jackshaft help

heintz54449 · 02-01-2009, 08:11 PM

i need ur help.
the mini i picked up yesterday (i am new to minis) came mostly in pieces. the the jack shaft measures out to 9/16. I know nothing about these so bear with me. one sprkt is 12t, the other is 24t. the bearings are non existent. this is were i get lost.http://www.oldminibikes.com/forum/im...s/confused.gif the holes in the frame are 1 3/8 inch. I have been looking at kits and cant figure out what holds every thing together. I realize the shaft diameter dosnt mater if I get a kit. how do I know the bearings r going to fit in the wholes with out any slop. also i havent seen any kits with a sprocket as big as 24t. if i go with a smaller sprocket with the bike be faster or slower? this is for a 6 year old so i dont need a rocket at this point. thanks for any info!

dean heintz

**125ccCrazy** · 02-01-2009, 09:58 PM

standard jackshaft and wheel bearings are 5/8 i.d and 1 3/8" o.d so they will fit your application..

I have never seen a 9/16 jackshaft but thats not to say there were some wierd stuff made over the years..

If you go smaller than 24 on the wheel side of the jackshaft you will have faster take off and less top speed..

If your gears are actually 9/16 i.d you may be able to drill them out with a nice new 5/8 drill in a drillpress on slow speed and then file the key slot deeper with a good bastard file..

ogygopsis · 02-02-2009, 12:00 AM

Dean, Buy yourself a couple of bearings like crazy is talking about 5/8 id x 1 3/8 od. You want the kind with a "snap ring" around the outside of the bearing. That is what keeps it in place once you install the jackshaft. Don't even attempt to drill your sprockets. Most likely they have been hardened and you will just burning up a bunch of 5/8" drill bits which money would have been better spent buying a couple of 5/8" id sprockets. The sprockets can be ground out to 5/8" id but that too would cost more than buying new ones. Ogy

**GTO** · 02-02-2009, 12:26 AM

The shaft should be 5/8" and you can get a coplete kit for $19.99

**steven Durham** · 02-02-2009, 01:04 AM

Dean : With a 6 year old using this mini bike what you need to do is know not guess what your ratio is. To do that you need to give the two fixed numbers the number of teeth on the clutch and the number of teeth on the rear wheel sprocket. From that point you can select the two sprockets on the jack shaft to get a limiting ratio for such a young rider. A kit may not be the best choice for you.
And if you are not using a micrometer or a caliper to measure you existing jack shaft you are just guessing. I would say your shaft is 5/8 inch and the snap ring type of bearings will fit your mini bike. You can buy the parts cheap enough outside a kit and select what will work for you best. Feed us back some information on your rear wheel and clutch sprocket and then a good set of ratio can be worked as required. Also Welcome to the OMB forum. Steve

**minibikedude** · 02-02-2009, 12:36 PM

If you don,t have a micrometer or caliper. Just use a 5/8 open end wrench and slide it over the shaft it is snug it is 5/8. If you are measuring were the bearings sit on the shaft it is worn down for the bearing seizing up causing it to read 9/16. I bought a bike and the shaft was wore down to almost 1/2 were the bearing sets in place caused from a seized up bearing.

heintz54449 · 02-02-2009, 06:20 PM

first, thanks for all the info! I did some digging and found some pix of jack shaft placement and that helped me understand things much more

as for the fixed gears the clutch is a 12, and i thought i counted 71 at the wheel. but i may have missed one.

**steven Durham** · 02-03-2009, 12:42 AM

Dean : I would recommend using a 18 tooth sprocket on the jack shaft from the clutch's 12 tooth. And going to a 12 tooth on the jack shaft on the wheel connection side. That will give the mini bike lots of power and less top speed .
But you need to try and set up a throttle stop so the mini can't be run wide open . One thing I did not factor in is the wheel size I assume the wheels are 5 or 6 inch if the wheels are larger this ratio would not be that good but most 6 year old riders would not fit a mini bike that tall.
Steve

heintz54449 · 02-03-2009, 08:05 AM

thanks for the info steven. now I have someplace to start. as far as the thrittle stop, i have had the same thoughts. im not sure what size tires are on it either, and we are in another cold snap so it will be a few days before I get out to puttz on it. as for his height, he is tall for his age so i think he should fit with out to many issues. thanks again
dean