Crankshaft Dowel Pin 'C' Engine

[Don Gale]

This proved my Dad, and mentor's wise council, that "the only problem with experience, is the price one pays, to earn it".

"Experience is what you get right after you need it"

One of my Dad's most frequent quotes was "What have I overlooked today that will raise hell tomorrow?"

[Mike Horton]

David, all, it has always been my "opinion", that the dowels were for indexing the flywheel, and crank, to maintain any balance effects present, and that for the most part, the gland torque, holding the flywheel to the crank, in tension, was the greater of the design fitments, just my $.02

I've been there on the diff carrier, as well. A 36 HP used part was used in the repair, back in about '66, due to my "broke college student budget", in my old '57 T1 Normal coupe, an "outlaw" for sure, as it had a "pea shooter" Abarth installed, a Talbot mirror, and aftermarket 5" German chromed wheels... I wish I still had those wheels.

[Don Gale]

David, all, it has always been my "opinion", that the dowels were for indexing the flywheel, and crank, to maintain any balance effects present, and that for the most part, the gland torque, holding the flywheel to the crank, in tension, was the greater of the design fitments, just my $.02

Ditto. The offset dowel further provides one way indexing for balance purposes. If the gland nut is properly torqued, the dowels should not normally see a shear force. Perhaps they were originally designed as a fail safe for a fault condition? I also think the OE pins were soft enough to shear before causing catastrophic damage to the crank or flywheel should the gland nut loosen or fail (just speculating).

I remember torquing a JC Whitney gland nut on a 1500 cc VW motor on the kitchen floor in my apartment back in the 70's, after assembling the case on the kitchen counter when I was single and could get away with such shenanigans. The head of the gland nut snapped off about an hour later and shot across the room & hit the wall. I replaced it with an OE nut from the dealer. For my next 356/912 rebuild, I will be using a Scat gland nut and washer & NOS German dowels.

[Harlan Halsey]

The 356 flywheel to crank connection has alwas been one of the most critical and sometimes troublesome features of the 356 engine. The connection was originally sized for the 25 HP pre war KDF/VW. Then it had 4 dowel pins and the 4 dowel pins continued through the 36HP VWs. Porsche, using the VW crank case, added 4 more dowel pins and hoped for the best. If you want to see what Porsche really thought in 1959, unconstrained by cost, look at the 40 HP VW engine. The diameter of the joint is much bigger. (Or for that matter look at the 130 HP 911 engine.)

When everything is perfect, the 356 connection does work, even in race cars, but when it is not the flywheel comes off.

It is pretty disappointing when thinking to fix a simple leak to find that a complete teardown is necesary. If it were my engine I would send the crank and flywheel to Armondo at Custom Crankshaft Repair in Texas, or Ted Robinson in Santa Clara, CA, could probably do it using the drill jig Bob Garretson made decaders ago, but he is pretty reclusuive.

[Mike Horton]

Don, check the flywheel, if it has been cut much, my only spare 912 flywheel, for my current spare project, came with a stock gland bolt, which had been "ground" thinner, due to the larger diameter of the Porsche 200mm clutch plate springs. My compromise for that, was to use a 1600, 200mm VW clutch plate, as the springs were smaller in diameter, and the chromoly AA gland bolt, as it has a thinner head height, just sayin'. I'll have to let you know later, as I'm just now waiting on powder coating on tins, to pre-install-run my fresh engine.

This is planned as a short term installation in my 912, while I go through the original, and is not a "cooking" engine, rather a "simmering", cruising, engine... time will tell,

[Mark Sabbann]

Edit: The oversize dowel pins explain the torn/deformed gasket at the broken one, probably tore when the previous mechanic tried to push the gasket over the larger pins.

Here's a what if- What if the dowel holes in the new soft iron gasket were lightly enlarged with a Dremel and then the perimeter was pre-split at each hole to allow the gasket to better accommodate the large pins and eliminate the stress resulting from forcing the gasket into place. Or would the gland nut torque shred the now 'star' gasket? OR not cut the perimeter but just carefully enlarge the holes? It would require a hole diameter increase of about .014". (.35mm/25.4mm/in = .014in)

[Vic Skirmants]

That gasket is also part of the sealing method to prevent a leak. Oil COULD migrate past the splits and under the gland nut washer.

[Don Gale]

Edit: The oversize dowel pins explain the torn/deformed gasket at the broken one, probably tore when the previous mechanic tried to push the gasket over the larger pins.

Here's a what if- What if the dowel holes in the new soft iron gasket were lightly enlarged with a Dremel and then the perimeter was pre-split at each hole to allow the gasket to better accommodate the large pins and eliminate the stress resulting from forcing the gasket into place. Or would the gland nut torque shred the now 'star' gasket? OR not cut the perimeter but just carefully enlarge the holes? It would require a hole diameter increase of about .014". (.35mm/25.4mm/in = .014in)

As Vic said, you want to keep the gasket contiguous without any splits or open gaps. Buy a few spares to practice on. I have successfully used a hole punch for gasket making. Buy a new punch, once worn, they tend to fold the material vs cutting cleanly. The flywheel has a 1.5 degree taper at the crank interface and will make contact outside of the dowel holes first, so it's important the outer perimeter of the gasket is not split or open.

Later VW type 1 1600cc engines do not use a gasket, the flywheel mounts to the crank directly. VW resolved the dowel pin leak path seal issue by machining a piston seal o-ring groove inside the large bore of the flywheel and seal against the o.d. of the crank with an o-ring. Perhaps the machine shop you sub the work to could make that mod. Being close to the centerline, that amount of minimum material removal would not affect balance, but you will probably want to have everything balanced anyway -- crank separate, flywheel separate, crank and flywheel assembled, flywheel and pressure plate together and indexed.

[Mark Sabbann]

The seal feature of the continuous gasket makes sense. I have a hole punch and can practice with it on the old gasket but it is metal.
So the suggested groove for an O-ring would be machined into the flywheel just above the tapered/beveled seat where the metal gasket sits?

[Don Gale]

Here's the flywheel/o-ring groove from my 73 VW Beetle "practice motor". The o-ring groove starts approx. 1.5 to 2mm from the crank face. The o-ring is metric, 3mm cross-section diameter (.118") actual. The inner diameter size will vary VW vs. 356, I don't have that number handy. Machine the groove depth approx. 75% to 80% of the actual o-ring cross-section diameter. The width of the groove 110-120% of the cross-section diameter. Use a 70 or 90 Duro Buna N or 90 Duro Viton o-ring. Size the i.d. of the o-ring to the o.d. of the crank as close as you can find. Slightly larger i.d. is better than slightly smaller for ease of installation. Grease the o-ring lightly. Don't use a gasket. Keep the flywheel and crank mating faces clean and dry. This will provide stronger metal-metal contact shear strength vs. with a soft iron (or Heaven forbid ) the former paper gasket of the early pre-A design. It will also eliminate the leak path from the front bearing thru the flywheel. The oil exiting the bearing toward the seal has a non-trivial pressure at higher RPM's. I personally prefer the o-ring design over the soft iron gasket for numerous reasons. Others may differ.

[Martin Benade]

I think the Dremel tool route will be best, assuming your crank is good, if you will be using the iron gasket.

[Martin Benade]

Don, how much leeway is there for correct o ring volume? I hear they are not compressible.

[Don Gale]

Don, how much leeway is there for correct o ring volume? I hear they are not compressible.

Yes, that is a valid point. A lower Duro o-ring may rupture during installation when too little volume is specified when the joint is fully assembled. A higher Duro o-ring may impede fully compressing the joint. A 120% groove width by 75-80% groove depth based on actual o-ring cross-section is pretty standard, allowing for a token .005"-.010" radius at the bottom of the groove and .003"-.005" round over at the outer edges of the groove, with a slight 3-5 degree taper of the groove sides. This is the Parker spec. for face seal and piston seal o-rings and provides approx. 90% o-ring squished volume vs. actual volume of the empty groove, so there isn't a concern in this case for rupturing the o-ring based on material compressability. I spent several decades doing o-ring seal designs of all types, both vacuum and high pressure gas, liquid, and oil at one of New Mexico's major weapons research labs and have never had a problem with these parameters. For piston seal o-rings, one must pay close attention to the gap between the 2 cylindrical contact surfaces, lest the o-ring be under compressed or try to extrude thru the gap at higher pressure, but shouldn't be an issue for a 356/912.

[Martin Benade]

You sound like you know everything one would need to know to successfully add an o-ring. I like it! Those dimensions are for a square-cornered groove?

[Don Gale]

Those dimensions are for a square-cornered groove?

Yes, round o-ring in a square/rectangular groove. Silicone vacuum grease is best as far as not causing the rubber to swell. I have also designed round o-rings in a triangular and trapezoidal 1-sided/2-sided captured groove for special applications. I have occasionally used square or rectangular o-ring stock with little volumetric clearance for high pressures, c-shaped thin wall open silver plated stainless rings with or without an inner spring for extremely high pressure seals. I have found 90 Duro Buna-N to be good in a static seal for up to 10,000 psi helium, nearly 100% volumetric fill by o-ring vs groove cross-sectional area, a regime we will probably (hopefully) never see in a 356 (ha!).