R975 Radial engines

[Malcolm Towrie]

From the photos, it looks like a classic fatigue failure to me, affecting all the bolts. If so, it is an age-related failure as it takes a certain number of stress cycles to grow the fatigue crack across the bolt to the point the remaining intact area is not strong enough to withstand the tensile load and the bolt fails.
A characteristic of fatigue failure is the fracture surface shows a relatively smooth area with fine, often crescent-shaped lines spread across it. The remainder of the fracture surface has a rougher crystalline appearance, which is caused by the sudden final tensile failure.
Bob, can you post a close-up photo of the fracture surface, the closer the better?
Malcolm

[Bruce Parker (RIP)]

If my 216 Chev or Ford flatty goes bang-stop it's 'oh shit' (been there, done that). I expect if the same occurs on your radial R975 that takes it to a whole new level.

[Tony Smith]

Originally Posted by Bruce Parker

If my 216 Chev or Ford flatty goes bang-stop it's 'oh shit' (been there, done that). I expect if the same occurs on your radial R975 that takes it to a whole new level.

Depends if your R975 is fitted to your Tank or your Aircraft.

Tank, you just stop and hold up traffic. Aircraft, yes it takes you to a whole new level, alright.

[Bruce Parker (RIP)]

Originally Posted by Tony Smith

Depends if your R975 is fitted to your Tank or your Aircraft.

Tank, you just stop and hold up traffic. Aircraft, yes it takes you to a whole new level, alright.

I was thinking more what budget my wife allows me for the repair.

[Maurice Donckers]

Adrian you are right , if you turn it over with mags on , and it fires , that will be the end of the engine .
I have also seen engines which had a tow start (to free it up I think)with water above the piston, yep , this creates the end of the crankcase . the bolts won`t help then .

[David Herbert]

Malcolm,
When I said that I didn't think that bolt failure was age related I meant old age ! These failures are I think mostly fatigue failures caused by the bolts being rather fragile. The fact that the design was upgraded for the '46' version would I think support this as the upgrade would negate their function as a fail safe in case of hydraulic lock. In both air and land applications of this range of engines there were mandatory precautions laid down to avoid the occurrence of hydraulic lock and my understanding is that, at least in the British army these were generally followed.

I would ALWAYS crank a radial engine over with the mags off before going for a start.

David

[Bob Phillips]

Thanks for all the input. After a week of searching I have found some of the original crank bolts. I also include a photo of the bolt installed.
bolt1.jpg

bolt2.JPG
Having now measured up these bolts I am still a little curious about the design. The threaded holes i n the crankshaft are just over 1 inch deep, but the threaded portion of the bolt is only a half inch long. The diameter of the bolt just above the threaded portion shrinks down to less than a quarter inch. The hole through the gear is at least 20-25 thousands bigger than the diameter of the bolt passing through it. So there is no support against the gear if the bolt is stretch or strained, all of the gear alignment depends on the bolts being tight and being centered by the taper fit between the bolt and gear. Though it has the projecting flange in the back to keep it centered on the shaft.
I don't think it would be prudent to install used bolts in this repair, they have already had 50 plus years of stress and strain. It seems more prudent to install new, high grade bolts, even though that will mean either making them or modifying existing cap screws. Comments and thoughts please?