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Thu Jun 28, 2012 5:38 pm

I have been working on my 356B that had been converted to 12 volt years ago. I find the ring gear has been heavily damaged and the twelve volt starter is not engaging into the flywheel properly.
Do I buy a new flywheel or has anyone had their flywheel machined to take a new ring gear.
I am interested in suggestions and experiences.
Ron Jackson

Thu Jun 28, 2012 5:51 pm

If you have ring gear damage, it is more than likely caused by using the old 6V starter running on a 12V system. (Most folks never changed the starter during the conversion to 12V, just ran the old 6V one on 12V). The starter solenoid engages very hard, slams its pinion into the ring gear, and the starter starts turning extremely fast, sometimes not meshing properly with the ring gear. Grind, Grind, Grind . . .

You can have the flywheel ring gear replaced by experienced machine shops such as Rimco or Competition Engineering or others, and then have them re-balance it and the clutch. Use the 109 tooth 6V ring gear as originally equipped. They will maching off the old gear, and heat the new ring gear and press it on the flywheel, and spot weld it into place, locking it down.

Then, I recommend an appropriate 12V starter with a 6V pinion. I use a geared 12V starter. I got mine from Zims Autoteknic. Its indexed properly for the Porsche transaxle, and it works great.

Thu Jun 28, 2012 6:14 pm

Rimco can put a new ring gear on it.

Thu Jun 28, 2012 6:47 pm

Mr. Jackson:

I hope you won't mind if I ask a question...

You don't mention what kind of starter/pinion combo was in the car.

And as a sidebar, I will note that a 6v starter w/a 12v solenoid will not damage a good flywheel.

Thu Jun 28, 2012 8:51 pm

Perhaps so, but a 6v motor running on 12v is potentially drawing twice as much current (Ohms Law - if resistance stays fixed, current goes up proportional to the voltage increase). The starter motor is not really going to like that very much, but I guess replacing the stressed starter motor from time to time is cheaper and easier than replacing the ring gear. But why not fix it right the first time?

Fri Jun 29, 2012 3:53 pm

Thanks to those that responded. The 356 is a relatively new project for me. The bendix was kicking out on start up so I pulled the starter but did not note the part number. Will do that when I am back home in 2 weeks and post it then. I reinstalled the starter as I was working through a variety of wiring problems under the dash board and need power on to resolve an absence of headlights that the last owner never solved. At that time I removed the existing 12 to 6 volt system and ugly wiring plus the 4 way flashers and intermitent wiper system to go back to stock traceable wiring. I have headlights now after changing the column mounted switch, burnt wires and am waiting on a replacement for the headlight switch which was tempermental and a headlight relay that did not suit the wiring diagram.
Jerry mentioned 109 tooth flywheels for 6 volt. As a novice to details I am curious in the differences between 6 and twelve volt ring gears and then the differences in teeth in the bendex of 6 and 12 volt starters.
Possibly the last owner put in a rebuilt 12 volt unit that did not mesh with the 6 volt flywheel! I think he tried and gave up hence the car came up for sale.... He had also changed all the bonding straps looking for an answer.
I have had pricing from Rimco for supply only and would look at having a local machine shop rework the flywheel rather than pay hefty shipping charges from and back to Canada plus the taxes. I do have a source for balancing afterwards. That all said would I be better off with an aftermarket flywheel in the 450 dollar area plus taxes and shipping.
First I need to confirm teeth counts for both the starter and the ring gear and if necessary can the bendix on the overhauled starter that I have be changed to suit.
Thanks Ron

Fri Jun 29, 2012 7:12 pm

Mr. Henning:

As the current (amps) increase is a dividend of voltage/resistance relationship, the difference between the two is not appreciable (and certainly not "twice"). Thus, the additional stress from current is negligible and, in practice, is less owing to the greatly reduced cranking time to start.

In reality (in my case this is 30yr./100+K mi. worth) the combination of good bushings and 6v. starter, pinion, and ring gear with a 12v. solenoid is no shorter lived (or trouble prone) than an all-6v system.

Mr. 356ron:

If I am not mistaken (not unknown, however), there is one less tooth in the 12v. pinion. Not sure about the ring gear.

Fri Jun 29, 2012 8:53 pm

ensys wrote: As the current (amps) increase is a dividend of voltage/resistance relationship, the difference between the two is not appreciable (and certainly not "twice"). Thus, the additional stress from current is negligible . . .
Very good. You stated the concept of the voltage to current relationship accurately, but, sad to say, you then failed to follow through with the correct math.
The exact formula is "I=E/R" (as you stated above in so many words) or restated: "current is equal to the ratio of the voltage divided by the resistance". If the resistance is a fixed value (the 6v starter motor windings stayed the same), and if the voltage in the system is increased from 6v to 12v (a factor of 2x), then the current in the system is also increased by a factor of 2x. More important, the power consumed by this upgrade increases as a square of the voltage increase, or is 4x the power consumed over the 6v scenario. (power is equal to the voltage squared divided the resistance).
To make matters worse, we now have a 6v starter in a 12v system drawing twice the current, when a properly designed 12v starter on 12v would only be drawing HALF the current, means you are asking your poor little Optima battery to supply 4 TIMES as much current as it really should!
In my mind, I would hardly call that "negligable"!
Now, I'm not sayin' you can't do it, just sayin' you shouldn't do it. Kinda like pushing your engine shift point to 2000 RPM past it's rated redline . . .

Sat Jun 30, 2012 9:15 pm

Hot Dog! Math Fight!!!! Lemme grab my slide rule and some cream pies.

Mr. Henning:

Clearly, there is no moss on your calculator. Can't say the same for my brain, but let's press on.

While your math is correct, it is about potentials. In practice, the force required to spin the engine (the amount of work done) is the same whether powered by 6 or 12v.

If the amount of work is also constant, then the amperage becomes a variable when measuring consumption during the task and it should be about half or equal to the draw of a 12v starter.

My pie awaits......

Sun Jul 01, 2012 6:41 am

The gear mesh could indeed be your problem. The 6V VW/Porsche ring gear has 109 teeth. The 12V VW/912 ring gear has 130 teeth. The 12V starter SR17X that is used in all the 911/912/914 mates to the 130 tooth flywheel and will not work with the 109 tooth flywheel.

In '65 Porsche converted to 12V systems on the 356 and used a 12V starter that mated with the 109 tooth flywheel. The '68 VW 12V starter uses the 130 tooth flywheel.

The 6V starter can be used in a 12V system, but it does indeed turn very fast. It will not, however, chew up the flywheel unless it was already bad or there was insufficient mating of the bendix with the ring gear.

If the flywheel teeth are chewed up it is cheaper to buy a used or rebuilt flywheel than having the ring gear reworked.

SV

Sun Jul 01, 2012 9:37 am

Rimco charges $130 to R+R a ring gear.

Sun Jul 01, 2012 1:39 pm

ensys wrote: While your math is correct, it is about potentials. In practice, the force required to spin the engine (the amount of work done) is the same whether powered by 6 or 12v.

If the amount of work is also constant, then the amperage becomes a variable when measuring consumption during the task and it should be about half or equal to the draw of a 12v starter.

My pie awaits......
Incoming Pie Alert! :wink:
Tell me - does your 6v starter motor running on 12v seem to turn your engine over noticably faster? I would bet it does. Do you think it takes the starter motor the same amount of horsepower to spin the engine at a slower speed on 6v as it takes to spin the engine at a faster speed on 12v? It takes MORE HP to crank the engine at a higher speed. More HP means same motor is doing MORE WORK, producing more output, as a result of the higher input voltage. More HP, more power, faster cranking speed, HIGHER CURRENT, more internal heating, wear and tear on a starter operating outside its design limits. (But it does a better job at grinding down ring gears!)
In this case, there is no such thing as more for less.
Awaiting return volley . . . (make it crow if you think me wrong)

Sun Jul 01, 2012 10:22 pm

Mr. Henning:

Before I holler "uncle" or "duck", let us make sure we are on the same page about a couple things.

First, a 6v starter is built with heavier gauge wire (as is the rest of the harness) than those in a 12v system. This quality assures a lower resistance (and less heat) and that neither stator nor windings can be "overstressed" by an increase in voltage/amperage (tho one could make a case for increased end bushing loads).

It is also the case that the I=V/R calculation measures the capacity of a system, not the draw to do a given unit of work. So at the given resistance of the 6v motor, its capacity for current actually doubles under 12v.

Next, to produce a given unit of work (I=P/V,where P is the work in hp/watts, or spinning an engine), a DC motor running on 12v. will draw half the amps of one running on 6v.

So I hope you will understand that the source of my confusion is that the obvious conclusion here is that the 6v. starter is, in reality, understressed by a factor of what, half, by conversion to 12v.

Finally, the primary reason a 12v spins faster is because of the stronger magnet field created in the windings.

I think its time for "duck"......

Sun Jul 01, 2012 11:34 pm

ensys wrote: Mr. Henning:

Finally, the primary reason a 12v spins faster is because of the stronger magnet field created in the windings.

I think its time for "duck"...... It's been my limited experience (45+ years in the trenches) that a 6 volt starter, hooked up to 12 volts, spins much faster than a 12 volt starter, hooked up to 12 volts. Don't know how to 'splain it, but "quack quack" :lol:

Mon Jul 02, 2012 3:28 am

This is a most interesting thread indeed. Friendly technical banter which I really appreciate. I have kept for the past 43 years my 'A' on a 6v system. During those years i have experienced the slow Rrr,rrr,rrr, of the motor when the car was hot so changed the bell housing bushing. I have experienced the slow rrr,rrr, rrr when cold changed all the earth leads and checked all the brushes.

Then again, the battery used to go through batteries till I bought an 'accumate' battery conditioner which really helped things and I also made sure the fuel system pump and carbs are as near perfect as possible.
So even now sometimes the starter motor is slower on turn over than at other times when it almost equals a 12v normal car system. This I have never been able to sort out but the car always (well so far ) starts up.

I really liked the explanation of available items by SV. I have one question though. If you run a 6volt starter should the solinoid be changed to 12v or am I up the creek here. Can a solinoid with the wrong voltage 'slam' a gear against the ring gear instead of just pushing it against it. As I understand it if the starter gear comes up against the face of the flywheel instead of the tooth gap it will still turn the flywheel enough to engage into the next available gear form.

Sorry if this does not make much sense.

I will stay 6volt though and hope!

Roy

Mon Jul 02, 2012 3:33 pm

Well, Mr. Staggs, this one is easy to explain: as noted, the gauge of wire in the windings are larger in a 6v than a 12v starter, so it is the case that the magnetic field created is stronger in a 6v running on 12v than a 12v running on 12v.

Mr. Mawbey:

Yes, using a solenoid of incorrect voltage will cause all manner of problems, including too-rapid engagement, reduced chance of proper engagement, and, of course, ring gear damage.

On the other hand, the addition of a 12v solenoid to a 6v starter (and, of course, the normal 6v Bendix gear) will provide trouble-free service in a 12v system. I have used this combination in my Coupe driver for enough time (30yrs and over 100K mi,) and with the kind of starts one sees in a regular driver, to state with confidence that one has no reason to expect anything like abnormal wear to either starter or ring gear from this configuration. Just mind your bushings, as normal.

Mon Jul 02, 2012 10:06 pm

What I understand that you are saying is that a 6 volt starter powered by 12 volts spins slower than a 12 volt starter powered by 12 volt. Yes? no?

Mon Jul 02, 2012 10:20 pm

I should have been clearer.... The larger ga. windings provide a stronger magnetic field, thus spinning a 6v starter on 12v faster than a 12v on 12v.

Mon Jul 16, 2012 5:48 pm

Thanks to those that have given me info on my starter compatibility question. When I got home I took my starter out and counted my ring gear teeth. It has 109 teeth. I have been told that the 6 and 12 volt starters have the same number of teeth so is mine a 6 or 12 volt bendix? The electrics are 12 volt.
My starter is an AL/EEF 0.5/6L4
Thanks Ron :?: