Name That Crank

[Jacques Lefriant]

Hi Guys
1. I know CJ has a lot of motorcycle experience and is technically informed. his comments on roller cranks are very well informed. . However SPG cranks were German manufacture. The SPG crank was basically a simple version of the Porsche pushrod Hirth crank .Even at the time the use of the "Hirth coupling" made the manufacture of these Hirth cranks expensive and not suitable for mass production. The Hirth crank achieved great success in the Spyders/ Gt race cars. there were some early problems with bolt fractures from being overstressed but the differential tie bolt seemed to eliminate that. The Hirth coupling is a proletary version of a zero angle bevel coupling similar coupling are used in the manufacture of turbines and are more commonly called Curvic couplings. Hirth cranks can be serviced and restored to optimum condition and if done on a regularly basis impending disaster can be averted since everything can be inspected. When SPG cranks were new they could have been bought for less than it cost to repair a plain bearing crank. I am at a loss to comment on Al's conclusions I would think the reason that Porsche used the next size over bearings was to allow them to better machine the crankcases that VW supplied since it did not make the bearing stronger by only very little. Porsche did not have the crankshaft manufacturing design experience and relied on firms like Hirth that had done aircraft and pre war racing cranks. back in the day airplanes had to survive with an oiling system that would allow the plane to be upside down. I am told that plain bearing aircraft engine use silver plated bearing to survive. To me the big problem with the SPG and to a lesser extant the Hirth was the steel used and the thermal processing to harden the components. Most failures can be contributed to abuse like the piston hitting the head or valves in the 4-Cam(mostly due to improper assembly). severe detonation r in the case of the SPG drag racing clutch engagement. Long term wear/fatigue failure is common to the majority of the Hirth crank that I have taken apart and could have been predicted by periodic oil analysist or a good ear. Some Hirth cranks just need the rollers replaced and an occasional cage to restore to factory specs. Also Ab's comments do not add much to the discussion since for the same application like early 1500 engines the roller versions had a smaller slower turning pump and did not have coolers installed only in the Spyders. 4-Cam roller crank engines feed the crank thru small jets to minimize the amount of oil since too much would heat the oil even engines that had these small jets plugged did not experience any starvation and the cams and drag levers which had bigger supply holes if plugged would instantly destroy. These are my takes on the subject and i beleive i have seen more of this than Al and Ab and i wish they would not give erronious info.
j

[Ron LaDow]

Engine internal rolling element bearings were a wonderful solution to pre-war ( that's "WWII" ) tech limitations to clean oil and workable plain bearings. What we consider 'normal' engine life-spans weren't in the picture.
Unlike plain bearings, which, theoretically, never have metal-to-metal contact, rolling element bearings do, and they have that contact under "line" (one-dimension, for those who read "Flatland") contact.
Per Ab, it takes a skid leaving or entering the load plane to 'wipe' the surface off the roller, or a detonation at TDC to smash the roller (line contact, remember?) into the journal to start the spalling. And all it takes is one; you've got dead meat thereafter and quickly.
To be clear, money talks, BS walks. I make the following offer:
I bought mine from a noted enthusiast 40 years ago, which enthusiast did nothing to mislead me. I opened the crate and the plastic bag once to confirm it had been serviced and welded by Jim Wellington. It was then re-bagged, the crate was re-closed and I bought good used plain bearing crank for that engine.
I paid $500 for that thing at the time; it is yours for $500 plus shipping right now.

[Albert Tiedemann]

Jacques:

I do not intentionally offer comments that could be construed to be erroneous.

In anti friction bearings, the rolling elements do carry the load and the oil dissipates the heat.
Logically, more oil volume provides more capacity if used as a heat sink like in air cooled engines.

In the rod/crankpin anti-friction bearing scenario, the rolling elements are accelerated and decelerated as stated and can be proven to be approximately sinusoidal if a reasonable assumption regarding rod length is made to avoid mathematical prowess beyond differential equations. Check early Porsche Panorama issues in the sixties for a proof under my pen. This action of speeding up and slowing down the rolling elements will produce sliding [as Al mentioned] or smearing and this condition does effect the life of the element. Other factors, like ones that you mention are also to be included as well as unknown factors like the heat treatment quality of selected metals. 52100 has always been the go to material for anti-friction bearings. I do not know what was used, but it would have been called out on the Hirth drawings. Well connected personnel in the trade might have access to same and this would go a long way in limiting speculation on the judgement of the designer who chose to use the materials from which the components were made.

Yes, I am certain that you have seen more than any of us participating on this forum regarding the subject at hand, but I am, perhaps, one of a few participants that have actually taken these Porsche crankshafts apart and effected a repair to same.

And to close the loop of the statement that the comments did not add much to the discussion, I may as well say that I designed and had manufactured for Heinz Bade a tool for loosening and tightening that differential tie bolt[different pitch same hand threads] you mentioned that was proofed at 960 lb-ft torque [limit of test set up]. A 350 ksi maraging steel, heat treated to 55-57 Rc [900-925 degrees F for 3hrs, air cool] was used.

[Craig Richter]

This has been an intriguing discussion between some very smart, well-educated engineers. Thank you. But most of this discussion is way over my pay grade, except the unanimous agreement that roller cranks are a least 80-year-old technology, don't last a hoot in 356 motors, and we now know where to find a nice one pretty cheap. Dennis, I'm afraid you've just got an interesting door stop.

[C J Murray]

We all agree that plain bearing cranks are best assuming they are given adequate oil supply.

The points of dispute are...

all roller cranks in all engine designs fail quickly. FALSE!

any engine with a roller crank will fail quickly if the engine is often operated at low rpm. FALSE!

As far as skidding rollers are concerned, don't they skid a little all of the time due to the difference of diameter between the inner and outer races?

How do wheel bearings survive?

[Dennis Vogel]

This has been a fascinating discussion with regards to the merits of roller bearing cranks. Mostly over my head.

In digging through a newly acquired copy of The Maestro's The ABC's and 912's of Porsche Engines he has photos and brief mention of SPG roller bearing cranks, and even an MC piston. Both found in our engine. He doesn't seem to speak of them in context of Super 90s though. So while perhaps Porsche parts originally found on 356 engines, I am still skeptical that they were original to mine.

[C J Murray]

MC pistons are not original, they were an aftermarket performance piston. Usually they required a lot of clearance so they caused oil consumption. Many also had grooves in the skirt for oil retention which resulted in cracks at the stress riser the grooves caused.

[Dick Weiss]

I have a couple of refinished MC big-bore sets in my archives if interested; Also 2-Arias big-bore kits and NPR kits + new spare cyl's.

[Jacques Lefriant]

Hi Ab
Don't take this the wrong way. I have more reliance on the guys in the trenches like Vic than theoretical statements that give me a headache. I was in the world of theoretical Physics worked on the Saturn V did high end numerical analysis for solving Differential Equations etc. from my recent survey of Hirth cranks that have been in service the ultimate ones were the 82mm 4-Cam ones. The most prevalent failure that retired the cranks seems to have been fatigue or spalling as Ron said. I had not seen true brinelling in my limited sample. almost all cranks throws had frosting due to contamination in the oil some cranks/rods were worn from impinged material in the cages but not on the roller path. extreme cheek damage was somewhat common due to piston pin misalignment or in some cases replacement rods did not have the proper flank treatment. only in extreme cases was there any discoloration due to elevated temperatures and brittle failures like the PB ones. Many 4-Cam racing engines on tear down have evidence of piston to valve/head collisions but causation to crank failure has not been established. most cranks in service had cage pocket wear which would increase the false motion of the rollers. From a reliability point of view repair to the cranks(sleeving/build up one or two out of 7 journals) should not be done since statically the other journal are about to fail. only servicing(restoring new tolerances by either using oversize rollers with new cages and or new rods/races is beneficial to the longevity. The best course would be to use new cranks. Since it was economically prohibitive even for Porsche in the day to construct more cranks we are left with bad choices. As Ab choose to use a special steel to build his tool you could make the crank from that steel and would even at $20 a pound only add $3K to the cost. I think an economical 4-Cam roller crank can be done using new developments from the bearing companies but will still be prohibitive for pushrod engines although there may be an occasional SPG that could be used for say 10,000 miles just for fun.
j

[Ron LaDow]

My doorstop is going elsewhere (after I confirm it's suffered no harm for being stored where it is). The buyer is knowledgeable and while I have no idea regarding his intended use, he's smart enough to use it appropriately.

[Wes Bender]

It's probably the lifetime warranty that you offer on such things.

[John Clarke]

Hi All
This has been a great topic to follow, with top engineers ( You do it for me Jacques , working on the Saturn 5) Respect, arguing or discussing the merits or otherwise of the roller cranks. We all have our different opinions on these very different cranks, but what we mustn't do is fall out with each other! Thought I was an engineer, I'm 2 Bob!
Cheers Jay