Sexton R975-C1 a mess

[LRDG]

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]

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]

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]

Originally Posted by David Herbert

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.

It strikes me that the oil passing from the main bearing into the hollow crankshaft is having to fight against centrifugal force, although after that centrifugal force is certainly helping it get to the master rod brg.

Originally Posted by David Herbert

Even at 800rpm big end lubrication is marginal.

Certainly - with a hot oil idle pressure maybe around 10 psi and weak centrifugal force, I can see that.

Originally Posted by David Herbert

There is no oil supply to either end of the slave con rods, they are lubricated only by splash.

The attached extract from the C1 manual below shows passages for force feeding oil from the master rod bearing to the slave rod big end bearings.

Originally Posted by David Herbert

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.

Do you know what the boost is above 1500 rpm? The doubling of the compression ratio infers it is 10-15 psi. It seems such a small impeller for such a large engine, even with the 10.15:1 gearing.

Originally Posted by David Herbert

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.

We will be. And that brings up another problem - finding space for and keeping track of the parts!

Originally Posted by David Herbert

Have fun, David

We will.

Master rod and artic rod oiling.JPG

[David Herbert]

Sorry LRDG, I was wrong about the oil supply to the slave rods,I should have looked it up like you did ! And yes, I should have said 'greatly increased' rather than 'doubled' when trying to stress the value of the supercharger. The C4 had a different supercharger impeller and ratio in an attempt to widen the torque curve but it is hard to feel the difference between them when driving them.

David

[LRDG]

David,
How interchangeable are C1 and C4 parts? I'm thinking about crankshaft, rods, pistons, rings, and jugs.

Malcolm

[David Herbert]

LRDG,

If you go by part nos there is not much interchangability but a lot of that reflects improvements in materials and fits. The difference in pistons seems to be primarily about wrist pin retention. Crank I think is interchangable, C4 still has the redundant front oil pump mount in the front housing. Rings need to go with the piston. Cylinders bolt on but there are lots of bits on the outside that are different (air deflectors, throttle linkage, rocker covers, oil recovery pipes) Supercharger gears and impeller are different but not sure about the housing. It is about 20 years since I looked at this in detail but complete engines were fairly cheap then so I never needed to build a mongrel.

You are about to become an expert in all this I think.

David

[carl brown]

Your engine definitely failed due to a worn out crankshaft main bearing, resulting from no oil supply due to incorrect operation as David says.

You can fit a C1 crank in a C4 but not the other way round. The C4 crank isn't machined to take the C1 oil pump drive gear or lock ring.
Con Rods are the same in both engines but master rods different.
Pistons are basically the same but there are different rings depending on chrome or steel cylinders.
I can help you with parts if you can't find anything your side of the pond.
Carl