Squint Scale Curtiss Model K Aero I4

[Roger B]

I think the prop inertia is underestimated, an acquaintance of mine had a full size Merlin on a test stand, this was fitted with a ~0.5m (18") diameter flywheel probably 50 - 75mm (2-3") thick.

[RReid]

That's an interesting data point, Roger, and I tend to agree with you and Bent re. prop inertia.

On the other hand, my 2 cylinder Halfa engine, with smaller diameter cylinders, does run fine with a rather small flywheel. But being DOHC I believe the friction in that valve train is less. The Curtiss has relatively stiff springs pushing the "push/pull rods" down onto the cams, in addition to the normal valve springs, plus those rods have to pass through a couple of alignment bushings each. I suppose it all adds up.

There is also the fact that those push/pull rods pass directly through the intake tract, which, as you pointed out some time ago, is far from ideal for best performance. But of course that was a feature that I wasn't willing to compromise with some out of proportion work around.

But who cares, she runs! 

Thanks Bent. I certainly can't argue with that!

[Zephyrin]

I'm pretty sure that after running in, you'll be able to do without the additional flywheel, which has been the case with several of my engines; and the compression also becomes much more perceptible when turning the engine by hand with the reduced drag...
great job with this engine, congratulations!

[RReid]

Thank you Zephyrin. I hope your prediction regarding run-in time proves true!

[RReid]

Well, the engine converted itself to a "display only" piece when it broke the crankshaft today just outboard of the 2nd conrod, bending at least the #3 and #4 conrods in the process. I guess a third main bearing would have been a good idea.

[cnr6400]

  sorry to hear this. Gotta be a heartbreak. Well, as you say, a heck of a display piece.   

[crueby]

Oh no!  So sorry to hear that!

[Kim]

Oh my!  That's terrible, Ron!   :'( That must have been quite frustrating, infuriating, and sad all at the same time.  I'm so sorry to hear this.  It was such a cool and unique engine.   

Kim

[Vixen]

Oh Ron!!

What terrible news, such a shame after all the toil and effort you have invested in this unique engine. I can fell the hurt and your pain from over here.

No, the story must not end here. Remember when you were a young lad and you fell off your bike, and bloodied your knees and elbows? What was the advise you were given? "Pick yourself up, dust yourself down and get right back onto that saddle".

Same here.   Most of your unique engine will be undamaged and can live again. So pick up the pieces, analyse the failure. Was it crankshaft whip? Was it a big end bearing failure? Was it a hydraulic lock (fuel or coolant) in one of the cylinders? Did the bigger flywheel contribute due to out of balance loads? Would a smaller flywheel at either end of the crankshaft have helped? Would a centre bearing have helped? What else are the bits telling you?

You have made one slender crankshaft, you can make another, this time with some small improvements. It may seem a daunting prospect at the moment but you can do it; if you push yourself. Then this unique and historically significant engine with it's "push me pull you" valve gear, will live once again.

Have the faith and go for it.

Mike

[john mills]

sorry to see the failure  but it has been terrific to see it run
it would be a pitty to see you leave it. I would encourage you to have a look at what might make it better and re make  whats needed.
weather  middle bearing could be added. a different material for the crank shaft what you se as improvements  .
It is always dissapionting to see a failure but you have done a lot of work  a bit more would be worth it . to have it running again.
John

[Roger B]

Sorry to hear about your broken crankshaft   

As others have said though it is an opportunity for some R&D work   

[Art K]

Ron,
Sorry to hear about your crank's unfortunate demise. it reminds me of an early don't quite recall if it was Austin or Morris used a two main bearing crank that was quite sturdy because of this.
Art

[Vixen]

Hello Ron,

My 1932 MG J2 was a four cylinder, overhead cam engine with a two bearing crank. It only got over revved once, the crankshaft and conrods suddenly appeared through a hole they had smashed in the side of the crankcase. Real big boy's tears, I tell you. An MG J2 engine was as scarce as hen's teeth (most suffered the same fate). Fortunately, I was able to fit a three bearing engine and gearbox form a later 1937 MG TA push rod engine. You have to do what you need to do to get running again

Cheers

Mike

[Admiral_dk]

So Sorry to hear the sad news - what a shame to see such a unique Engine be 'lost' ....

I hope that you will try some of the suggestions to make an improved version - but I will not blame you if you decide not to do so ....

Best wishes

Per

[RReid]

Thank you to all of you. I really do appreciate the support, encouragement, suggestions, anecdotes, and advice.

Here are a couple of pics showing the break. In the close-up you can pretty much see the smooth surface where a crack propagated, ending at the rough surface left where the crank pin just snapped. It's perpendicular to the plane of the throws, suggesting a vertical flexing between TDC and BDC. Even a tiny stress riser would be enough to get the party started. I was careful to leave a small radius at each pin/web junction, the failure appears to have been just about at the inner end of that radius.




The only other damage as a results seem to be these two bent conrods.