R975 Radial engines

David Herbert · #1 16-12-20, 12:37

I like Jordan's suggestion a lot. The split tapered collet will be squashed down onto the bolt and there would be no scope for relative movement. I think that this is the ideal solution IF there is enough clearance for the slightly taller head that will result.

As for using standard countersunk Allen screws - I think that the standard countersunk, flat top head will be rather larger diameter than will fit into the recesses in the timing gear. If this is so it may be possible to turn down that extra diameter and leave enough parallel portion to drill for a lock wire. Doing this will not compromise the strength of the screw as the modification is just to the outer end of the head. I think that locktite would be a very bad idea as removing them would be near impossible if they did fail.

Another option would be to turn a small taper onto standard cap screws. I think that this could be done without getting too near the bottom of the hexagonal hole (compare with a countersunk screw ?) but the bearing area to the gear might not be big enough.

Of course the other option would be custom made CNC machined bolts to the original design but in a more modern high tensile steel. They don't need airworthiness certification and they are quite simple so it is do-able.

I don't think that these failures are age related but simply that the original design was too fragile. It may be that the bolts that are failing now were not torqued correctly but we have no way to check this. I know of several engines that have failed this way and been repaired "with standard Allen screws" but I don't know how they overcame the taper issue and am not now in touch with the people that did the repairs. I have not heard of one of these repaired engines failing again but that proves nothing.

David

Maurice Donckers · #2 16-12-20, 16:08

The idea behind these screws was that if you cranked the engine over with a hydrolock , these bolts would shear , and not the whole cylinder would be pushed from the crank case , and create more irrepairable damage

Jordan Baker · #3 16-12-20, 16:16

Quote:

Originally Posted by Maurice Donckers

The idea behind these screws was that if you cranked the engine over with a hydrolock , these bolts would shear , and not the whole cylinder would be pushed from the crank case , and create more irrepairable damage

After re reading I am thinking this too. Bob as you said no other damage was done.

David Herbert · #4 16-12-20, 16:19

That may have been the idea but in real life the cylinder gets detached from the crankcase causing lots of damage. I have seen the results and the eight bolts were still OK.

David

tankbarrell · #5 16-12-20, 18:51

If you start the engine, i.e. mags on with hydro lock, you will likely lose a cylinder or two.

If you crank it over with mags off, you will probably see the benefit of the shear screws.

Malcolm Towrie · #6 17-12-20, 03:28

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) · #7 17-12-20, 03:50

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 · #8 17-12-20, 05:13

Quote:

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.

Bob Phillips · #9 17-12-20, 03:48

Some good ideas here... I like the idea of conventional cap screws and tapered washers..but have not found any in the 5/16 size yet. It would be easy to make a tapered washer or even have custom bolts machined. I am not sure about the idea that these bolts are intended to shear off under hydrostatic lock up. I am sure that that is one situation where they might fail.There is no mention in the detailed OH manual about any such intentions.
I do have an engine which was badly smashed up inside due to oil in the lower cylinders, when it warms up I will pull it apart and check out the crank gear and related parts. Sorry, I cannot post any other photos but you will note there is a similar pattern of failure on them all. As failures go this one is certainly less traumatic then lots of other types, provided no other issues resulting from this, turn up.