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  1. #1
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    Default 21 Tooth Front Sprocket???

    Slowly getting my 1969 T-120 Bonneville built. Looking for opinions or thoughts??? Anyone got some ride time on a 650 with 21 tooth front sprocket? Like how did it accelerate, pull, etc? Did you like it or hate it? Rear sprocket is stock number of teeth. Bike is set up bobber style. I weigh in at whole 130 lbs. myself and where I will ride the most is down in South Georgia so roads are pretty much flat, not many hills out on the highway. Yea I know "just try it man" and it may be fine for you, each to his own. Thanks

  2. #2
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    i've got a '72 650 (5 speed) with 21t front and 43t rear. LOVE it. Great highway cruising gear, and with the 5 speed, still gets out of the hole pretty well.

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    your rear sprocket is probably a 46, so you could also put a pre unit 43 tooth drum/sprocket on the rear and make it a better freeway bike.

    for every one tooth of the drive sprocket it's the same as 2.1 on the rear sprocket.
    Last edited by Torch; 12-04-2015 at 8:09 AM.

  4. #4
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    well, i'm running a 21 front on my own 72 5-speed T120, but i have a morgo on it. works great.

    using the stock 47-tooth rear sprocket and the old 19-tooth gearbox sprocket, i would run about 64 mph at 4000 rpm. just changing to the 21 gives me the same speed at around 3700. doesn't sound like a lot, but it's significant. initial acceleration is just fine if you use more rpm to avoid lugging the motor, but also that first gear is much more useable now in ordinary riding.

    this gears it for 120mph at 7000.

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    Good luck with longevity running any of these motors at sustained HI RPMS, many people just assume you can rev the shit of these old britt bikes like its a Honda or something. The factory race teams figured the stock alloy connecting rods were shot, or worn out after 1 500 mile race. Alloy rods are not well known for long term health. Yet i see people taking old motors, not cleaning out the sludge traps, and then firing them up and running the piss out of them. GRENADES! Keep em bleow 5000 RPM unless ready for serious engine building.
    On most stock brit bike restorations I use a 20 tooth with the stock rear sprockets (For 1970 and earlier). If you are running a 16" then definately a 21 or 22 tooth. It all depends as well on tire profile size. The old high profile 16" was actually only 1" difference than a regular stock 18" if you measured. (Measure from Axle center to ground, and compare, its the tire height or profile that also has an impact)
    On most bikes such as Unit Triumph, you need to carefully notch the hole behind the clutch basket to allow removal and replacement of the primary sprocket and more importantly the CS oil seal. Sealing up the trapdoor then requires some skill. I can email you a picture of how that works or looks if you want. I have run up to a 23 tooth CS on some chops. Early rear drums on unit triumphs have the sprocket cast in, later have a bolt on sprocket. And then there is options on the rears for Bolt on, or adapt a custom sprocket from something else. There are several online calculators for figuring out your ratios. BSA was the king of gear ratios with a wide variety of gear box alternative ratios, Primary ratios, CS and final drive ratios. Dont assume your stock box is the same as all others. There was a surprising number of gearbox choices, and keep in mind on Triumphs a 5 speed is still 1:1, not an overdrive. 5 speed allow more gears to stay in the power band. 5 speeds are more brittle than a 4. For comparison look at how Harley gears the 883 different than a 1200cc, and Buell the XB9 vs the gearing on a XB12. There IS a reason.

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    I agree Grenaded a few

  7. #7
    SteveCox
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    Sir,
    Any chance you still have a sketch of the crankcase mod required to fit a 21t sprocket through the access port? Gotta be easier than splitting the cases ...
    Yours hopefully
    S

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    Quote Originally Posted by SteveCox View Post
    Sir,
    Any chance you still have a sketch of the crankcase mod required to fit a 21t sprocket through the access port? Gotta be easier than splitting the cases ...
    Yours hopefully
    S
    You dont have to split the cases, just remove the gearbox and slide the sprocket on that way if you dont want to notch the access hole.

  9. #9
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    Quote Originally Posted by datadavid View Post
    You dont have to split the cases, just remove the gearbox and slide the sprocket on that way if you dont want to notch the access hole.
    Thats BS,,, Its a serious PIA to pull the gearbox cluster,. these are not cassette style like a 500 unit, BSA A65 or Sportster. Even though I am experienced at gearboxs on trumpets, I find it a PIA and it can take a lot of time to index them correctly, get everything stuffed back in, and then find out it leaks. Pulling the gearbox cluster is a major PIA.

    As to the Diagram, I can post a pix, possibly tomorrow. have done a ton of them, and if done right no issues. Basically what I do i slid a CS gear over the main shaft, take a sharpy and trace around it and do it in such a way you dont take out any of the screw holes for the trapdoor,. Then use a angle grinder with a cutting/grinding disc, and then die grinder with a carbide, followed by a grinding stone to get the saw tooth pattern.

    On stock - restorations I generally run a 20 tooth, 21 on chops and have up to a 23 for a guy who did long road trips. Keep in mind clearance as well in the chain cavity in the case, and a case saver is good to protect the case, (Metal strip). A tensioner on the frame whether a tensioned roller skate wheel, or jack screw and skid pad to prevent chain whip.

    Its a holiday here so, busy with family so will snap a few photos hopefully tomorrow for you.

  10. #10
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    Quote Originally Posted by Dougtheinternetannoyance123 View Post
    Thats BS,,, Its a serious PIA to pull the gearbox cluster,. these are not cassette style like a 500 unit, BSA A65 or Sportster. Even though I am experienced at gearboxs on trumpets, I find it a PIA and it can take a lot of time to index them correctly, get everything stuffed back in, and then find out it leaks. Pulling the gearbox cluster is a major PIA.

    As to the Diagram, I can post a pix, possibly tomorrow. have done a ton of them, and if done right no issues. Basically what I do i slid a CS gear over the main shaft, take a sharpy and trace around it and do it in such a way you dont take out any of the screw holes for the trapdoor,. Then use a angle grinder with a cutting/grinding disc, and then die grinder with a carbide, followed by a grinding stone to get the saw tooth pattern.

    On stock - restorations I generally run a 20 tooth, 21 on chops and have up to a 23 for a guy who did long road trips. Keep in mind clearance as well in the chain cavity in the case, and a case saver is good to protect the case, (Metal strip). A tensioner on the frame whether a tensioned roller skate wheel, or jack screw and skid pad to prevent chain whip.

    Its a holiday here so, busy with family so will snap a few photos hopefully tomorrow for you.
    So you would rather split the crankcase, is that what youre saying Doug? Even if that doesn't help one bit?
    And experience doesnt always=skill, manual dexterity is necessary too.

    Anyway, i dont think the OP will do neither.
    Of course its easier to just notch the fucking opening, i just dont see anything like that is going to happen. He will be happy with a 20t sprocket. He is probably capable of installing that without needing diapers.
    Last edited by datadavid; 11-27-2021 at 11:43 PM.

  11. #11
    reneamcmahangm1410
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    every one tooth of the drive sprocket it's the same as 2.1 on the rear sprocket snow rider 3d

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