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Thread: Donor Gearboxes

  1. #1
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    Donor Gearboxes

    So I’ve just finished building my first engine and have my eyes set on the next. I’m hoping to get most of the CAD work done over the winter and maybe start manufacturing next summer. So at the moment I’m just trying to source a suitable crankcase, gearbox and clutch so I can get the ball rolling when I run out of things to do on other projects.

    The plan is to use 4x Chinese YZ85 barrels and crankshafts with a custom lay shaft joining it all together. Hopefully it’ll make 100+hp

    So what kind of gearboxes do you think would be a good or bad idea?
    -I keep seeing cheap Hyosung GT650 motors which are rated at about 55nm and have 6 gears
    (Cheap but unsure about the quality)
    -I’ve also been looking for some 600cc bottom ends but everyone wants the earth for them
    (Too expensive and most have oil fed through the centre of the shafts, I’m unsure if I can delete this feature without sacrificing reliability)
    -I could use a 450 MX gearbox
    (Probably too fragile, plentiful and light weight, may need a bigger clutch, probably only 5 speeds)

    Here’s some pictures of the last engine.
    Click image for larger version. 

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  2. #2
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    24th April 2016 - 19:07
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    This sounds interesting... I'd be curious to see some sketches of the layout - v4/square//stepped?
    re gearboxes ebay is a happy hunting gound . recently i bought a late (ish) low k gsxr1000 gearbox complete with shift shaft, selector with guides and detent , bearings and even a front sprocket all for 139 usd. Gear ratios will be a deciding factor with those cylinders - a gsxr 600 gear set likely will be the ticket. peak torque values with your engine won't be too high - maybe a 400 gearbox would be ok, esp if you speed up the primary ratio ( and save a bit of wieght for the same horsepower capability).

  3. #3
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    I’ve found a few cheap gearboxes, the only downside is I need the cases too so I can get all of the shaft centres and other dimensions. The layout will be a cross between an rg500 and an rgv500, I don’t think the yz85 cranks will handle being geared together so they will all drive the layshaft rotating in the same direction which will drive the clutch. once I have the pieces I’ll be able to sort a 2D cad drawing and see how it will fit into a bike. I’m thinking 90 degree V and firing the cylinders in pairs.

    All I want to do is make the cases, 4x primary gears and a layshaft, nothing too ridiculous.

  4. #4
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    25th March 2004 - 17:22
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    I'd steer well clear of a Hyobang. There's a reason why you see so few of them on the road compared with just a few years ago.

    You'll probably have to spin it a bit faster to get that sort of power, probably peak at 12 at least. Top end parts should handle it fine as long as confidence in crank.

    600 clutch would handle the power.
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  5. #5
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    20th January 2010 - 14:41
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    TL or sv Suzuki.
    Suzuki always had the best boxs
    there was one on TM as while back complete with shifters for $125

    The other option is Ducati the 900/916 were all pretty much the same and cheap as well if you look around.
    you want something narrow, Across the frame 600s are not narrow.

    For the crank find yourself a two RD200 twin cranks
    they have lab seals they have a kind of odd girdle set up but far easier then trying to gear together 4 yz85 cranks
    they also have a spineless primary
    i have the dimensions somewhere.
    Click image for larger version. 

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    They run 20mm pins and commons sizes for the YZ's 14mm piston pin

    The other option is to do a single crank like a NSR500 they have more room than a twin crank and are a simpler set up.
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  6. #6
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    13th June 2010 - 17:47
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    Gearbox width Suzuki V twins vs across the frame fours is pretty much identical. I'd still look for a GSXR600. Even the early ones will work in your application.
    But in pretty much every 6 speed I've seen there's oil up the shafts. Where a gear in the center of the box runs on a bush on the shaft, you've got to lube it.
    A small oil pump driven off the back of the clutch wouldn't be hard to arrange. You could even semi dry sump the box. Oil level well below the spinning gears with pressure fed shafts and a spray bar.

  7. #7
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    24th April 2016 - 19:07
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    This project sounds really neat. a few years ago a nephew got a new KX85 and it was left with me on the farm for a while - what a neat engine .. It'll be a rocket. each crank having something like a 9 or 10mm wide gear meshing with a single 20ish mm wide layshaft gear would be a tidy and light setup. do the yz's have a bridged exhaust? the bottom cylinders might give the bridge a bit of a hard time being on the thrust side. 20:1 maybe

  8. #8
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    Just trying to visualise this whilst scoffing an éclair.

    So the layshaft would drive the primary clutch and the RH cranks would both drive it, so will need to be a fair size gear, then the LH cranks would pick up in the centre in same fashion? Then lop the tapers off both RH cranks which will make them a different weight.

    Will make it fairly wide. And fairly long even if you stack the layshaft. And have to drive cylinders backwards. Or have I got this wrong?
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  9. #9
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    5th December 2010 - 23:03
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    Well I’ve got a 1997 gsx600f gearbox lined up, so maybe that’s the ticket. $400 and on the other side of the country (Auckland) I’d rather see it first though.

    I’m thinking about stacking the gearbox to make it a bit shorter and lopping the tapers off the RHS cranks. I’ll only know the size once I order the Chinese crank and cylinder and have the gearbox in my possession. Surely it can’t be any bigger than an RG500

    There’s a shop down the road that will hobb and harden the gears for me from en36 at about $300-400 a gear or I can make them myself and have them harden them for me for about $40 each.

    I’m going to lost foam cast the cases again but pay a foundry to pour it this time.

  10. #10
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    I’m thinking a cross between these two set ups in terms of the crank gears. Except offsetting them slightly so I can run the nuts recessed inside.
    For the oil pump i have a bunch of Honda C50/pitbike ones floating around I can use as I want the sprocket side of the engine to be have a cassette style gearbox and I will drive it off one of the input shaft gears with a plastic/steel gear. Kind of like the last engine pictured.

    Click image for larger version. 

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  11. #11
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    gear spacings - years ago i measured lots of race bike gearboxes and noticed to get the best out of a higly strung 2 stroke 1st was pretty high and the rest had diminishing drops in rpm as you shifted up. if this bike is headed for manfeild then i reckon you might need to have first close to 50% of top gear and a gap no bigger than around 15% to second 12% to third and so on. i've previously found a motorbike gear ratio site that had all the gear ratio's of just about every sport bike made. could be worth a look. it shouldn't be too hard to find a set of cases to measure up if you went with something like a sports 600 gearset. I want to come and watch this race... do the std yz cranks have keys or splines on the drive end? $40 to case harden a gear sounds real good.

  12. #12
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    Quote Originally Posted by el chupacabra View Post
    I’m thinking a cross between these two set ups in terms of the crank gears. Except offsetting them slightly so I can run the nuts recessed inside.
    If the shafts are splined, just retain the gears with circlips. Least space needed. Yes, splining required but you can save a little $ by machining the gear blanks yourself. Any time spent at home is cheaper than having someone else do the whole job.

    Been there with close ratio conversion gearsets for Aermacchis. Last batch arranged by someone else - and roughly twice as expensive as the gearcutters did it all.

  13. #13
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    The late model yz85 cranks are splined, but the earlier ones have a single keyway. I guess I’ll be able to figure the total width out when I have the crank in my hand. They are only $140 each shipped for the early model cranks, so pretty cheap and I’m not too concerned about hacking them up.

    The plan will be to have it drop straight into my other RGV250 using the stock engine mounts as it will fit an Aprilia too and there might be a demand for something like this way down the track if the prototype isn’t too much hassle to sort out.

  14. #14
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    Thank you aliexpress Click image for larger version. 

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  15. #15
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    So you are going to run the cranks facing each other in 180 deg rotational symmetry. [Edit, should that be axial?] Its not so much the gearbox as the cranks you may have to stack and run the front counter rotating (forwards) to the rears (geared off) which would help thrust on lower as Jimmy alluded to.
    Also means easier on the brain for ignition if running a 4 way ignitech with two pulsar coils on one flywheel? Heck. We need power to generate enough for the ignition.

    I'm having the odd beer so my already diminished brain power is even more shakey compared to these other two real world experts. If they say something, Listen.
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