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TRactor Motor Crank Breakage

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martx-5

martx-5

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Over on the Miata forum, they're talking about MGs vs TRs. Sort of the which is better Chevy or Ford. Well, one poster brought up the subject of TR3 crankshafts breaking (between 3rd & 4th journal?) at sustained high rpm. I remember reading something about this years ago, but it supposedly happened at low speeds and quite unexpectedly. Is there any truth to any of these stories, and if so, what were (are) the causes, and is it still a concern. In all my years on this forum, I don't remember this topic coming up. Is this one for the mythbusters??
 
Not really.

The top TR4 racers in the 1960s were turning 7000rpm on stock cranks and rods. They would cycle rods out every 5 races and cranks every 8 or so.

The last two people to really campaign these cars in SCCA used nitrided stock cranks and spun them to 7200rpm.

With forged cranks and other lightweight components, they'll spin to 8k. They however don't make power up that high, but it's nice to have a safety margin. The bottom end can take the revs better than the valvetrain. The camshafts with their unsupported journal in the front bounce around like a wet noodle and cause problems at really high revs. This issue is further compounded by the fact that high lift racing cams have a smaller base circle and it weakens this area.
 
I basically agree with Monkeywrench but the myth, IMO, is not totally busted. TR3/4 crankshaft breakage was not unknown under racing conditions and at much less than 7000k. Although the top teams may have been using stock cranks, their engines were extremely well prepared and particularly well balanced, which may have been as important in keeping things together as cycling cranks and rods. Crank breakage may not be a problem at 7000k, but there is little room for error.
Tom
 
I was under the impression (and I can offer no citation for this) that there was a particular RPM at which sustained running presented the risk of breakage.

IIRC not a particularly high RPM, more like 2800 or so. But as I said, I do not speak from any experience other than flogging a TR3 for 40 years w/o such a problem.
 
My TR4A crank was turned .030 under on the mains back in 1980 when I first got it and rebuilt the engine. It hasn't broken yet. Perhaps I should knock on the wood dash....


Dan B
South Charleston, WV
 
It definitely happens. Just a few years ago, one of our local vintage racers brought part of a crank to club "show and tell" night. It started as a stock crank, but had been nitrided (for increased strength), radiused and so on. ISTR he said he was running a harmonic dampener, but I could be mistaken on that point.

Stock crank breakage is enough of a problem that some folks are still developing better crankshafts for these old TRactor motors.
tr4crank2012-1web.jpg


Note that high rpm is not the only problem. The crankshafts have resonant frequencies where torsional vibrations grow much larger (greatly increasing the stress within the crankshaft). There are supposedly several rpms where the cylinder firings tend to stimulate the resonances, one of them only slightly above the factory redline of 5000 rpm. At least in theory, the original fan hub helped act as a dampener to reduce the resonant vibrations (kind of like putting your hand on a ringing bell). If you remove the fan and run for extended periods above redline, you need to add a dampener. This is written up in one of Kas Kastner's more recent books. IIRC he also recommended having the crank magnafluxed (tested for cracks) every few racing seasons.

One last thing, "stock" may be something of a misnomer. They start with a stock crank, but it gets significantly modified for racing. According to Kas, just the nitriding will add 50% more strength.
 
Shouldn't be a problem with normal everyday driving. After all, you can make all 5 MGB main bearings out of one TR3 main.
 
They're still mostly stock. Most guys aren't doing a lot of lightening to them and just add a bigger radius and nitride them.

There have been forged cranks available since the early 80s for TR engines.

Chris Marx doesn't seem to use a dampener, but he seems to be the exception. We're using one and it sure makes sense.

There are cranks available with dummy mains, but I don't see the reason for that.

The center main support flexes at high rpm, but that's fixable.

The bottom ends can take the revs fine. As I said before, the camshaft is the weakest point in the whole chain. What these engines really need is someone producing billet cams out of much stronger metal.
 
The cranks do break. I have one sitting in my garage. I don't k now if the supercharger had anything to do with it cracking. My new crank is nitrated. I hope it lasts.
 
On regular bases, I have been driving a tr3 since 1972. I remember a time when this guy back in about 74 wanted to sell me a tr3 with a hard top and wirer wheels and the only wrong was a broken crank. He wanted 100.00; it was in a field about 50 miles from my house, so I past. I thought about it and decided to get it, but it was gone. Like most the tr3 of that time, it had been rode hard and hung up wet more than once. Anyways, the implication I got was that when you ran them at a kinda high RPM then down shifted up and down in that process more force was placed on the crank, and they were more likely to break. I am not a physicist, but it made sense to me. I have been using the same crank for 43 years and sometimes I will red line, but I do not run it up, back off, down shift, and run it up again. Well may be once.
 
sp53 said:
Hey Randall for give my ignorance, but are all three of those crankshafts in the picture for a tr3?
Click to expand...
Yes, or TR4.
 
The top one is a standard crank (covered in either rust or rust preventative). As you can see, the TRactor motor only has 3 main bearings, there is no main bearing between cylinders 1 & 2 or between 3 & 4.

The center one is an improved design. For reasons I don't quite understand, the fake main bearings are supposed to make it stronger and more rigid. It also weighs less than the stock crank (but is still fully balanced of course).

The bottom one is a further development, even lighter and with improved oiling passages. Here's another view
tr4crank2012-2web.jpg
 
Do you apply the dye yourself, or is that from a shop?
 
I once had a crank on my Morgan SS that had cracked through enough of its thickness that you could wiggle the end back and forth and watch the oil squish in and of of the crack. Scary, but that's what overly-high RPM'll do with a stock unit. The engine got a Moldex crank when rebuilt - at least I think that was the name of the firm it came from.

Another bit of crankshaft trivia is that with the most typical break location being at the back, it's sometimes possible to have a broken crank and drive home anyway, with the shattered ends bearing on each other enough to transmit power. A friend did this for 5 or 10 miles in his TR2, and I've heard of it being done elsewhere. The friend said it was pretty nerve-wracking to listen to and he did surface streets in LA only, didn't try the 10 freeway. Back when a good sidescreen TR was a $400. car.
 
Moldex and Crower both make cranks for Triumphs. With enough orders, they'll make a forged or billet crank for anything.

There are one or two companies in Europe making forged crankshafts.

I'm really surprised no one has stepped to the plate concerning camshafts though. I suspect there aren't enough racers running these high enough that it's not a widespread issue, yet.
 
Monkeywrench said:
I'm really surprised no one has stepped to the plate concerning camshafts though. I suspect there aren't enough racers running these high enough that it's not a widespread issue, yet.
Click to expand...

From what I understand, cams for the TR3-4 generally only break when the regrind has gone into base shaft. Supposedly that is why the front half is so thick, the factory found unacceptable levels of flex before they made it thicker. I know that when Larry Young (no relation AFAIK) did a run of Tiltech cams a couple years ago, he offered both regrinds on stock cams and new ones from billet for that reason.

Another reason is that, apparently, most of the common regrinds are actually milder (in terms of max valve acceleration and jerk) than the stock cam.
https://www.tildentechnologies.com/Cams/TriumphCams.html

I'm really looking forward to trying his "street" cam after all the positive comments I've heard about the racing versions. Unfortunately, at the rate I'm moving, it will likely be several years before I get around to building an engine to put it in.
 
Hi Randall,

There aren't any billets available for the TR engines. There are new cast blanks, but the gear is hobbled incorrectly. This has been known for sometime and everyone that I know of that has tried one has had problems.

To get the lobe lift that is required for a real racing camshaft, the base circle needs to be decreased. There really isn't a way around it. A billet would just make the entire unit out of much stronger material. Using stronger metal would also mean better control of the valvetrain at higher revs.

There is only real cam that I can think of that was designed for smaller lifters that a lot of racers were/are using. With the availability of tool steel lifters in the Triumph size, racers can run aggressive lobes to take advantage of the larger diameter.

Remember, while these are undersquare engines, to make horsepower you need to either spin it higher and / or make it more efficient. Parts that can run all day at 6500rpm may not last long at 7000+rpm.
 
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