TR4/4A Busted crankshaft

[mctriumph]

A good friend called up with the news that his Tr4 had broken a crank. Pretty fresh build, but he does some AutoCross
and hits 6000 fairly often. Stock rods , it is proved once again, are CRAP !! Have you ever weighed a stock rod??
Save you the trouble 930gr +- 6gr. It may as well be a brick. Modern steel rods have ARP bolts and now cost less than
the expense of reconditioning old rods.They weigh 660gr, Even if math is a swear word to you, this is a LOT lighter.
(nearly 10 oz) The loads imposed on a 60 year old crankshaft by this extra mass at high RPM is measured in TONS!!
I say that the only place for stock rods, at this point in history, is the scrap pile! If you build a motor and fail to invest
in the good stuff it a mistake. All of our LBC"s are old enough to need all the TLC they can get.
My buddy now has to try and save his block which will involve line boring (new main center cap) and a full build from there.
Seems like just the other day(2 years ago), I said "ya need steel rods dude" He is getting them now.......
Mad dog

 

[auprichard]

There's a reason there's a red line on that tach. You can't blame the rods - unfortunately the blame lays squarely at your friend's feet for over-revving a stock engine.

My smallmouth TR3 has steel rods: my TR250 has steel rods and a steel crank.

 

[CJD]

I have never run my TR's hard, but from a couple decades reading the forum, the crank is the limiting part. You are right, in that reducing rod weight goes a long way, but remember that centrifugal loads go up exponentially, so you still have to respect RPM.

To improve the reliability of any crankshaft, you can smooth all the burrs and make it smooth with a grinder. Machine the journals with larger corner radii, and then have the crank shot peened by an aviation company with mil-spec equipment.

 

[Sarastro]

I think that what you say has some validity, but as to the the expense of reconditioning stock rods: I paid $6 each (USD, of course), to resize the big ends, $6 to install and hone the small-end bushings. Balancing was not itemized, but it was about $100 for all the rods, pistons, crank, and flywheel. Not so bad, I think.

As for the rest, I think that stock rods are adequate for anyone who doesn't drive like a lunatic. Remember, those 660g rods are still more than 2/3 the weight of the stock ones, which probably helps but is still significant. Centrifugal force is proportional to mass, so the stock rods are a factor of ~1.5 over the lightweight ones, in that respect, but I suspect (and I admit I haven't run the numbers) that the piston forces are dominant, by far, if you drive full bore at high speed. In that case, lighter rods probably won't save you from the consequences of your lead foot. Might help your crank last a little longer, but probably not a lot longer.

 

[Sarastro]

Well, I probably should modify my earlier post.

I pushed some numbers around, and found that the forces from the piston and the centrifugal force on the crank, due only to the connecting rod, were roughly the same. A little surprising, but there it is.

A couple things to keep in mind, though: (1) the crank is spinning not only the connecting rod but, of course, its own weight. That is substantial, and makes the difference between the two types of rods less significant; (2) the centrifugal force is proportional to RPM squared, so if you halve the RPM, you get only 1/4 of the centrifugal force; and (3) if you get off the freakin' gas pedal, you've reduced the torque on the crank significantly, as well. I conclude that the way to keep your crank in one piece has more to do with treating it gently than anything else. That's what I do--I like my car so I don't beat it up.

 

[Hamish Racing]

I can recommend Maxpeeding rods and a light weight flywheel and everything properly balanced.

and in action

 

[bobhustead]

What did they run for cranks, pistons, and rods when they were finishing endurance races when these cars were new? Did cranks fail? Dunno, but thought it could shed light.
Bob

 

[mctriumph]

The full mass of the reciprocating assembly goes down, STOPS then goes up and STOPS again.
As the RPMs go up, the loads on the crank go up NOT linearly, but logarithmically. These loads
will have the effect of bending the crankshaft snapping it like a twig at some RPM .
In a perfect world ,rods and pistons would have little or no mass to accelerate/decelerate several
times a second. Here on Earth we do what we can to make stuff light and strong.
The engine in my 59 Tr3 was dyno tested to 6800 with a HP peak of 155 on pump gas. Trick pistons
steel rods ,deep nitrated crank. Has 30K on it now and I shift at 6000. so as to take it easy on the ol girl.
Hamish is correct Maxpeeding rods are sweet! Bob asks a good question about history .Kas spent a lot
of time on his rods making them as good as they did, but still had failures,it was just part of racing.
Mad dog

 

[Sarastro]

Well, logarithmically is less than linearly. I suspect you meant exponentially, but I'd need an explanation of how the laws of motion could produce literally exponentially increasing forces. (I'm best convinced by hard info and numbers, not hand waving.) Perhaps, I don't know, but it seems very improbable to me. And I certainly won't put words in your mouth.

My problem with your initial post was really that you gave no solid information to back up your claim, and then concluded that stock rods are trash. They might be inadequate for what you are doing, but not for most of us. The needs of racers aren't really our standard.

 

[karls59tr]

A good friend called up with the news that his Tr4 had broken a crank. Pretty fresh build, but he does some AutoCross
and hits 6000 fairly often. Stock rods , it is proved once again, are CRAP !! Have you ever weighed a stock rod??
Save you the trouble 930gr +- 6gr. It may as well be a brick. Modern steel rods have ARP bolts and now cost less than
the expense of reconditioning old rods.They weigh 660gr, Even if math is a swear word to you, this is a LOT lighter.
(nearly 10 oz) The loads imposed on a 60 year old crankshaft by this extra mass at high RPM is measured in TONS!!
I say that the only place for stock rods, at this point in history, is the scrap pile! If you build a motor and fail to invest
in the good stuff it a mistake. All of our LBC"s are old enough to need all the TLC they can get.
My buddy now has to try and save his block which will involve line boring (new main center cap) and a full build from there.
Seems like just the other day(2 years ago), I said "ya need steel rods dude" He is getting them now.......
Mad dog

Just as a point of interest I remember reading on a forum years ago about TR3 crankshafts breaking at the 4th journal and it was due to lugging the motor when in 4th gear. Apparently some sort of sympathetic vibration is set up at that point that causes the break in that spot. So not just over revving to 6 grand will do it. When the crank broke on my TR3 I had just gone over a railroad track....the straw that broke the camel's back??? What a gawdawful banging but the car still ran till I pulled over and called a tow truck.

 

[mctriumph]

It all comes down to the three main journals on a crank that begged for FIVE!
Crankshaft flexing is a real thing and it is deadly to nodular iron parts. Also there is
NO harmonic dampener like all modern motors, opening another can of worms.
And the factory never X rayed these cast lumps, they break at faults in the castings.
The miracle is that so many still run, 60 years on after the flogging they received
by wild eyed youths(us).
Mad dog

 

[CJD]

The TR2-3 crank is forged steel. Surely they didn't go cast iron on the TR4??

"The Crankshaft (Fig. 8) is forged from molybdenum manganese steel, being provided with balance weights which are an integral part of the crankshaft throws, adjacent to the three main bearings."

 

[malbaby]

You could also fit a harmonic balancer.

 

[mctriumph]

The TR2-3 crank is forged steel. Surely they didn't go cast iron on the TR4??

"The Crankshaft (Fig. 8) is forged from molybdenum manganese steel, being provided with balance weights which are an integral part of the crankshaft throws, adjacent to the three main bearings."

You are mistaken sir. It is nodular iron, look at the casting marks
Mad dog

 

[CJD]

Not me...that's straight out of the TR2/3 Triumph service manual. My TR2 crank does not have casting marks, but only forging parting lines. Plus, I had to re-drill the flywheel locating pin and mounting bolt holes because someone ran the flywheel loose...and I can vouch it is definitely steel! If the TR4 is nodular, then that is a change they made in later production. It seems strange they would up power for the TR4, and then weaken the crank??

 

[mctriumph]

The only Tr2 crankshaft I have ever had in my hands, had a casting fault. A bubble the size of a
a pencil eraser. Quite deep and fatal. It was from a 55.
In our race car we used a Miata engine, also a cast nodular iron crank. Modern and X rayed ,
But this is a 5 main engine and with steel rods, good to close to 8000 RPM.
Mad dog

 

[LarryK]

I have seen SCCA racers run TRs with success, but not stock rods. Soft bearings and watch rpms. Seen them come in and switch bearings between races. Never saw problems with stock TRs in Solo/Autocross when revs are watched.

 

[mctriumph]

Also not discussed is the fact that metals fatigue over time due to repeated stresses.
NOTHING lasts forever. This includes crankshafts. The fact that the steel rods sold today are less than 1/2
the cost they were 20 years ago is wonderful IMHO.I firmly believe that using them when possible
will keep Tr's on the road longer.
Mad dog

 

[CJD]

Even though it seems like a crank "just breaks" all of a sudden...it is always a fatigue fracture that kills cranks. You will see the tell tail "beachmark" striations on the face of the broken parts. Failure will always begin at the corner of one of the bearings, where the stress is concentrated. If you want a crank to live the longest, machine a large radius in all the bearing journals, and then have the entire crank shot-peened. Fatigue only happens in tension...and almost always at the surface of the part. Shot peening compresses the outer layer of metal, leaving it in compression. That way the surface sees less tension, so less chance of fatigue starting.

One problem I found when looking for a shop to shot peen is the so called "speed shops" have no idea what shot peening really is. They all think it is mixing steel shot with sand and walnut shell. That is the worst thing you can do to a crank!! Each sand impact sets up a stress riser to begin fatigue faster. You must take the crank to an aerospace contractor specializing in mil-spec metal surface prep. The machine is a couple hundred thousand $, with 8-20 nozzles that shoot precision shot at the crank while the crank is spun and raised up and down. The speed and size of the shot is important for the type steel being treated. All journals are protected by only duct tape, while the radii are left open to the shot, believe it or not.

McT...because forging is a non-precise impact process, you get a lot more imperfections than in most well-cast cranks. The difference is that a bubble like you saw will kill a cast crank, but it can live a long life in a forged steel crank. Of course, if you are racing and need rock solid reliability at high RPM, then you want to select a forged crank with no visible imperfections, even though this is difficult with most forgings.

Triumph nitrided their cranks. This process drives a layer of nitrogen into the surface of the crankshaft...but only about .002" deep. It does little for fatigue resistance, but really reduces wear. The older nitriding process is outlawed by the EPA, as the crank was dipped into very caustic chemicals. The modern alternative is called Ion Nitriding. In ion Nitriding the parts are place in a sealed container and electrified while in a nitrogen gas. The parts light up like a neon light as the nitrogen bombards the surface and becomes impregnated...again only .0025" max into the steel.

From what I have seen, the stock TR rods will long outlast the crankshaft. But, anything to lighten the rods...and pistons...can only help the crankshaft. Rods, being steel, can benefit from shot-peening. Balancing the assembly within a gram also goes a long way to reducing loads on the crankshaft, as it prevents uneven bearing loads.

Above is 4 years of Engineering Mechanics Materials higher education in a nutshell.


Post script...If you look through the archives, I once said that with meticulous prep as mentioned above, I would be comfortable twisting a TR crank to 6k regularly, with an occasional visit to 7k. I based those numbers on the improvements gained when prepping a known crank, good to only 5k stock. Randall told me that I was talking irresponsibly, as the crank will never go above 55k without problems. From what I hear from you guys tracking your cars, I was pretty close!?! I used to take a nodular Chevy 400 crank with a 5500 redline, and after prepping turned 7k regularly. (the prepping on that crank was even more substantial, with lightening holes and mirror polishing). I sold the car to a guy who is still running the engine weekly in the quarter mile, with the shot peened crank happily spinning along and pushing over 600HP on a cast crank! Someday I'll go into how fast I could spin a prepped forged Chevy crank!!

 

[mctriumph]

Got a pic at the autopsy note the broken main cap