Sprite Race Gearbox Rebuild

Discussion in 'Classic Mini' started by Minidave, Mar 18, 2021.

  1. Minidave

    Minidave Well-Known Member
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    Dec 22, 2009
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    The owner brought this to me last November, saying it was his backup gearbox, so "no hurry". He knew I would be down for a while with my knee surgery.

    Since I'm kinda stuck for parts on my two engine builds I thought I would go ahead and get this knocked out. The tag on the gearbox says "broken third gear" and boy they weren't kidding.

    When you haven't done one of these for a while you forget all the little tricks you learned on the last one, so you have to learn them all over again - the hard way. This one proved no exception - I forgot you have to put it in 2nd gear before you try and take the mainshaft assembly out thru the back of the case or it fouls the reverse gear. Guess what I forgot to do?

    Ok, so here's what it looked like when I took the cover off....

    IMG_20210316_171247R.jpg

    Yep, that's third gear missing almost all of its teeth. The layshaft fared a little better, only losing about 1/3 of it's teeth....I'm going to replace all the gears with straight cut rather than helical - these gearboxes were built for an engine making maybe 60 hp, they're now pushing close to 3 times that much!

    I started by taking off the shift housing and tailshaft assembly....the red rag is just there to sop up oil.

    IMG_20210316_170040R.jpg

    IMG_20210316_170855R.jpg

    I made a special tool to remove and install the bushing in the tail housing, but I can't find a reamer to make it to fit the shaft, so I don't replace them any more - they don't seem to wear badly anyway. I do replace the rear seal.

    Today, I managed to get the rest of the gearset out of the case. First you remove all the shift rods and forks - being careful not to lose any of the detent balls or pins. Then you drive the input shaft and gear forward - it won't come out of the case this way but you have to get the snap ring retainer off. Then you drop the laygear down into the bottom of the case by pulling the shaft out of it. Next - if you've remembered to put it in 2nd gear (which I didn't) - you remove the mainshaft assembly out the rear of the case in one big chunk. Then drive the input shaft back into the case till it's free, and lift the laygear out. The reverse gear shaft is held in with a threaded pin, after you remove that the shaft slides right out and the gear comes out easily.

    Once everything is out you can see the real carnage....

    IMG_20210318_124542R.jpg
    Now, the REAL fun part of the job, removing the 2nd and 3rd gears off the mainshaft. To do this you have to press in a spring loaded pin, rotate the lockplate to match the splines on the shaft, then slide the lockplate up while not sending the pin into orbit never to be seen again. Installation is the reverse as they say - BUT - it is one bitch kitty to do! The other gear has not one but two pins!

    IMG_20210318_124538R.jpg
    Maybe after lunch I'll tackle that......
     
  2. ScottinBend

    ScottinBend Space Cowboy
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    May 4, 2009
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    You have to wonder how they came up with these convoluted ways of assembly.....
     
  3. Minidave

    Minidave Well-Known Member
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    10lbs of crap in a 5 lb box? :)
     
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  4. Minidave

    Minidave Well-Known Member
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    Ran into a snag on this gearbox.....turns out it may be either a Morris Minor box or has the guts out of a smooth case Sprite box in it. The mainshafts are completely different, and the gears are smaller too - probably why it broke.

    So, we're stopped for a while.

    Clancy says he has three "cores" in his garage and we can get what we need from one of them or simply rebuild one of them for Bugeye Dave's car, but he's out of town till Monday or Tuesday.....

    In the first pic you can see the pin I have to push in, so I can rotate the plate with the tiny hole it it till the splines line up, then I have to carefully lift that plate while not shooting the pin out from under it off into outer space. Once that's all out I can slide the gear off the roller bearings.......except this one has brass bushings! RuhRoh!

    IMG_20210319_121945R.jpg
    IMG_20210319_151828R.jpg
    IMG_20210319_152004R.jpg

    Then when I compare the gears the inner diameter on the bad gear it's quite a bit smaller than the new ones - that are made to run on roller bearings. So - we need a completely different mainshaft - I'm hoping the case is the same for either one - and I think it is. Anyway you cut it I get to strip down another gearbox......

    Wait till you see the fun shenigans I get to have to get 2nd gear off the mainshaft - there are two pins, a top plate and two small plates under that! See the last pic.....

    late gearbox.png
     
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  5. Minidave

    Minidave Well-Known Member
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    Clancy got me another gearbox, so I stripped it down and now have the right mainshaft to build Bugeye Dave's gearbox. More as it happens.....
     
  6. Minidave

    Minidave Well-Known Member
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    Ok, so here's the difference between a smoothcase or Morris Minor gearbox and the later ribcase - BugeyeDave bought this transmission from Vicky Brits already rebuilt over 25 years ago. At that time it was just a street driven car and didn't become a full on racecar till much later in it's life - that's when it blew third gear to bits! So either they put smoothcase stuff in this ribcase case - which was not uncommon way back when as the case is a lot stronger too - or like I said, it's a MM transmission.

    So Clancy had dropped of another transmission earlier for me to rebuild but frankly I though the case was suspect - it had clearly broken and been rewelded and while it looks fine, I don't know that I would trust it to be straight and true. We decided to rob the mainshaft from it to use in BugeyeDaves trans, so I stripped down the new mainshaft so I could change the gears from helical to straight cut.

    In the first pic you can see it uses the same pin and plate arrangement as the earlier gearboxes, but when you get that plate off you can see a row of needle bearings - that's the big difference.

    IMG_20210320_101016R.jpg
    IMG_20210320_101605R.jpg

    That and there is a boss machined in the later mainshaft for the 2nd and 3rd gears to ride against that the earlier trans doesn't have, instead it relies on a brass plate between the gears.

    IMG_20210320_103120R.jpg

    Once 3rd gear is off, I flip the shaft over and take out the harder plate - it has two pins, and some small half moon plates that ride under the pins and fit inside the plate on the top.

    IMG_20210320_102548R.jpg
    IMG_20210320_102558R.jpg
    It's a lot of bits to juggle around when it goes back together - but I've developed a secret way to get them off and on now. I use some assembly lube to hold the needle bearings in place on the shaft while I carefully slide the gear down over them.

    Then there's a pic of the mainshaft to show the assembly all done.

    IMG_20210320_133718R.jpg
    Last pic shows the input shaft and laygear installed, however, to put the mainshaft in the case you have to pull the layshaft out of the laygear and let it drop down into the case so the mainshaft gears will clear those on the laygear. Once the mainshaft is in place then you raise the laygear back up and slide the layshaft into it, making sure not to drop the end washers out of place, cause they can bee a bugger to get back in with all the gears in place.

    IMG_20210320_144705R.jpg

    After all that the next phase is to get all the detents and springs in place - I counted 5 pins and 2 balls that have to be put in the exact right place for them all to work correctly. Once those are all done, I'll put the tailhousing on and shim the rear bearing, then install the shift linkage and lastly I have to shim the front cover bearing too. Before I put the sidecover on I have some plates I made to hold the detents in the case while I shift it thru the gears to make sure they all work. If they do the side cover goes on and it's done and ready to race. Still a ways to go on this one.
     
  7. Minidave

    Minidave Well-Known Member
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    Much better, this one won't break unless he does something wrong....

    IMG_20210320_164137R.jpg
     
  8. Crashton

    Crashton Club Coordinator

    Jun 4, 2009
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    What a lovely set of straight cut gears. This tread reminds me of building my last Spidget transmission. Make sure the detent balls are all there & in the right place. Oh man that one just about killed me. :Dead:
     
  9. Minidave

    Minidave Well-Known Member
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    Yeah, they're pretty easy once you remember where they all go!

    I have all the forks in now, ready to shim the rear cover, then add the shifter housing.....should have it done this afternoon sometime.
     

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