Just a footnote to this discussions on sparkplugs. I was out rooting through one of my trailer, looking in boxes I put away 25 years ago and I found some more, radial/aviation style engine sparkplugs. The first another Champion C63, but with a twist. check out the twin electrodes. It the first one I remember seeing.
Second I found a couple of cartons of plugs for the Light Tank (Locust with Lycoming air cooled aviation engine) which are also Champion plugs. Note they are single electrode and unshielded.
Here are a few pictures.

Attachment 113298 Attachment 113299 Attachment 113300

If anyone finds spark plugs with electrodes like the above, single or double, the thin piece welded to the threaded portion is platinum. Do not try to change the gap as they will fall off. However if the plugs are scrap it makes it easy to separate the platinum which is well worth doing - check the price !

David

$25.000/ kilo, here

You have a helluva stash of parts, Bob!

The Brits liked those "fine wire" plugs shown in your third photo. Ive seen them on Ferrets and an FV432. Like David said, the pins are platinum or ididium and they are a high-end plug for aviation use. Good for burning off lead deposits, which I found out aviation gas still has, much to my surprise.
Nowadays they cost over $100 each, so $1000 for a four-banger aircraft engine! Total overkill for a museum vehicle.

Malcolm

Most AvGas sold in Canada at present is 100LL. I.E. 100 octane Low Lead (max 0.56g/l of lead).
See https://www.shell.com/business-custo...uel/avgas.html for more information about AvGas.

I would like to revive this discussion of radial engines. The forum has covered a wide variety of topics from history and , identification of different engines, to common problems even fuel grades. In post 31 Dave Hebert mentioned that one issue was the failure of the 8 bolts holding on the crankshaft gear, meaning the rear gear case drive was disconnected from the front section. Thats what this new addition is about. I would appreciate input from forum members as I am looking for solutions to this problem. Anyway here is the story.
ABout a month ago, one of our fellow MV enthusiasts had his C4 equipped M18 out for a number of successful drives. At the end of the day when he went to restart it to garage it for the night, it would not start. The starter would run but the engine would not turn over. The engine could be turned using the cooling fan but there was no connection back to front. A magneto was pulled off and the fan rotated but the mag drive did not turn. So the engine had to come out and the rear case removed for a look see. It had, as Dave had pointed out, sheared off bolts otherwise fairly clean and had no apparent collateral damage. The bolts were removed without difficulty and the gear seemed to good enough to reuse. pictures enclosed.

Attachment 118237 Attachment 118238 Attachment 118239

Note the unusual shape of the sheared bolts, they are essentially a tapered shoulder cap screw with a square driving head on top with holes to put through lock wires. The manual identifies C1 and C4 crankshaft bolts as
"Screw, machine, sq-hd, dld-f/1kg-wire, alloy -S, 5/16-24NF-2 (19/32) x 1 7/32 overall lgh" it is part number G104-0102120 or CO-200439 , also B206663. Not quite sure what all of the description means, can anybody translate ?

Now before I ask you for your respective opinions on how to repair this, lets look at the later -46 engine gears. The Wright R975 started out with much less power than the finished C4 version. It appears that one of the weak links remained the 8 bolts holding on the rear crank gear- a design unchanged since the mid 1930s?? By contrast, the late 1940s designed 975-46 was built much heavier to handle its boosted horsepower. Some of this has been previously referred to as we noted the heavier case construction, heavier rear main bearing etc. If you check out the next photo you will see a rear crankgear for a -46 and the much heavier method of attaching it. While it uses 8 capscrews, they are flat not tapered. The gear/crankshaft have an aligning dowel on one side. Most importantly, there is a heavy metal projection attached to the gear which is drawn into the rear of the crankshaft witha heavy bolt. A LOT stronger than the C1 and C4 version!

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.

Attachment 118240 Attachment 118241 Attachment 118242

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.

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

Attachment 118243 Attachment 118244

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)

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

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.

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

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.

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

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.

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.

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

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 .

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

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.
Attachment 118357

Attachment 118358
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?

I have no knowledge here other than being a mechanic. The broken bolts have all failed with a line radiating out from the center which would indicate to me fatigue from the gear being loose and causing failure due to repeated shock loadings, in a rotational direction.
I have seen a Continental engine conrod that had the flats that the conrod bolt head sat against and it was not machined square. This would have led to a conrod bolt failure. There can be many reasons for failure. One of them can be faulty new parts.
I guess I'm about to be educated
Btw. Allen screws / bolts are usually grade 12..... and forged. Machined threads on bolts are usually in the realm of junk bolts.

bolt decoded?
 

I believe it's this.

Screw, machine, sq-hd, dld-f/1kg-wire, alloy -S, 5/16-24NF-2 (19/32) x 1 7/32 overall lgh

sq-hd = square head
dld-f/lkg-wire = drilled-for locking-wire (note it's an l not a 1)
alloy-S = alloy is steel
5/16-24 NF-2 (19/32) = 5/16 dia 24 TPI Fine, class 2, 19/32 thread length (.594") perhaps?
1 7/32 overall lgh = 1.219"

Anybody have any info on the "class 2"?

"Class 2" refers to the fit of the thread - its precise form and tolerances. It should be possible to google it but search "American National Fine thread" not UNF which is based on NF but not precisely the same and was not standardized until post war.

David

So David, I assume then that there is no indication of tensile strength in the description.
What are your thoughts on what those breaks indicate.

This thread is fascinating and super informative!!!! Thanks to all those contributing!!!!! As a C4 operator I’m learning new things about keeping my engine in top form.

1. As an M18 owner I can tell you that in normal automatic shifting operation of the DTD Hydramatic transmission in an R975 C4 engined vehicle is fairly kind to the engine crankshaft. The shifts are smooth going up. Clutch operation in a manual shift set up with a R975 C4 will, as suggested, stress the crankshaft bolts a lot.. BUT, and this is a big one, if you downshift an M18 going to a lower gear from high speed to that lower gear you will get a head-jarring ‘bang’ as the engine rpm is increased by the transmission gear change. There is actually a noticeable ‘bang’ sound when that is done too. I suspect the downshifting may be the source of shock and ultimate failure of the crankshaft bolts in C4 engines in M18s. Normally one would expect an automatic transmission to be ‘kinder’ to the engine crankshaft but my observation as an owner is that the downshift causes a shock just about like the clutch dumping in the manual transmissioned vehicles.

2. I will alert the M18 owners I know that the engine rpm should be kept at 1000 rpm after warmup to prevent main bearing oil starvation and premature wear and engine damage. Almost everyone idles them at 600-800 rpm, not 1,000 rpm.

3. BOB: Was that C4 that you were “sorry got away from you” the core in 2007 or 2008 that I brought from you?

4. Also, anyone see any issue with using AV gas in a lower compression C4 built with the lower compression tank pistons? I know that higher octane gasoline is reputedly for detonation reduction but it seems to have other benefits. We use AV gas exclusively because one, ethanol in MOGas damages fuel pump rubber diaphragms and will cause corrosion from separation in time. Also, we have noticed ‘smoother’ operation in the R975 and super quick starts even when the engine is below 0 C. Also, using AV gas instead of MOGas has the key side benefit that the fuel in the fuel tanks does not ‘rot’ by evaporation and polymerization into the orange garbage old MOGas becomes after sitting 6 months. M18s do sit quite a while between gigs and regular operations though we try to start and run the engine to warm up temps every few months at a minimum to keep the seals in good condition.

5. Further, since it's an AC engine we ignore the M18 Lube Chart for the M18 specifying 30 wgt motor oil (vehicle type) and use AeroShell 100 aircraft engine oil for the higher cylinder head temps found in aircraft engines run in tanks.
daninnm

:thup2:

Lynn,
No indication of tensile strength. The description of the bolts is just that, not a full specification. "Alloy Steel" is the hint that these are not low grade material but that is not much help. Somewhere there will be a full specification of both the exact alloy and the properties required of the finished bolt but finding it will be a challenge. It might be possible to have a bolt analyzed but while real ones are still to be found it is cheaper to use them and be extra careful with torquing them up.
My personal opinion is that the failures are fatigue failures caused by the bolts flexing over a long period and not that they acted as a safety device. This could be due to them being wrongly torqued when the engine is built up (allowing a small angular movement of the gear) or that they were not really suitable for the constantly changing engine speed in an AFV and that this was not addressed until the '46' version. I wonder if one could fit a '46' crank and gear into a C4 ?

Dan,
I would strongly advise that one gives the engine some gas when downshifting, just as you would with a manual transmission. This is not a fully auto transmission, just easier than a manual - the driver has to help !

I agree with your views regarding fuel. You don't need the extra octane rating of AV gas but it will do no harm other than to your wallet. The ethanol content is certainly relevant though and I think that you have made a good call.

Have a safe Christmas everyone
David

Thanks again for all the comments and input. Dave and Lynn, I think the current rating for tensile strength in grade 8 bolts is 150KSI ( 150,000 pounds per square inch) vs 120 KSI for grade 5. Regardless of the actual value I am sure you would agree that modern high grade bolts would be preferable to the original and now stressed and aged bolts. I think that C4 ( and C1) owners should expect to replace these bolts at any point when they have their engine apart for overhaul or repair.
I have thought about trying to use the -46 improvements on the older style crank, indeed it may well be possible to simply use the rear of a -46 crank on the front of a C1 or C4. The issue I think may be one of correctly balancing miss matched parts, cranks, pistons etc. This is something outside my experience but as I have extra -46 cranks it may happen some day. Maybe this would be a good point of discussion for a future post.
Dan, thanks for your kind comments about this thread. There are not many places you will find informed discussion about 975s, and the goal is to keep as many of these fine old engines alive and running. You are also correct about my old C4- I take satisfaction in knowing it went to a good home.
Your comments about proactive measures to keep these engines alive and well ( high grade fuel, higher idling speeds etc) are right on. While radials are high maintenance powerplants there is nothing better than the sound of one starting up, with its coughing, belching and smoke. You gotta love it!
Best wishes to everyone for the Christmas season!