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The text following is part of a series of articles written by John S. Henry on the restoration and maintenance of air-cooled Volkswagens. While his experience is exclusively with the Beetle, many of the techniques can be applied to other models.

This text is copyrighted and duplication, re-distribution or publication is prohibited without consent of the author.

Article: Using a 12volt Flywheel in a 6volt Transmission
Last updated: 10/26/01

Using a 12volt Flywheel in a 6volt Transmission

Symptom: You are trying to put together a drivetrain from 6 different "for parts" cars and the flywheel on the engine won't fit inside the bell housing in the tranny; you are trying to mate a 6 volt (pre '67) tranny to a 12 volt engine

For the most part, Beetle parts are very interchangeable year to year. Give up originality and for the mainstream years anyway, a fender is a fender, a seat is a seat, a door is pretty much a door; this is one big reason why the car was so successful and popular. But there are a few "snags", years where things a significantly different in form and or function. One such area in the drivetrain is the "6 volt" transmission. (Actually, in countries outside the US, the "12 volt" transmission was used on 6 volt, post '66 cars). In '67 on the US models, the transmission bellhousing was enlarged, the flywheel got bigger (from 109 teeth to 130 teeth) and the 12volt starter was added. This created an incompatibility between the "old" trannies and engines and the "new" ones.

But VW enthusiasts being as determined as they are, found ways to overcome that incompatibility, in particular when using the old, robust "6 volt" tranny with the newer and more powerful "12 volt" engine. All that was really needed was for the bellhousing to be enlarged and the brass bushing in the tranny for the starter nose to be replaced with a "12 volt" one. This allowed the 12 volt starter to be used in the 6 volt tranny.

I found myself making just this swap many years ago, and this is, by far, my all time favorite VW trick. And the story of how I happened upon it is just as good, so, get a cup of coffee and I will tell the story (as opposed to getting into some long winded technical "how to:" text). Read it, and you will know everything that you need to do this yourself.

My very first car was a '68 Beetle. I bought it after high school graduation, it was to be my "college" car. The problem was, I didn't really come from an "automotive" family. My parents thought that a car, no matter how old, was something that you bought, put gas in and bought tires for every few years, and that was it. But my brother and I were born engineers, and given a few tools, would attack anything.

Well into my 6 year "tour" of finer South Carolina colleges and universities, when I was attending Clemson University (which I did, by the way, eventually graduate from), my '68 started popping out of 4th gear. At first it would only do it under maximum load, like trying to accelerate uphill on the highway, but eventually it got so bad that it you couldn't even go straight on the highway without keeping a hand on the shifter. When I drove the 240 mi. "home" from Clemson to Charleston SC, I would wedge a notched-on-one-end 1 by 2 between the firewall and the shifter, just below the knob. Eventually I decided that I had to replace the tranny. I got a "used" unit at the junkyard for $100 (this was in about 1983) that the "salesman" said "yeah, it'll fit in your car". I read my John Muir and took my car down to the Charleston Air Force base "hobby shop".

[For those of you who haven't had the privilege of growing up an armed services "dependent", the auto hobby shop was a godsend. It was basically shop space that you could rent hourly or daily; they had a full range of tools, lifts, welders, and a machine shop to provide services. You could get an indoor, heated "bay" for about $3 a day, or a covered outside "carport" type bay for $.50 a day. In the outdoor bays, you could leave your car overnight for up to 2 weeks. They also had a paintbooth with compressor and guns for $8 a day. I spent a lot of time there.]

I opted for the $.50 deal since I really had no idea how long this was going to take and mom said she would come pick me up if I had to leave the car. I had pulled the engine quite a few times before and had gotten pretty good at it. I took my time and had the tranny out in couple more hours. I already knew that the tranny was a pre '68 and that I would have to swap the axles and axle tubes. Luckily I, tried to "mate up" the tranny to the engine outside of the car and it was then I found that the flywheel was too big for the tranny bell housing.

I panicked.

I called one of the two VW shops I had dealt with in the area, "Hanks Garage", because I knew he had lots of parts and could sell me a "smaller" flywheel (I presumed that was all I needed). I had mom come pick me up and drive me to Hank's. I told Hank why I needed the flywheel, and a smirk came across his face. He was one of these guys who you knew knew everything, you just had to pry it out of him a little at a time.

He went on to explain how the flywheel is made of a cast iron/steel and is much, much harder than the alloy of the tranny bellhousing. He said that all I needed to do was to mate the engine to the tranny as much as I could; just start the nuts on the bolts, and then use a big ratchet on the pulley crank on the other end of the engine and force the engine around so that the flywheel teeth "cut" the bellhousing. I should keep tightening the nuts a little at a time and then turn the pulley some more.

I was thrilled. Hank was in his 50s and had been working on VWs all of his life. As far fetched as it seemed, I had no doubt that what he said would work. And they had huge ratchet handles at the hobby shop. Just before I raced out of his shop, Hank called me back and sold me the brass bushing that I needed to put my 12 volt starter into the 6 volt tranny. Hank was a great guy.

I don't remember exactly what time of year it was, but it was one of the 10 1/2 "hot" months of the year in Charleston. I bolted up the engine to the tranny, got the right socket for the pulley bolt and a huge ratchet from the tool counter, and started working. I worked the engine around and around, tightened the bolts and worked some more. Once I started to perspire a little, I unbolted everything to see what progress I had made. I had etched a thin, barely 1/16" line around the inside of the bellhousing; there was a tiny pile of filings at the bottom. "No problem" I thought, I'm the last person in the world to quit something for lack of effort. I bolted up, turned, tightened, turned, tightened, turned more, tightened, and turned. I stopped, breathing hard, the chest area of my T shirt was soaked, people working in the stalls next to me were starting to stare, wondering exactly what I was doing. I unbolted the tranny. I had gone about 3/16", the pile of filings was only slightly bigger. I tried to imagine Hank doing this. He was one of those guys who always had a bulging, dirty plain T-shirt under an unbuttoned denim work shirt on which you just barely make out "Hank" over the pocket. There were as many beer cans laying around in his shop as there were old valve covers. It didn't make sense. I couldn't be doing anything wrong, but this was WORK, and Hank couldn't do this "all the time" like he said. I flopped down on the passenger seat inside the car. I gazed back at the missing back seat I had removed to uncouple the shift linkage.

That's when I saw the battery.

HEY! I always wanted to see a VW engine run "on the ground". I snatched the battery out of the car and set it beside the tranny. I propped the engine up on some wooden blocks. I took the heavy cables out of the car and wired the starter, then I used a piece of stray speaker wire to hit the solenoid. The bendix shot the gear all the way across the 3/4" or so gap between the engine and tranny and hooked about 3/16" worth of teeth on the flywheel. OK!, so all I need is some kind of gas can and a wire to the coil right? Grab an empty soda can from the trash, drain some gas into it, a piece of fuel line, some duct tape, prop that up, loosen the nuts a little so the flywheel spins freely- I hit the solenoid and the engine instantly started and came to an idle.

Somehow a beam of light from the heavens shot down right through the corrugated steel roof of the hobby shop and illuminated the little air-cooled. I beamed. The people wrenching nearby who were staring now put down their tools and walked over. I reached down and goosed the throttle arm once. The engine practically stood up on one head (with the tranny attached). "Ok, don't goose the throttle on an engine running, sitting on the ground" I thought. I tweaked up the idle screw a few whole turns and cautiously tightened the nuts a little. A grinding noise started but the engine hardly slowed down. I took this time to explain the Chevyheads that had wandered over that I was pre-stressing the bellhousing to accommodate a "high performance" clutch setup; a common practice with VW enthusiasts. They asked me if I raced the car. I pretended not to hear and leaned over to tighten the nuts a little more as the grinding had subsided. Within 5 minutes or so, I had the nuts completely tight, the tranny mated snugly to the engine, still running, no grinding noise. I put a foot firmly on the right head and gave the carb a little tweak and pulled the wire from the coil.

I separated the engine and tranny. There were lots of filings in the bellhousing. I got a rag and cleaned everything out really good. I also cleaned up the flywheel (from which I had removed the clutch plate and disk) really good. I looked closely at the teeth on the flywheel expecting to see some damage but they looked untouched save for a stray filing or 2 lodged between the teeth. I blew out the whole thing with compressed air and I think I replaced the rear main seal and gland nut and put a new clutch disk in while I was in there. I tapped the new starter bushing in.

In a couple of hours, I was driving away from the auto hobby shop, and going straight to Hank's place. All the way there I couldn't wait to give HIM some information. I pulled into the dirt parking lot, I saw him look up and do a double take when he saw it was me. He actually strolled outside to meet me like he knew I had something to tell him.

I told a friend back home that story sometime in the late 80s and learned, sadly, that Hank had taken his own life in the last year and the garage was closed.

Copyright© 1998; John S. Henry

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