R975 Radial engines

[Bob Phillips]

So if I do this correctly you should see two pictures of a C1/C4 type gear out of the engine and without the view obstructed by the camshaft. You will see the gear bolts up tight against the rear of the crank. It has a small projecting ring on the back which helps align it in the crankshaft. No other attachment ot dowels etc. The -46 in the third photo is much heavier.

C1geara.jpg C1gearb.jpg gear2.jpg

So here is the problem. I am unable to locate any of the original type bolts. If you have any or recognize these, or know of a source I would like to hear from you. Have already checked aircraft supply places and engine rebuilders who declined to sell parts.
So option two..given the fact that the gear is designed to use a tapered shoulder bolt, could we not use conventional tapered socket cap screws in grade 8? They are readily available. They should be just as strong. They would not have the square top and tie wire holes but why not use locktite to hold them in? That would prevent any movement or shifting of the bolts, and the only downside I can see is if the bolts were to shear off it might be a pain to get out the broken bolts. What do you the readership think of this suggestion? All opinions welcome.
Another question has to do with the cause of the failure. The owner checked the detached gearcase to see if there were any obvious bearing failures, or increased resistance to account for the breakage. Everything seemed as normal. Could it just be fatigue in 75 year old engine rebuilt 20 years ago? Aside from checking the gear case, cleaning the sump and putting in clean oil in the engine what other things need to be checked out? Again all opinions welcome.

[Bob Phillips]

Cap screws, tapered shoulder and flat style. available in grade 8.

capscrewgrade8a.jpg capscrewsockethead.jpg

[Jordan Baker]

What about using tapered washers? They come in a variety of sizes. This way you could you use a regular grade 8 bolt/cap screw

Drilled for locking wire (dld-f/lkg-wire)

[David Herbert]

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]

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]

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]

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]

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.