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Rear chain to belt conversion for pre 65 trannies


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  • Rear chain to belt conversion for pre 65 trannies

    This post is about converting rear drive chain to rear belt for pre 65 transmissions

    This ain't a chain vs belt discussion

    Some background

    In the early '90s, I had a rigid evo that I built with a pre '65 4 speed ratchet top tranny. I wanted a rear belt drive, and Karata was pretty much the only game in town that made complete bolt-on rear drives for about any HD that didn't originally come with one. I believe the kit was like $650 at the time for a BT with rigid frame, kick only

    For BT's, the Karata rigid kit I needed came with a front pulley, rear pulley, and the rear belt. It was an easy bolt-on. I had that setup on that rigid evo for a few years. Later on, I built an evo softail and used the same tranny and rear belt drive kit: It was a direct changeover and I NEVER had a problem with that rear belt conversion. I'm talking thousands of interstate miles, almost 500 miles one way in the summer heat and then dirt roads to parties

    So for something a little different:

    I wanted a rear belt drive kit for my current project, 60 pan in an OEM 1970 swingarm frame

    Understand, I have an open belt drive, pre 65 tranny with ratchet top, no inner primary or bearing support

    So I went to the Karata site, and while the site is up, they don't really sell rear belt drive kits anymore. Bottom line, they no longer sell the conversion kit I needed or parts for them anymore: I couldn't find any other belt drive companies that sell conversion kits anymore for older (pre 65) tranny setups

    So I said fuck it, I'll get one together, one part at a time

    The following is for a SWINGARM rear belt conversion (I'll go over a rigid frame conversion at the end)

    I based my conversion idea on the 1980-82 FXB Sturgis ("B" for belt). It was a dual belt (primary and rear) that HD tried out. It only lasted two years because the primary belt had a reported habit of failing (This was because of mostly home mechanic error by misadjusting the primary belt)

    After research, I found the FXB used an early style clutch basket, same as used on the older 4 speed (pre 65) ratchet tops, the only real dif being the main shaft on the FXB was longer for the use of an e start.

    So I'm thinking I'll just order the tranny pulley for an FXB, and it should fit just right on a pre 65 tranny main shaft as the splines are exactly the same. The hardest part to source was finding the 1981-1982 FXB trans pulley: V Twin had them at one time but quit stocking them. And no fucking where on the net had them. But I contacted a Jay, and he found some about a month later thru V Twin (They started stocking them again) I'll post part numbers/costs at the end

    So I get the trans pulley, cool: And it fits right on my ratchet top tranny, pre 65 main shaft perfect. Keep in mind the trans pulley is a 33 tooth

    So then my next question was, what tooth rear pulley should I run? I went to the Karata site and found a chart with the ratio info I needed: 33 front, 70 rear. So I go to ebay and get like a 90's era rear pulley, 70 tooth that will fit my rear rim (3/4 axle)

    Now what size belt?: 1 1/8" wide, 125 teeth, 14mm . It took me forever to figure out what belt size (tooth count) I needed. Now to find one: Found out it's the same as what's run on a 91-99 sportie: I found one on ebay, cool

    Since I am now running a 1970 swingarm frame (not a rigid) an extra step is involved: You have to have a swingarm that has a kick out on the left side that allows for both belt and rear pulley clearance: I found a used 1982 FXB SA on ebay: Fits my 70 frame perfect. Paugho makes a replica SA that will work, but I don't know the number right now.

    So I go to put everything together..........

    The trans pulley fits the main shaft perfect, BUT: Since the FXB pulley is like two inches wide,(the Karata version was only 1.5" wide) the bottom of the clutch basket hits the pulley: I was confused for a while and then figured that since this pulley was originally made for the longer e start main shaft (1981-82 FXB) that is why it's touching: I have a non-e start main shaft, which is shorter: SOLUTION: I cut the pulley back to 1-3/8" wide from the outboard side = perfect fit

    Rear-wheel spacing was easy. Just ensure that the belt tracks perfectly on both pulleys w/o running off. The belt doesn't have to run exactly center on both pulleys. Just be sure it doesn't run off the side of either or contacts any part of the frame. Triple check the alignment, then do it again just to be sure: It was an easy deal for me, no issues. Once you get the belt adjusted, it's fire and forget. Interesting note, my 1.5" wide belt fit just right running a tin inner and outer primary cover on an older build using a Pan motor

    So now I have a dual belt, kick only, swingarm panhead: I also found that the rear belt guard on all factory rear belt, 4 speed swingarm HD models bolts right on, and ebay has plenty of them (Got one for $30)

    Want to talk smooth?: Trust me, the ride is……

    Costs/Part numbers?

    33 Tooth 14MM front pulley: VT 20-0449 $130
    (1990's era 70 tooth rear pulley, 14mm EBAY $70
    Rear swingarm from 81-82 FXB $75 (ebay)

    Rear belt, 125 tooth, 14mm $160

    For those with a rigid frame:
    You will save $$ as you don't need a swingarm: If I remember correctly my old build in a rigid frame (no inner/outer primary, pre 65 tranny, kick only) had enough clearance for the rear belt and pulley in the Karata kit (1.5" wide): But the rear belt you will need for a rigid frame will need a rear belt will vary

    Hope this helps for those wanting a rear belt drive on their rides using older trannies

    Pics coming

  • #2
    Forgot to add
    Costs/Part numbers?

    33 Tooth 14MM front pulley: VT 20-0449 $130
    (1990's era 70 tooth rear pulley, 14mm EBAY $70
    Rear swingarm from 81-82 FXB $75 (ebay)
    Rear belt, 1 1/8" wide, 125 tooth, 14mm $160 (Used on 91-99 sporties)


    • #3
      Price is right and pretty painless.


      • #4
        Yeah: It was fairly painless, a plus

        Some pics

        Also, you can run a tin inner and outer primary with it
        Attached Files


        • #5
          Way cool mod. Can you post up a full side shot of the rigid?


          • #6
            I no longer have the rigid, sorry


            • #7
              stretched rigid frames with belt conversions are a dream compared to that long ass chain vibrating slapping and slinging oil everywhere!

              Utilized a Sturgis Shovelhead inner primary on my daily rigid.
              I had to run a static roller (skateboard wheel)to take up some of the slack of my overly long 1.5" belt

              I'm setting up a rear belt drive right now
              Shovelhead "drop seat" frame dimensions
              Panhead shoe strum swing arm.
              Alternator tin style inner primary
              5 speed with a kicker kit
              But using a Sportster front pulley (It's already narrow)
              A 66 tooth x 1" rear pulley
              and a pan head swingarm.
              Going to have to run a sprung "chain" tensioner to take up the slack of the ,too long , 125 tooth 1-in belt. (Cut down from an inch and an eighth spare that was hanging around)
              My coworker tells me there's a shorter belt available for a European model Sportster. But I have yet to find one in stock anywhere.
              Managed the stuff of 150 tire back there, and a disk brake, and a pulley.
              Had to turn down the built-in offset of the pulley,
              And run counter sunk bolts on both sides of the wheel.
              It's tight.


              • #8
                Interesting you can run a sportster tranny pulley on your 5 speed. Splines must be a match: No way it would work on a 4 speed? Its odd the 125 rear belt is too long: I beleive the reason is your choice of pulley size

                The Karata BT kit came with a 33 front, 70 rear: I ran that combo back when and it was highway and around town friendly on my 4 speed, which is what I based my current conversion on

                Since you are running a very small rear pulley (66), what is your front pully tooth count? This is possibly why your belt is too long. Or maybe your panhead swing arm is too short for the conversion?

                More importantly, let us know your front pulley tooth count: You need to get your front and rear pully ratio right for how you want to ride before you go any further: It would really suck if in 1st and 2nd you are lugging around, or at 50 mph you are running high rpms.

                Even though you are running a 5 speed, I have found they arent that much more giving in 5th for highway speeds. Running a 66 rear means you need a REALLY large tranny pulley to get your rpms right in all gears and its size might not clear the swingarm pivot point I dont know your tranny gear ratios, so not much more to add here)

                I don't believe anything less than a 125 tooth rear belt is out there (unless you have it custom made) I searched EVERYWHERE for a rear belt doing research for this belt and I don't remember seeing anything less than 125 Running a belt idler like you are doing should work though to take out the slack

                What type/make frame are you running?: If it is a SA build, you can get yourself a used SA for any Big twin SA bike up to year 1984 that came from the factory with a belt (It doesn't have to be a FXB) The SA should bolt right on to the frame and will really cut down on the work, no counter sinking, and a 125 belt should fit fine w/o an idler

                Kudos, great job so far, just consider that front to rear pulley ratio: It can get expensive getting belts and pulleys that wont work


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