R975 Radial engines

[Don Phillips]

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"

[Lynn Eades]

Anybody have any info on the "class 2"?

[David Herbert]

"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

[Lynn Eades]

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.

[daninnm]

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

[David Herbert]

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

[Bob Phillips]

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!