bearings.

David Herbert · #3 22-01-14, 12:29

The 30psi at 800rpm is the oil pressure generated by the pump. There is almost no pressure resulting from the pump actually at the big end. The pump gets it to the main bearing, from there it makes its way by centrifugal force. Even at 800rpm big end lubrication is marginal.

You must remember that this engine was designed to be started, warmed up in a very controlled way (1200rpm) and then run at max revs for the rest of the day. If allowed to idle at 500rpm it will destroy itself! There is no oil supply to either end of the slave con rods, they are lubricated only by splash. As there is quite a small angular movement in the joints between the master conrod and the slave conrods that is ok but they can not withstand the hammer action that results from huge play in the big end bearing. The damage you show was not caused by general wear, it was caused by idling at too low a speed.

Incidentaly the low compression ratio is effectively doubled by the action of the supercharger above about 1500rpm which is how it manages to make nearly 400hp.

You will have to completely dismantle the engine to make sure there is no swarf anywhere and to check / replace bearings that have been damaged.

Have fun, David

David Dunlop · #4 22-01-14, 17:12

Somewhere here in town, there is a junk yard I have been passed so many times I have tuned it out at the moment, but it has had one of these engines leaning against a wall or fence in their yard for decades. It might have been the old Princess Auto yard, or a place across the street. If it comes to me shortly, I will let you know.

While it is in my mind, although Wright was the original company that designed this engine series as the R975, the same engine was also built under licence by Continental as the Continental R9-A and in Spain by Hispano-Suiza as the Hispano-Suiza 9Q, so lots of parts by various makers are out there.

In North America, the engine was used extensively by both Beechcraft and Cessna, so you may want to check their local dealerships for information as well.

David

LRDG · #5 23-01-14, 01:12

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