Hirth Cranks

Discussion of 4-Cam Type 547 engines (and all the Fuhrmann racing variants) and cars that powered them.
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Joris Koning
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Re: Hirth Cranks

#16 Post by Joris Koning » Thu Jan 27, 2022 7:22 am

Hi Rainer,

Very interesting thanks or sharing. In my notes I have the crank material as 17CrNiMo6 for the throws with the two wheels as 30CrNiMo8. How does that compare with what you found?
'56 Coupe
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Rainer Cooney
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Re: Hirth Cranks

#17 Post by Rainer Cooney » Fri Jan 28, 2022 11:58 am

Hi Joris, A couple of cranks I did needed to be re-hardened so my concern on material composition was the rod journals which were what I had shown, I never had any web material tested. I also had the tie bolts tested and they came in the same. Obviously this material is a '50's alloy, with 28 elements listed in the resulting test, and the exact same can't be had these days but it's equivalent would be 4140. All this testing was done in 2001 by a friend who in addition to being a 356 owner was the Dean of Engineering at MIT. He gave the crank material to some students to use as a project and the results I got back, most of which is WAY more information than I needed, would fill a small 3 ring binder. I really got a kick out of all the electron beam photos although, again, have no idea as to what I'm looking at. Here's a list of the total elemental breakdown.
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Jacques Lefriant
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Re: Hirth Cranks

#18 Post by Jacques Lefriant » Fri Jan 28, 2022 2:54 pm

Hi Rainer
what was the take on the quality of the steel used? would a better steel be recommended? after HT what was the Rockwell C value of original/ground.
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Re: Hirth Cranks

#19 Post by Rainer Cooney » Fri Jan 28, 2022 6:09 pm

Jacques, Material science is a wonderful thing and more than 70 years has passed since the manufacture of the first 547 roller bearing crank so there are certainly better materials and treatments of these that will produce a better quality crank. There was mention during the analysis of the material that it was leaning toward 4150 steel but I think today if I were making these I'd go with 4340 vacuum arc remelted steel which will result in a more homogeneous steel with less inclusions. Having said that for 1950's technology the Germans , I think, did pretty good. The proof of this is the way Billy used to repair these by grinding away almost 1/3 of the total area and sleeving them on the outside. Some of his customers were very enthusiastic drivers to the point of him shutting them off work due to damaged components as if they were readily available. When I visited with Bill in Jackson he had a "wall of shame" with the destroyed parts from some of his customers who though nothing of over-reving an engine. Of all the broken pieces he had I never heard of or saw a broken crank. I myself have no problem beating the hell out of an engine but one must know what it is you have in there and have confidence it will take the abuse. Rockwell of the original cranks on various areas of bearing surface were 58 to 63 RC. I took a really damaged crank ( junk) and kept grinding and checking every .005" per side down to to .060" which resulted in 48 RC. You can safely remove .015" and still have 58RC and that's per side for a total of .030". I never went beyond .015" total with out re-heat treating them.

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Re: Hirth Cranks

#20 Post by Rainer Cooney » Fri Jan 28, 2022 8:01 pm

Rainer Cooney wrote:
Fri Jan 28, 2022 6:09 pm
Jacques, Material science is a wonderful thing and more than 65 years has passed since the manufacture of the first 547 roller bearing crank so there are certainly better materials and treatments of these that will produce a better quality crank. There was mention during the analysis of the material that it was leaning toward 4150 steel but I think today if I were making these I'd go with 4340 vacuum arc remelted steel which will result in a more homogeneous steel with less inclusions. Having said that for 1950's technology the Germans , I think, did pretty good. The proof of this is the way Billy used to repair these by grinding away almost 1/3 of the total area and sleeving them on the outside. Some of his customers were very enthusiastic drivers to the point of him shutting them off work due to damaged components as if they were readily available. When I visited with Bill in Jackson he had a "wall of shame" with the destroyed parts from some of his customers who though nothing of over-reving an engine. Of all the broken pieces he had I never heard of or saw a broken crank. I myself have no problem beating the hell out of an engine but one must know what it is you have in there and have confidence it will take the abuse. Rockwell of the original cranks on various areas of bearing surface were 58 to 63 RC. I took a really damaged crank ( junk) and kept grinding and checking every .005" per side down to to .060" which resulted in 48 RC. You can safely remove .015" and still have 58RC and that's per side for a total of .030". I never went beyond .015" total with out re-heat treating them.

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Jacques Lefriant
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Re: Hirth Cranks

#21 Post by Jacques Lefriant » Fri Jan 28, 2022 8:08 pm

Hi Rainer
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Re: Hirth Cranks

#22 Post by Rainer Cooney » Fri Jan 28, 2022 10:58 pm

Well that's certainly an interesting part. Is this the cause of failure or the result? What do the remainder of the crank pieces look like. In my previous post about the strength of original cranks I wasn't trying to say they're unbreakable just that when properly maintained they give good service. I've had the pleasure of repairing more than a few holes in cases, some rather challenging, which were the result of broken cranks. A lot of these engines were run until destruction and probably ran real good, nice and loose, right before they blew up.

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Re: Hirth Cranks

#23 Post by Jacques Lefriant » Sat Jan 29, 2022 11:27 am

It looks like a typical fatigue failure like the broken PB ones. OK who is going to the first to tell how the other coupling was removed? i was going to use it for tooling.
 

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Martin Benade
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Re: Hirth Cranks

#24 Post by Martin Benade » Sat Jan 29, 2022 11:34 am

Waterjet?
Cleveland Ohio
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Jacques Lefriant
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Re: Hirth Cranks

#25 Post by Jacques Lefriant » Sat Jan 29, 2022 12:37 pm

Hi Martin
close try again.
 

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Martin Benade
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Re: Hirth Cranks

#26 Post by Martin Benade » Sat Jan 29, 2022 1:18 pm

Laser?
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Joris Koning
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Re: Hirth Cranks

#27 Post by Joris Koning » Sat Jan 29, 2022 1:38 pm

Edm?
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Jacques Lefriant
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Re: Hirth Cranks

#28 Post by Jacques Lefriant » Sat Jan 29, 2022 1:48 pm

Joris goes to the head of the class. yes it was a wire EDM notice the entry path that is how thin the cut is .
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Harlan Halsey
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Re: Hirth Cranks

#29 Post by Harlan Halsey » Sun Jan 30, 2022 6:21 am

Here's some lore and conjecture. German GP cars of the 1930s used Hirth roller cranks because they were the most reliable racing cranks. A factory racing crank needs only to finish a race reliably. For example, Le Mans is a 12-hour race so the crank must last 24 hours or about 3,000 miles. Porsche, of course, was very familiar with these cranks. The Carrera engine was developed exclusively as a race engine in the early 1950's, and naturally used the Hirth roller. In the Spyders these racing engines developed an outstanding reputation for reliability and power. So much so that they were often installed in other racing chassis. But in this application the engines really needed a life of only a few thousand miles before a rebuild.
It is rumored that when it was proposed to install the Carrera engine in street cars, the Porsche engineer in charge of maintenance objected strongly. I thought that that objection had to do with the inaccessibility of the #3-cylinder forward spark plug in a street Porsche. While that may have had something to do with it, I suspect that it had more to do with the maintenance requirements of the Hearth roller, and possibility the lifetime of the chrome lined aluminum cylinders. We now know that Hirth Rollers went about 40,000 miles on the street before failure, say 30,000 miles before needing a rebuild. Required maintenance on a street Carrera should have included a crank rebuild every 30, 000 miles! Race cars much more often. (At which time the aluminum cylinders and pistons would have been replaced too.) A street Hirth roller rebuilt every 30,000 miles would probably go several re rollerings before major parts would need to be replaced. Porsche could have had a crank exchange program. (When Chuck Forge was running the Red Car in vintage racing, he had Jim Wellington rebuild the Hirth roller every few seasons.) But with people pulling the Carrera engines and replacing them with pushrod engines because the Carrera, with no torque below 4,000 RPM was so in tractable on the street, Porsche had little incentive to do anything. (Carrera engines were sitting in garages all over the country in the 1970s as people had them replaced with pushrod engines. I got one such engine for $250 at a swap meet. Not running of course, but today, running it's worth more. The crank survived on that one, but the cylinders did not. Vern Covert iron sprayed the cylinders and Jim Wellington rebuilt the crank. That one ran the MHAR about 15 times before I got around to replacing the valve guides.) Due to the lack of maintenance, a lot of the original 500 or so Carrera engines were run to destruction. That largely accounts for the scarcity of Hirth roller crank parts today.
By 1960 Porsche knew that roller cranks were no longer necessary or desirable in racing engines. The The original 547 Carrera engines all had Hirth rollers as did the first 692s, but 692/2 of 1959 was a plain bearing Carrera engine as were all subsequent Carreras.
Today, 70 years later, about the time difference between the Wright brothers at Kitty Hawk and the Boeing 707, others are trying to recreate the
Hirth roller or to patch the old parts with modern technology.

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John Clarke
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Re: Hirth Cranks

#30 Post by John Clarke » Sun Jan 30, 2022 8:07 am

EDM
Jacques
Is that 'Spark Erosion' by another name?
Jay
 

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