Name That Crank

[Dennis Vogel]

Can anyone identify the crank in this photo? I am not able to find any identifying marks on it other than a faint "4969" that looks burned onto the surface of one journal.



Our 1960 Super 90 engine appeared original, till I got to the pistons (marked MC HA) that have no bolts on the connecting rods (marked R33804A) . Now it appears the original camshaft (VW 111.109.111) and crankshaft are gone too.

My understanding from the late great Harry Pellow was that it would have had a Super 90 non-counter balanced crank that is no longer available, even as a reproduction. Used they seem to sell for North of $2,000. And then, I suspect you’re taking a leap of faith that they’re good.



I have some expensive decisions to make. Sigh.

[WilliamVaughan]

So what do you have against roller cranks? Evidently you have been nice to it. You didn't kill it.

[Martin Benade]

I'll guess an SPG roller. If it is, nobody seems to have any respect for them. The word doorstop will come up.

[Dennis Vogel]

So what do you have against roller cranks?

I have no opinion on roller cranks good or bad. I have no experience with them. Just disappointed that the engine wasn't more original.

We've never started the engine. It was in no shape to run when my dad bought it over 10 years ago.

Whomever had it before us wasn't totally nice to it. The crankshaft drive pinion is a bit chewed up. I think I identified that correctly. Runs distributor drive?

[Ron LaDow]

I'll guess an SPG roller. If it is, nobody seems to have any respect for them. The word doorstop will come up.

Really high-tech for 1935 or so, with a life-span to match. They are unreliable and when they go, they tend to take a lot with them.
Probably the cheapest decision you'll make here is deciding not to use it.

[Jacques Lefriant]

Hi Dennis
there is no need for an original S-90 crank. a SC or 912 would give good service you can then use the existing flywheel and you will have less problems finding bearings. If you wanted you could use the SPG. If you do be careful that the piston does not come too close to the rod small end sometimes you have to clearance the piston. also note the oiling feature of the main bearings. If you are going to abuse the crank you should pin or weld the rod throws. I know I will get a lot of contradictory advice but I only want first hand recommendations. I have used a few SPG and I think the big problem is they are not serviceable these days.
cheers
jacques

[GregVandenbussche]

a scat crank would almost be more economical if you plan to by something new...

[Vic Skirmants]

Dennis; the brass distributor gear on the crank is a good example of what happens when you rotate the engine backwards WITHOUT the distributor in place. The distributor drive gear climbs up the teeth and then chews on them if you keep rotating.

[David Jones]

How do you know that Vic ? As an aside that is an easy piece to obtain. There are millions of them out there as it is identical to the VW part.
A new one is cheap ...https://www2.cip1.com/ProductDetails.as ... D105%2D223
or
https://www.stoddard.com/54610211001.html.html

[Al Zim]

You have an SPG roller crank. To make it work you will need to modify the main bearings to spray oil on the rod bearings. You do not want to use the crank. The crank is pressed together and under severe load the rod journals will rotate causing the crank to be inappropriately timed. Nice to look! but worthless. al zim 2018/12/26

[C J Murray]

Roller cranks that are made correctly should last a very long time. I am always mystified by why roller cranks often fail in Porsches when they worked wonderfully in all types of high performance motorcycles. Plain bearing cranks are now the norm in all engines but a good roller crank should work fine.

When I look at an original Hirth roller crank design I do question why they made them so complicated requiring fo many precision machining operations that could be prone to inaccuracy. Maybe that is why they fail?

The SPG crank is Japanese, I think, and they have a simple press together design like Japanese motorcycles and Harley Davidsons. Yes they can twist under very extreme conditions but that is rare. Press together cranks should be welded if the use is to be extreme like racing but some engines don't need that step even for racing. An advantage for all roller cranks is that they need very little oil and the splash that enters the rod bearings through the slots in the rods should be more than enough.

All that said I guess their reputation suggests that a Scat is a better choice.

[Al Zim]

Porsche was a very experienced design firm. this was born out by the numerous designs they produced. The intricacy of the design underlined the reliability and longevity of their vehicles. Ask yourself why the 356 main bearings are the first outer oversize on the case? This distinctly added to the longevity of the early engines, and made their vehicles expensive. Addressing failures in the roller crank engines were primarily due to the way they were driven in the USA. Everyone here drove one gear higher than they should have causing lower RPM's and loading the crank so the rollers slid instead of spinning.
SPG cranks did not have pressure oiling to the rod bearings and required a modification to the main bearings to allow an oil stream to spray on the rod bearings to lubricate the bearings. Failure was inevitable. al zim 2018/12/26

[C J Murray]

Motorcycles with roller cranks were often ridden at too low rpm without failure. Porsche misjudged the stress level when driven "normally" which means, they didn't allow for misuse which all machines face when operated by "the public". Maybe the bearings were sized incorrectly or the wrong metals were used or the balance was wrong? Who knows, but they got something wrong.

As per roller rod bearings needing a lot of oil flow, there are many 2 stroke and 4 stroke engines that would disagree.

[Albert Tiedemann]

When I look at an original Hirth roller crank design I do question why they made them so complicated requiring fo many precision machining operations that could be prone to inaccuracy. Maybe that is why they fail?

An advantage for all roller cranks is that they need very little oil and the splash that enters the rod bearings through the slots in the rods should be more than enough.

What machining operations are you questioning and is the concern for the pushrod roller crank or that used in the 4 cam?

The rollers carry the load not the oil film.
The oil must handle the heat generated and perhaps that is why Castrol R40 was used by Bruce Jennings and Others campaigning the 4 cam car in SCCA racing in the sixties. More oil and a dry sump better manages oil temperature. More likely it was the acceleration and deceleration of the rollers entering and leaving the load plane and the resulting sliding and smearing of the rolling elements that promoted brinelled looking of the rod bores and the crankshaft journals. Now, for the land riding cages used in the connecting rod bores of both roller bearing crankshaft versions for the 356 Porsche, there were, to my knowledge, only 3 machines in the world in that era that could produce the cavity for housing the crankpin rollers and these were single purpose machines.

[C J Murray]

Hi Ab. My comment has to do with the 4 splined joints which are vastly more complicated than the interference fit that is generally used in built up cranks. Traditionally a taper fit was used with a nut used to pull the taper into its interference fit. I'm not positive who started the straight interference fit method but it does simplify machining and assembly and takes less space because no nut is required. This is a simple and elegant design solution that works.