After about 50 flights running a 14x6 @ app: 9600 RPM on ground .The thing sounded worse than usual & then stopped with a helluva bang - prop flew 100 ft ! TheEx rocker arm adjuster broke at the junction of the arm & the jamb nut. The broken off nut jammed between the arm & cover. --Bent push rod ,bent valve . Repair made all O.k .About 10 more runs ,then no power. This time the inlet adjuster broke in the same manner,fortunately it made no contact with the cover . Questions . Has anyone else had this failure ? Why the failure on top of the arm ? Any fix ? My O. 91's have all run well for many years. I don't think I'm overtightening anything .,my internal torque wrench was trained for about 45 years and still works well . Thanking you in advance.ou

What you describe sounds like a an over torqued jam nut on that small shaft. I suppose after all those runs at that high of an RPM the little shafts just gave out. Most people do not set their 91 FSs for more than about 9K on the ground to prevent over revving in the air in a dive with a low tank of fuel.

Yes, I had the same exact failure while adjusting the valves. My TT .91FS was running poorly, so I removed the rocker arm covers, and proceeded to adjust the valve. Wow - you really can't torque those locking nut much or the adjustment bolt just breaks. I broke 2! Well after these broke I just dissassembled the entire rocker assembly and the push rods turned out to be bent - both of them. TT (Ace) was kind enough to cover it and mailed me the replacement parts at no charge. I was really careful when I went to torque it back the second time. It seems like really soft metal if you ask me.

I had the same thing happen to mine! After the 5th flight, I noticed the compression was leaking out the exh. valve. I went to adj. the lash and when I loosened the nut, the adj. screw w/nut fell off! It fractured right at the juncture w/ the rocker arm. Hmmm...

I checked the lash when new (before I ran it ), and it was o.k. so I left it as was. I figured a bad batch of screws, or a problem during assembly?
TT/Ace sent me a couple extra screws N/C...

I haven't run it since, and that was a year and a half (approx. ) ago...
I just put that engine in my new H9 P-40, and figured all was o.k. but now I'm wondering again!

I have never had trouble with the jam nuts. Some people just tighten things as much as you can. If you are one of these types I would suggest going to an industrial supply store and by some high strenght nuts. I had an OS, poor performance when propping below 9K, I proped the OS .91 for 9,500 to 9,800. But the TT likes to run over 10K on the ground. Those extra revs are bound to have some effect on the valve train, however I never had a problem except with a loose screw on the rocker cradle, the screw broke and the cradle came loose, TT replaced it under warrenty and a little Loc Tite keeps that from happening again.

I had and still have a feeling that the screw material is brittle...
I can't speak for others, but mine apparently was cracked during assembly, and when I went to loosen the jam nut, the nut with a piece of the screw fell off and landed on my work bench!

Hi,
In my TT91FS valve adjustment screws broke at first adjustment attempt. Replaced them with M3 metric 8mm long Allen screws. Engine ran fine until ca 40 hours. Now is beyond adjustment. Quits in the air without any reason, thou valve train is still OK.

Thank you everyone . MORE THOUGHTS. Looked at the threads , no obvious distortion & a fairly clean break.I think this means Brittleness . A file test shows the screw is harder than a standard Alan head cap screw ,music wire ,shank of a H/S drill bit and the adjuster on a Saito. An engineer friend does not like my reliance on "mechanics feel " & has loaned me a torque wrench .I have ordered a new part --Shall I trust it ( Rocker box cover off for a few flights )?
Make an attempt to re temper it ?Or make my own out of a regular Alan screw. I am going to make an attempt to get the jam nut torque specs ,which I think should be included with the instructions for setting the valves. P.S Put my 8 yr old O.S back on the plane & it showed its usual 9600 rpm and worked nicely

If you doubt the proper temper in the new adjusters, You can put them in a self cleaning oven for a cleaning cycle and draw them back to a softer temper.

Enjoy,

Jim

How hot does a self cleaning oven get? It would have to get over 600 degrees to temper most steel. Apparently TT had a batch of bad screw's. Wonder if they have some good ones to replace them under warrenty?

I wonder how tight is tight enough when it comes to the jam nut? A torque wrench might be a good idea, but what are the torque specs for that particular nut? Just a few oz/in? You could look it up I suppose, but does that general spec. take into account a too hard (or soft ) screw? (insert your own dirty joke here )
I always go by feel, and it's been many years since I've had a problem either stripping or breaking a bolt/screw, etc.
Some people have a better "feel" than others...I've tried to tell and teach people and some get it and others don't, ya know?

I would think that you'd need to know the alloy of the screw, and then the proper temp. and ammount of time to "soak" it to actually re-heat treat them? I don't know, but I just used the screws they sent me and "snugged" the jam nuts down "about that tight"

Just 1 or 2 clicks of the elbow torque wrench? Or perhaps 1/32 of a grunt?