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Discussion Starter · #1 · (Edited)
Original Post

Okay, I'm in the middle of removing my D2A4 transmission so I can swap in the H2A5 gearsets. I'm not breaking the shafts down, just going to do a straight swap of them and see how I like it. I've already got the donor H2A5 from the local Pick 'n Pull. What I'd like to know is if anyone here wants pictures of this procedure before I get too far into it? I've already taken care of some of the smaller stuff like the starter motor, speed sensor and clutch cable bracket etc. I can get pictures of those too when I'm putting it all back together. I'm also going to do the clutch while I'm at it if anyone wants pictures of that too.

I've also got a question for anyone out there who knows the answer. The Helm manual indicates that the transmission and mount hardware is supposed to be replaced. Is this really necessary? Is there something special about these bolts that you can't reuse them? Anyway, let me know what you all want to see and I'll try to grab it. By the way, this is my first time doing something like this so any advice would be welcome also.

21 July 2014 Update

Fellow Luders it's been a long time since I've visited. I dropped out of the scene because I sold the car I had put all that work into. I needed something more reliable at the time I sold, but I wish I'd been able to keep it. I checked out the guide that I'd made and noticed a lot of broken links for the pictures. I repaired the links, so the guide will be as useful as the day it was made. Hopefully a lot of you will still find it useful.

I never did achieve the gas mileage I'd been hoping for. There was some small improvement, but not the huge gains I'd thought I'd get. The car never got good gas mileage to begin with, which leads me to believe that there were problems with the car that prevented this. Should I ever get another 3rd gen Prelude, I will do this swap again and let you all know how it goes. Cheers!

Edit for the write-up. 11 Jan 2013 Update

I'm bringing all the pertinent information from this thread into the first post so as to make it very easy to access. Consequently, there's going to be a lot of double posts of pictures that are further down the thread because they're already there. Any discussion in this thread will continue at the back end, but all pertinent information concerning this gear swap and the results of it will be reposted to this first post regardless. Most of it is going to be copy/paste, but I'll make changes where appropriate. There's a lot of good, worthwhile information that was made available by a lot of people, so reading through the thread is still recommended, but all the essentials have been brought up to the front.

Update News:
Adding mileage numbers. This was a long overdue update. My gains are not as substantial as I had hoped for. My worst numbers with the current gear set is still better than the best mileage I ever got before doing this swap, but they're still not as good as I was hoping for. It is winter time. As far as I understand it, cars do not get as good mileage during the winter than in the warmer months. But it's also darker, so I use my headlights more. I have to wonder how much those pop-up lights may or may not affect mileage because of the amount of drag they produce when turned on.

I'm wondering if I should have gone with Prelude FD after all? It would only affect RPMs to a minor extent, but that slightly higher RPM might be what this particular motor needs to maximize efficiency and economy. Anyone is welcome to chime in with their two cents on these ideas.

Comments on Swap

I've been putting the car through its paces and I LOVE it! I was cruising down the interstate at 70 mph and the tach was reading exactly 3000 rpm. The more I drive it, the more I think I really can't tell the difference in acceleration, but then again I don't push my car that hard. Of course, 5th gear is going to be the most noticeable difference in regard to acceleration. But so far I really enjoy the new gear ratios. Some people might notice the difference, and in that if you're that worried about it then you should keep the Prelude FD. Otherwise, I think most people would be perfectly satisfied swapping all the internals over, mainshaft, countershaft, and differential as complete units, and keeping this process as simple as possible.

I confirmed the speedometer with a GPS and it is accurate throughout the entire gear range. You can use your old speed sensor just fine without having to swap for the Accord part.

As I have more information to add, I will make normal posts and then copy that information to this post immediately afterward. Enjoy! :smilejap:

Real World Results

I was cruising down a county road at 55mph in 5th gear. The tach was reading just about exactly 2375 RPM. The lighting was super low, but the trip odometer was reading 125 miles.

The fuel gauge reading doesn't mean too much yet. Every Honda from this era that I ever drove has a fuel gauge that reads super high for a long time and then drops down to 1/4 in the space of 50 miles.


You really can't see the big picture for a few tanks, because so much can change in one tank. Where and how you drove, weather, etc.
Like Andrew says, one tank won't tell the whole story. I'll put at least three up here and average them once I have the numbers to do so. I've been trying to drive like I normally do, which you can tell from the picture is fairly conservative. I push the car when needed, but I'm usually content to cruise at the speed limit. Numbers will definitely vary based on driving habits!

For now, I've got numbers from one complete tank of gas. I will share my average mileage from before and My new mileage. I've only had one tank to measure so far, so the number could go up or down with averaging. In the near future I will also make an pure highway miles run with an average speed of 70 mph. I will add those numbers once I have them

Average Mileage Before Swap: 24-26 MPG

New Mileage After Swap:

1st tank: 29 mpg
2nd tank : 28 mpg
3rd tank (half): 29 mpg
Freeway only (first run): 30.5 mpg

Best so far: 30.5 mpg
Worst so far: 27 mpg

These are not fantastic gains, in my opinion, but they are gains. But the increased mileage, when combined with the lower RPMs overall, make me very happy with the result and I'm happy I did this mod

Comparing of Gear Ratios

This was part of a list I made. I had collected some numbers and was trying to figure out how I wanted to do this swap. I determined that I wanted max MPG, so I chose to swap with the H2A5. 1st and 2nd gears are identical on these transmissions, so any swapping that would be of benefit would have to come from swapping gears 3-5. Swapping FD will affect how all the gears feel, including 1st and 2nd, regardless of the fact that those gears are identical between the two transmissions.

Comparing the Housings of the H2A5 and D2A4 Transmissions

Once I had the transmission out of my car, I made observations between the two transmissions. I completely busted the myth that you can directly bolt up an H2A5 to a B20/21 motor. It can't be done. Another good point was noted by Sciroccoboi16v:

Looks like the diff is mounted higher up on the d2a4 as well. Likely to keep good alignment despite our engine tilt. Putting the other one on would be bad for axle align.
Look at the pictures and see for yourself. There is no straight bolt-up solution for the B20/21 motors.











Tearing into the transmission and making the swap!
This is a direct copy/paste from the original post. Any editing I make, if any, will be minor.

Alright, the moment you've all been waiting for! I've got a lot of pictures for the complete breakdown and swap. I ended up swapping both complete shafts and the differential. I've made notes on many of the pictures. I'll give more details below the pictures where needed. And without further ado...




The manual makes this look super hard, but you don't have to touch this part of the assembly except for the four bolts holding it on.






There's two pieces to the highlighted portion, the shifter arm and the interlocker. I put them both right back on the rod to ensure these pieces didn't get mixed up.


You can see here where the shifter arm interacts with your shift forks.







I'll get an image up with the torque specs and bolt pattern when I do the rebuild write-up.




Don't sweat this part. It's easy to get the snap ring off and it's easy to get it back on. The ring sits in its own little groove and will stay put when you remove the housing. During the rebuild write-up I'll explain how to get the ring back on. It's surprisingly easy.


Don't lose those bearings! Tip the housing to the side and catch them as they roll out. Take the bolts out one at a time to save on stress. These look like they have to do with the detents on the shifter forks to hold your gear setting while driving.


Once you've got all that removed then you should be ready to lift the housing off. Hondabond is some really strong stuff! There's a few points around the perimeter where you car pry up on the housing. Be gentle with this and take your time. You don't want to mar any of the mating surfaces while doing this. Once the snap ring slips off the end of the bearing on the countershaft, the whole thing will lift off.

It's probably a good time to mention that you're going to want some plastic bags or saran wrap to cover your components and guard them from dust and dirt as much as possible. If you're going to use bags, make sure to use the black 50 gallon Hefty bags or something like it for the bigger components. Those gearsets are HEAVY!



Put this in a plactic bag


With a little wiggling this lifts right out. Put this in a plastic bag.


It's embarrassing, but it could happen to anyone, especially if this is your first time doing this. Do NOT pull from the bearing, or you'll be sorry! I couldn't find all the ball bearings so I had to swap in the bearing from my D2A4 shafts. They were the exact same part, with the same part number, so no worries.


I'm actually kind of glad the bearing came off because it was very hard to tell these pieces apart otherwise. Both shafts have the same bearings on top, and both splines are the same. The picture is a little distortorted at the bottom, but the input shafts are both the same size at 24 splines each.


Same story as the mainshafts. If you look closely, you can actually see the difference in gear size for 5th gear on the top of both shafts!


This was the hot button topic, and I know to some of you may think I have committed heresy, but I swapped out the differentials completely. Once the shafts are out, the differential lifts right out. I counted the teeth on the carrier gears, and they were both the same at 23 teeth. That told me that the speed sensor would work just fine with this setup. So, in the spirit of keeping things simple, I swapped out differentials. Love me or hate me for it, but that's what I did :)!







These are seriously easy to miss!


I thought for a minute that this mod wouldn't work because the gears were locked up. But upon further inspection I found that the synchro sleeves weren't in their neutral positions. I moved them to where they needed to go, and the shafts started turning.

And 88SE Lude is right about the fact that the courntershaft output gear and the ring gear on the differential are integral to each other for FD. If you move just the shafts without the ring gear you will feel the resistance between the gears. They will go together, but they will NOT be happy to do it. Match the countershaft to the ring gear for proper results!

And that concludes this portion of the write up. I tried to make things as clear and as simple as I could. I also chose the easiest possible way to do this sort of swap, which happened to be the most extreme way to do this swap in terms of changing your gearing. Other options means breaking your gear shafts down. I plan to do a little write up on that for things to watch out for when breaking them down and building back up. There's still the rebuild to do and I've got to get the transmission back into the car and drive it, but to all appearances this looks like it's going to be a successful swap. Enjoy!

Putting the transmission back together
Also a direct copy/paste with some minor editing, if any

Here's the second part of the gearswap: the rebulid! Things went well mostly. I was able to resolve all the problems that came up and get the transmission back together. I took lots of pictures and tried to be as detailed with this part as the last one.





My main recommendation here is to make some practice runs before you lay your Hondabond down. Practice putting the housing on and taking it off until you're confident that it's going on straight and not snagging on anything. The bearing popped off the mainshaft while I was practicing this and that's what you see down there in the bottom of the housing. Wasn't doing any harm there so I left it. Everything still went on straight.

Here's a picture from the manual.

Yes, I know it's expensive as hell! But don't cheap out on this part of the procedure. Go to the dealership, fork out your $15.00 and get some of this stuff.




You have about 20 minutes of working time once you start laying down the Hondabond. That's why I recommend practicing putting the housing on a few times until you're pretty quick about it. And this stuff is hard to squeeze out, too. It took me a while to lay a bead around the entire housing. My hands kept cramping up on me. Follow the pictures and the diagram from the manual and you'll be good as far as the Hondabond goes.





I could have left my mistake out of this write-up, but how would that benefit any of you reading this, especially if this were your first time trying something like this? My torque wrenches are of questionable quality, which is probably why this happened.

I was trying to follow the torque specs. 20 ft-lbs. isn't that much torque, so you can infer than Honda intended the Hondabond to be the main sealer, not the tightness of the bolts. I wasn't getting the click, so I kept on going. Then at one point torque stopped rising and the bolt started spinning. What was happening was that the bolt was stretching out.

As you can see, this happened with two of them. One stretched, one broke! I believe that Honda intended that these bolts only be used once. But we junkyard junkies don't often have the luxury of using new OEM parts. So remember, if the bolt starts to spin, STOP torquing! It's tight enough!

When I started considering doing this swap, this was the one aspect that kept me from trying it for a long time. This part is too easy! As long as you set the snap ring up before you torqued the bolts down, you can turn the tranny upside down, *click*, and you're done. So simple!






Each fork has an up, down, and middle position. All three need to be in the middle.




Should read "lockplate." I neglected this and that thing almost fell into the transmission! That's why I made this precautionary step.




The one on my transmission was covered in Hondabond, so I reused the Accord one instead. The manual says nothing about using Hondabond here, but use it if you like.




So there it is folks! All that's left is to put it back into the car and road test it! BUT... I have to wait on my clutch kit. That won't be here until this coming Wednesday. But I'm going to make a how-to thread for clutch replacement since I've already got the transmission out of the way, and I'm also going to make a how-to thread on removal and installation of the transmission once that time comes. I'll make amendments to these instructions as I find better ways to word things or more accurate information to post. Enjoy, everyone!

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Discussion Starter · #5 ·
I'm happy to see some interested folks. I started taking pictures today anyway anticipating that there'd be some interest. I will edit and post pictures as soon as I've got my transmission out of my car. I encountered some problems with the passenger side axle nut. I'm going to have to put the wheels back on the car and put it back on the ground to break torque on that one. Ran out of daylight today.

I'll likely get it done tomorrow. If not tomorrow, then definitely Monday. I will post pictures of the two transmissions side by side and make some observations about why a direct H2A5 swap would or would not work. We've all heard about how putting a H2A5 directly on a B20/21 is a giant myth, but I want to support or refute this claim with evidence (pictures). I'll go through the breakdown process, and most importantly, the rebuild process. I found a thread which went over how to break the transmission down, but not how to rebulid the thing or tips for reinstalling it. I'm on a bit of an adventure here so we'll see how this all turns out!

Bear in mind I'm not breaking down the gearsets. I want the Accord final drive because I'm trying to maximize fuel efficiency. But I suppose what I could do is break down the D2A4 gearsets since I won't be using them just to show everyone what they look like. Let me know what you all want. I'm going to take advantage of this transmission being off and apart as much as possible. I'm happy to contribute and happy to learn the process!

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Definitely interested. This was a project I thought about trying eventually, but in reality I have no idea what I'm doing with cars. Can't wait to see the results.

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I usually slide a screwdriver in the vent part of the disc brake and let it hold in place with the brake caliper and that helps loosen the cv joint nut. Rather than having to put on the wheel back on and drop the car to ground level.

But very interested in to this swap. I started tearing apart a d2j5 trans that had a bad cracked case. But I didnt get far lol.

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Discussion Starter · #12 ·
As far as I know, the only differences in the gearsets are the gear ratios. Other than that they're supposed to be identical. Here's some numbers I've found along the way for gear ratios:

As you can see, 1st and 2nd gears are identical, so there's no point swapping those in unless you have a damaged gear. 5th gear has the most dramatic difference, that and final.

There was one thread I found where someone did this swap but he wanted to keep the Prelude final drive gear, so he ended up breaking the gearsets apart to get the gears he wanted to keep. My goal, as stated, is efficiency, so I want the Accord final drive gear.

I'm still trying to figure out what the physical final drive gear is inside the transmission. I don't know if it's the gear all the way at the bottom of the countershaft, the ring gear connected to the differential, or both of them together. If anyone knows the answer That would be much appreciated! In the meantime, I'm going to try to get that axle nut off and see if I can drop that transmission before dark!

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Discussion Starter · #13 · (Edited)

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What's your goal with doing this?

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Discussion Starter · #15 ·
I usually slide a screwdriver in the vent part of the disc brake and let it hold in place with the brake caliper and that helps loosen the cv joint nut. Rather than having to put on the wheel back on and drop the car to ground level.

But very interested in to this swap. I started tearing apart a d2j5 trans that had a bad cracked case. But I didnt get far lol.
I put a tiny 6mm in the vent instead of a screwdiver, but it worked like a charm!

So I didn't have to put the car back on the ground after all. I spent this afternoon monkeying around with the driveshafts and ball joints figuring out the best way to take care of them. I got pictures of all that stuff. All that's left is the actual transmission bolts. I'm a little worried about those, though. Whatever shady DIY mechanics that got their hands on this car before me screwed up a lot of stuff. One of the transmission bolts is missing and one of the other ones isn't the same as the others. This joker mashed up the threads of one of the side motor mount holes and I'm concerned I'm going to find something similar with the transmission. We'll find out soon. It looks like I'm going to get rained out tomorrow, but hopefully that doesn't happen.

As far as my goals for doing this, I can think of three:

1. I seriously want to reduce my freeway RPMs and increase MPGs. I live in the middle of nowhere and have to commute everywhere. Cruising 70MPH at 4000 RPM sucks. Period.

2. I want the experience. There are questions I have about this particular mod that I've never found adequate answers for. This is a project I really want to tear into and find some answers and possible solutions to.

3. I want to make a meaningful contribution to this forum. I've gleaned a lot of great information from My car sat for about a year until I decided I wanted to fix it up. I got started a couple months ago and this site has been indispensable to me. Basically, I want to give back to this community.

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1. I seriously want to reduce my freeway RPMs and increase MPGs. I live in the middle of nowhere and have to commute everywhere. Cruising 70MPH at 4000 RPM sucks. Period.
OK that's fair enough.
Just checking as some people get their ratios all wrong and end up doing the opposite of what they wanted.
But be aware that over-geraing a car can hurt mpg if you go too far.
And if you're only getting 70mph at 4k rpms you already seem to have a problem !

2. I want the experience. There are questions I have about this particular mod that I've never found adequate answers for. This is a project I really want to tear into and find some answers and possible solutions to.
Well I've not done that particular mod and will need to read through when fully awake to see what you intend doing. I'm not sure you'll have any luck if you intend just swapping the whole shafts over. Swapping gears over is much more likely to work.
Doing things just to see if they work is fine by me, often why I do things. When I first fitted the LSD into mine it was mostly to see if it could be done and to show those who had done half the job (or paid others to do it) and never completed it that it was indeed very possible.

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Discussion Starter · #18 · (Edited)
Okay, I wasn't sure I was going to get it done today, but I managed to get the transmission out of the car! I've taken a lot of pictures and am ready to show everyone what I found. I set the two transmissions next to each other and made some comparisons between them. At first glance these are the observations I made:

I haven't opened up the transmissions yet. I just took and uploaded these pictures. But from these pictures, it's plain to see that there's absolutely no practical way to bolt the H2A5 to a B series motor. It would be far simpler (and cheaper) to open up the casings and swap the parts you want. I'll get into the transmissions tomorrow and show you all what I end up doing. Enjoy! :)

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Looks like the diff is mounted higher up on the d2a4 as well. Likely to keep good alignment despite our engine tilt. Putting the other one on would be bad for axle align.

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Discussion Starter · #20 ·
Looks like the diff is mounted higher up on the d2a4 as well. Likely to keep good alignment despite our engine tilt. Putting the other one on would be bad for axle align.
I hadn't considered the tilt of the engine, but now that you mention it, I can see what you mean. I'm excited to open these up tomorrow and start tinkering around with them. I'm also going to do the clutch while I've got the transmission out. Don't know if I should write it up here or create a separate thread for it. Anyone know if there's any need for a clutch job write-up?
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