Si 2017 Si Transmission into 2020 Sport Hatch

mrburgerUS

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Hey everyone!

You may remember my 2020 Short block build, which I am detailing in another post. I am currently waiting on work to smooth out before I continue to spend money on that project. In the meantime though, I purchased a used 2017 Civic Si Sedan Transmission with 48k miles on it for $1550. The sensors on it are damaged, but the actual transmission is functionally sound and moves smoothly. I will be documenting what needs to be done to swap a Si trans into a 6spd Hatch, as well as my findings along the way! Hopefully this doesn’t bite me later, haha!
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mrburgerUS

mrburgerUS

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I brought the trans home, after sitting in a rebuilders warehouse for a few months it is filthy. Not touched since being ripped from a totaled Si. There is a chunk of reinforcement ribbing missing, but no visible damage to the actual casing. The reverse lockout solenoid, backup light sensor, and speed sensor were all destroyed, and I managed to get a slight discount for that. The speed sensor managed to crack into pieces, so I had to fish it out with some specialty electronics pliers I have from Hakko. The gears are in good shape best I can tell without removing the whole case, with only minor notching to the gears from the speed sensor. The notches are very shallow and don’t bind thankfully, and a friend noted they may even be for oil. I have disassembled all the damaged pieces and partially drained the trans (some of it leaked onto the carpet in our basement, but the carpet is commercial and needs replaced anyways). I’m guessing based on the color and smell this is the original Trans fluid, which is okay but definitely shows who owned this before. I am at work currently, but I took some photos of the trans.

Current Notes for Hatch Owners:

The Si 6 speed trans fits onto the L15BA engine no issue, but the gotchas for this install so far are:

  • The Si trans for 2020 will mess up speed readings if used because the final drive is 4.35, but for 2017-2019 the final drive is 4.10, same as the FK7.
  • Using a used transmission means there could be parts needing attention, but since low mileage trans are still basically unobtainium and a new one is $4000 from Honda even a rebuild will end up cheaper.
  • The Si LSD is splined differently. As mentioned in other posts, the Si passenger half-shaft and driver axle spline perfectly and work on the hatch without issue. I will get part numbers later and post them when I verify fitment.
  • An Si trans will ALWAYS have a see-through differential. Spinning one side of the splines should spin the opposite side splines in the opposite direction, as on my former 2015 Si. This is how a functional LSD should work.
  • There are 3 positions for both of the change levers on top of the trans. I watched an Acuity video of the engine bay to find this, and checking function can be a pain.
  • Reverse lockout solenoids are $160+ from Honda. My trans’ solenoid has the connector missing, so I will have to replace that. Avoid buying a trans with a broken lockout solenoid unless you can get a deal because replacements are expensive.
  • Replace the speed sensor. The plastic is extremely brittle, and my thinking is any accident will destroy it and it will most likely need to be fished out much like mine. Plus, an OEM one is $21 last I checked.
Pictures of course!
Honda Civic 10th gen 2017 Si Transmission into 2020 Sport Hatch 8DC174A6-E67C-491C-A3C3-4CF40FB96C04
Honda Civic 10th gen 2017 Si Transmission into 2020 Sport Hatch 6C90FA61-BB1F-4C42-BA18-FF8008718DA8
Honda Civic 10th gen 2017 Si Transmission into 2020 Sport Hatch F80D7BEA-B359-46E5-BF2E-1A57EA9D6CE4
Honda Civic 10th gen 2017 Si Transmission into 2020 Sport Hatch 05930E5F-01F3-4395-ACEC-F262657F0F6C
 
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mrburgerUS

mrburgerUS

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Split the casing over winter break, and boy was I surprised! For 48k miles, this thing is pristine. Problem is, putting it back together has been a serious problem. The casing seats about 90%, but the snap ring for the countershaft WILL not seat. I am afraid I damaged something internal, or possibly there is something clocked incorrectly. Personally, I am half-tempted to take it to a trans shop and have them kill me on cost and put the case back on. Anyways, here are pics of the gears, all in great condition.

Honda Civic 10th gen 2017 Si Transmission into 2020 Sport Hatch IMG_2744.JPG
 

Jjakef96

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Any updates? And any tips for being able to find si transmissions locally?
 
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mrburgerUS

mrburgerUS

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car-part.com is how I found mine. Be very careful about transmissions being sold as Si. some people badge ALL 6MT trans as “Si”. I have not made and progress as my vehicle had a pretty depressing accident in late January, and it just came back this month. I have purchased a couple things for this swap though,
 


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I'm not sure the comment about changing the final drive will affect your speedo. Different FD will change engine to diff ratio but the diff to wheel ratio is the same. Speed is calculated at the wheel speed sensor IIRC. Cruising RPM should be higher but speedo not affected.
 

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I'm not sure the comment about changing the final drive will affect your speedo. Different FD will change engine to diff ratio but the diff to wheel ratio is the same. Speed is calculated at the wheel speed sensor IIRC. Cruising RPM should be higher but speedo not affected.
yes u right.
OP everything is the same & you can keep your original as spares. the only diff is the FD + LSD which is a huge +.
 

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There was another user that swapped in the LSD by itself I believe. Not sure which route would be easier/cost effective.
 

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There was another user that swapped in the LSD by itself I believe. Not sure which route would be easier/cost effective.
You would need at least the Si final drive as well. The number of bolts/bolt pattern is different between the hatch and Si in terms of how the final drive bolts to the diff.
 


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mrburgerUS

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Yep! Been running it since late 2021 no issues. I ended up doing a Si trans and what you’ll need besides the trans is a driver side axle and a passenger half shaft. You can also do the passenger outboard axle because I believe it’s slightly more rugged. The LSD has a beefier axle and higher spline count. This makes the car about 16x better, scientifically.
 

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Yep! Been running it since late 2021 no issues. I ended up doing a Si trans and what you’ll need besides the trans is a driver side axle and a passenger half shaft. You can also do the passenger outboard axle because I believe it’s slightly more rugged. The LSD has a beefier axle and higher spline count. This makes the car about 16x better, scientifically.
You got more info like YouTube or Instagram I can check out for this build?
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