[GUIDE] F35 to F23 (with LSD) Transmission Swap

Desert Sasqwatch

Goblin Guru
WERKS Racing can build one, if you can afford their prices - see the link.

And please keep in mind it's not the HP that kills transmissions, it's the torque output of the motor and how much at any given RPM. I would have to believe a 425HP Ecotec makes probably 400 ft lbs or there about. Based on the manufacturers numbers for torque capability the F35 is rated at 258 ft lbs - in a 3000+ lb car. In a Goblin, conservatively estimating, the F35 should be good for around 388 ft lbs - but again that's a new transmission rating. See this thread for additional information:
 

Scott Geyer

Well-Known Member
WERKS Racing can build one, if you can afford their prices - see the link.

And please keep in mind it's not the HP that kills transmissions, it's the torque output of the motor and how much at any given RPM. I would have to believe a 425HP Ecotec makes probably 400 ft lbs or there about. Based on the manufacturers numbers for torque capability the F35 is rated at 258 ft lbs - in a 3000+ lb car. In a Goblin, conservatively estimating, the F35 should be good for around 388 ft lbs - but again that's a new transmission rating. See this thread for additional information:
Thanks again
 

OptimizePrime

Goblin Guru
  1. It can be difficult to break the intermediate shaft retaining bolt loose. The input shaft will want to spin; I 3d printed a socket adapter so I was able to put a wrench on the input shaft. Piece of cake after that.
    1. For those without a printer, you can use Kent Moore Input Shaft Anti-Rotation Tool J-44377
  2. The new LSD will need bearings pressed onto it. You will need access to a press, or a shop should be able to do that easy-peasy. If you choose to replace the races in the trans case, it requires some creativity, special tooling, or both. Technically you should replace the case races, but if you're confident the new diff bearings match the old case races, it should work, and other have done this successfully before. I was able to slowly and carefully cut through them, at which point they will just fall out.
    1. Having done this a few times now, I would 100% buy the tools below and have the original diff spacers out in a snap (with a press) and use new diff bearings. Don't even bother with the rest of them it's not necessary.
    2. KENT MOORE TOOL J-44379 DIFFERENTIAL BEARING PULLER PLATE
      KENT MOORE TOOL J-44381 SHIFTER BEARING REMOVER
      Total was like $50 from ebay. Well worth the risk / time / effort
  3. The intermediate shaft seal retaining bolt cover is a bear to source. They can be had out of GM Europe, or you can slap something together. I actually 3d printed a replacement cap and RTV'd it into place. I had a catastrophic fluid leak that turned out to be a bad input shaft seal (on a new OEM part!). I thought it might've been my cap, so I ordered a GM replacement, but it turned out that was holding fluid fine, so false alarm there.
    1. Yoooo drop that .stl son, these caps are much more expensive then one would think ($34). My printer was down when I going through this so I just ponied up for it. Other times, I just re-used the original cap and siliconed the hole that was drilled.
A few comments above for people who are on the fence.

Thanks for the great explanation. I did have one more question. Can the F35 handle up to 425whp?
As Squatch mentioned, it's the torque but also keep in mind the Goblin is significantly lighter which changes the game. You could be fine at 425whp which is intense in these Goblins.
 

Rauq

Goblin Guru
Unfortunately I don't remember which of the intermediate shaft seal retaining bolt covers I ended up using. I printed three different tolerance cap and ended up using the one that was a slight interference fit. I skimmed the internal surfaces with RTV to ward off leaks due to porosity and sealed with RTV on both the cylindrical and flat surfaces (see attached pic). I printed with PETG for temperature purposes. Like I said, I was concerned that was the cause of my trans fluid leak, but the area around that cap was clean while below the input shaft seal had leaked, and the resulting leak picked up a bunch of grime that highlighted the source of the leak.

I also attached the spline to socket adapter. I think it went with a random 1 1/32" 12 point socket I had laying around, as it was the smallest socket that fit over the input shaft.

Last but not least, an F23 axle shaft seal cover. Made it a piece of cake to cover the seals for cleaning and painting the trans.


I'll dredge up a piece of unsolicited advice I've given before- consider the source of advice you're being given. Have they been there and done that, or are they racing from the stands? There are several +400whp Goblins around, and at least one over 500whp, and I believe most, if not all, started with an F35. As far as I've read, if any of them have swapped transmissions, it's been to an F23.
 

Attachments

OptimizePrime

Goblin Guru
It has begun.
Thank you for the guide! I have started this now myself. Just need to find a new seal cover for the shaft with the 8mm
Love to see it! Did you go with the 4.17fd? Looks like you're doing a full tear out with the shifter section also - I remember there are some plastic pieces you can replace that would stiffen up that section as the plastic wears and creates play. I'll try to find the link even though it looks like you may be using a zero-mile trans based on how clean it is.

Here you go:

Also, I see one of the bearings stayed with the top section. Be super careful putting the top section back on as the bearings can pop out or not seat right and get slung around (best case the magnet picks it up). If you take the plastic & bearings out, there is a top and bottom orientation so make sure it goes on the right way.

37627
 
Love to see it! Did you go with the 4.17fd? Looks like you're doing a full tear out with the shifter section also - I remember there are some plastic pieces you can replace that would stiffen up that section as the plastic wears and creates play. I'll try to find the link even though it looks like you may be using a zero-mile trans based on how clean it is.
I did go with the 4.17 final, It came out of a local junk yard to me with just under 100K for only $200! I cleaned it up a good bit prior to disassembly, and am going to paint it black once it's back together.

Here you go:
Thank you for the heads up. Going to look into this. I just wanted to take a look around inside being it's a used trans and make sure the synchros and all looked good. It had a brand new slave on it when I picked it up and it looks as if someone has been inside of it before. I cannot tell why, but the head of the bolt that holds the shaft had some marking on it, and it had a part number on the seal I cannot cross reference.
37630

Also, I see one of the bearings stayed with the top section. Be super careful putting the top section back on as the bearings can pop out or not seat right and get slung around (best case the magnet picks it up). If you take the plastic & bearings out, there is a top and bottom orientation so make sure it goes on the right way.

I did this to not knock it off or out of the place. Figured it was safer gently set away than around me digging around.
 

OptimizePrime

Goblin Guru
I'm intrigued by this from the link you sent.

That makes both of us! The counterweight would still need to be chopped off to fit which is unfortunate. Cheaper than the DF short shifter!
 

newbtrying

Member
I have a Quaife LSD from an F35, is this something (hopeful thinking) that can be dropped into the F23? I think I probably already know the answer to this lmao
 

OptimizePrime

Goblin Guru
I have a Quaife LSD from an F35, is this something (hopeful thinking) that can be dropped into the F23? I think I probably already know the answer to this lmao
Nah but you can sell that LSD for a pretty penny! It would probably cover the cost of the F23 anyway. The Team Green Insert (not a true LSD) is a budget friendly option for your F23 if you didn't want to fork out the cash for the F23 Quaife. Honestly you could just run your F23 without LSD and save up or decide if it's worth it.
 

newbtrying

Member
Nah but you can sell that LSD for a pretty penny! It would probably cover the cost of the F23 anyway. The Team Green Insert (not a true LSD) is a budget friendly option for your F23 if you didn't want to fork out the cash for the F23 Quaife. Honestly you could just run your F23 without LSD and save up or decide if it's worth it.
I'll spare the details, but I'm expecting power in the 500 hp range once I get my engine reassembled and in my Goblin. Because of this, I can't really choose any other route than "go big or go home". Because of this, i'd be purchasing the Quaife and If I choose to go with the F23 I'll just slap it in before I install it. (what a comical oversimplification o_O) At the moment, I have an F35 transmission that I have to decide if it's worth rebuilding, or even possible... looking at Performance Autowerkz, but if they still offer this service, it's extremely expensive. Thanks to this fantastic walk-through, I think it might be easier and cheaper to go the same route as the OP.

My other concern is maintenance. The F23 would be easier to fix if I ever break anything down the line.
 

OptimizePrime

Goblin Guru
I'll spare the details, but I'm expecting power in the 500 hp range once I get my engine reassembled and in my Goblin. Because of this, I can't really choose any other route than "go big or go home". Because of this, i'd be purchasing the Quaife and If I choose to go with the F23 I'll just slap it in before I install it. (what a comical oversimplification o_O) At the moment, I have an F35 transmission that I have to decide if it's worth rebuilding, or even possible... looking at Performance Autowerkz, but if they still offer this service, it's extremely expensive. Thanks to this fantastic walk-through, I think it might be easier and cheaper to go the same route as the OP.

My other concern is maintenance. The F23 would be easier to fix if I ever break anything down the line.
I put down 400whp, 400ftlbs (probably 500hp) on my F23. JReed and others are in the 600whp range. You'll be just fine on the F23 as long as you boost ramp and don't drive like a dingus (most of the time anyway).

The weight of the Goblin changes the abuse capacity in our favor! Go for the 4.17FD :)
 

newbtrying

Member
I put down 400whp, 400ftlbs (probably 500hp) on my F23. JReed and others are in the 600whp range. You'll be just fine on the F23 as long as you boost ramp and don't drive like a dingus (most of the time anyway).

The weight of the Goblin changes the abuse capacity in our favor! Go for the 4.17FD :)

love to hear this! Before I get started searching after work, does anyone have any leads on purchasing an F23 with 4.17 FD? I see DF has some 0 milers with 3.84 FD for a reasonable price.
 
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