GTA91

iTrader: (1)

Guys, I am wanting to swap my 10 bolt drum rear out for a 9 bolt 3.27 posi unit from a '88 GTA. Is this a direct bolt in?? What will I have to change? How do I calibrate my speedometer to work with different gears, and are there any aftermarket gears for the 9 bolts? Last thing, can I swap the rotors and calipers onto my drum rear instead of swapping entire rearends? Thanks!

Kevin91Z

iTrader: (1)

Yes its a direct bolt in. You will need to change the speedo gears in the trans for a 3.27 rear end. The only aftermarket 9-bolt gears are 3.70, and they're very expensive. The last question I dont know. I went from a 91 10-bolt 3.42 disc brake rear to an 89 9-bolt 3.45 disc brake rear, and the disc brakes swapped with no sweat, because they are the same.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

GTA91

iTrader: (1)

We are always meeting somewhere aren't we.(its me Jeremy) Anyways, I'm not sure what I'll do yet, who would know if I could bolt the discs from the 9-bolt rearend onto my 10-bolt drum rear? Also, do I just got to a dealer and ask for the speedo gear for a GTA with a 3.27 rearend or what?? Does it matter that I have the 5-spd and not the auto???(for the speedo gear) Well, thanks again man, you're helping me all the time. BTW-nice car pic and webpage!!!! Good time to boot!!

sparks383iroc

The swap will be a direct bolt end, but the '88s calipers will probably be different. They installed the newer type brake calipers on the '89 and ups, '88s were a different setup (different backing/mounting plate, larger pads, different rotor). At least this is what I've seen, there's no telling what GM might have done in between years and the calipers might be the same. Do you have access to the rear? If so, check the calipers and see if they're a match.

Sparks a flyin'.

After re-reading your post, I saw that you are wanting to swap drums for disc brakes. I don't know the answer to that one, sorry I couldn't help.

[This message has been edited by sparks383iroc (edited October 16, 2000).]

Kevin91Z

iTrader: (1)

Ok, you're putting a 3.27 rear end with a 5-speed. My dad is doing the exact same thing, and we found you need a 7 tooth and a 19 tooth speedo gears. Or a 9 tooth and a 23 tooth gears. But the 19 tooth and the 23 tooth are not made for electronic speedos, from the research we've done so far... We may have to use a cyberdyne box to convert the signal.

Tony89GTA

iTrader: (7)

Actually GM makes a 19 tooth electric speedo gear. I have one at home, it came from a 87 V6 firebird with a T-5.

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89 GTA 350 5-speed
(Jet hot coated) SLP headers,flowmaster force 2,no cat's,SLP cold air,MSD 6al,jet chip stage 2,march pulleys,ported plenum,air foil,B&M ripper shifter,180 stat,alston SFC,spohn LCA relocation brackets,MAT relocation kit,free mods.

G-Tech times
0-60 5.3s
E.T. 13.91@104.8MPH (2300ft)

GTA91

iTrader: (1)

Thanks guys, Anyone know yet if I can put those discs on my drum rear though???? Can I get that 19 tooth electronic speedo gear from the dealer then?

MILT

GTA91: you will also need the proportioning valve from the 9-bolt car as well, to correclty balance the brakes...

GTA91

iTrader: (1)

MILT-
Thats what I thought, do I need to use that master cylinder also. I wasn't sure.

Kevin91Z

iTrader: (1)

Ok, I was wrong. The 7 tooth & 19 tooth combo is for a 3.4x ratio rear end. For a 3.2x ratio rear end, you need a 7 tooth & an 18 tooth combo. Or a 9 tooth and 23 tooth combo. So do the 18 or 23 tooth gears exist?

Tony89GTA

iTrader: (7)

If you have the stock 245/50R16 tires then you still use the purple 7 tooth drive gear and white 19 tooth driven gear combo for the 3.27 rear end.

Kevin91Z

iTrader: (1)

How can you use the same speedo gears for a 3.4x and a 3.2x rear end? That doesnt make sense.

GTA91

iTrader: (1)

CONFUSED NOW!!!!!!!!! What will I need then. I'll just wait, let me know.

Tony89GTA

iTrader: (7)

Sorry about the confusion but I asked this question before in a previous post and that was the answer that I got, check out (What T-5 speedo gears do I need for 3.27) also if you look on the TPI board and look for the question (3.42-3.73 for 305) in there MARK_ZZ3 has a link to a chart that shows all the speedo gear/gear/tire combinations.

lonsal

iTrader: (2)

OK, now for the answer on the 9-bolt disc swap onto the 10-bolt drum brake rear end. No, you can't swap them. The 10-bolt drum brake rear end has a hub dia of approx. 6.25". The 9 bolt rear end has a hub with approx. 5.88" diameter. The disc rotors won't fit on the drum hubs. Second, the 9-bolt rear end backing plate that mounts the calipers won't bolt to the drum rear end. They are different. You will have to swap the entire rear end to get the disc brakes. I am told that since 89 the proportioning valve is the same for drum brake cars and disc brake cars. The older drum brake pv's had a fitting that comes out the front of the pv. When changing pv's on those vehicles the line needs to be re-bent to the new location which is on the bottom of the pv. However, I have swpped the disc brake rear end from a 88 Iroc to my 90 RS convertible and am still fighting a problem with the proportioning valve. It continues to have too much rear brake bias (rears locking prematurely and brake light coming on). I have even swapped in the 88 Iroc pv and STILL have the same problem. Good luck with the swap, Lon.

GTA91

iTrader: (1)

Hey- I was told buy another guy that he did the swap... putting discs on a drum rear. He said he made a template and drill ed the holes to mount the calipers and such on the drum rear?!?! Now what. He said he performed it on a Formula.

84FTA

It'd be much easier and faster to just swap the full rear-end. No matter what you have to drop the whole rear, you can't swap in posi gears into a non-posi rear nor swap in 9-bolt gears into a 10 bolt housing. Also, if you wanted to swap the housing itself, you can remove the axles but it would be very hard to get the spot weld off from the top of the axle housing with it still on the car. As for speedo controls, I don't know, you will have to investigate that. Unless you're dropping gear ratio's drastically (4.10 down to 3.27's) then it is ok to wait a bit while doing the swap.

The rear end is a basically direct bolt in. You do not need to redrill and fit adapter plates to switch to rear discs because ALL the mounting hardware is on the other rear end. As for the prop valve and master cylinder, it is rumored that for early (82 -88ish ) had the same master cylinder as the later J65 disc brake cars (88, after the recall on the earlier calipers). It would probably be more worth while to buy a wilwood or similiar adjustable prop. valve. It will save you time and cost around just as much.


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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration


[This message has been edited by 84FTA (edited October 20, 2000).]

[This message has been edited by 84FTA (edited October 20, 2000).]

GTA91

iTrader: (1)

84FTA-
I don't think you'd have to drop my drum rear to add discs to it. Also, I NEVER said anything about putting non-posi gears with posi gears or 9 to 10 bolt! Where did that come from? So can I put that rear in my car and use the same car's prportioning valve and rear lines? I'm about ready to sell BOTH rears and just buy a 4th gen one with discs and 3.42's! What should I do??????

84FTA

It sounded like you were doing the swap for the gear and the 9-bolt strength. I assumed you were swapping the rear anyway because 9-bolts are stronger. If you're just looking for rear disk brakes, then you can create your own adapter plates and run rear disks. The only thing I could think of is you wanted to somehow not drop the full rear but install the 9-bolt and the disks....excuse me if I'm incorrect.


Anyway, short simple answer is yes, you can swap a full 9-bolt rear in place of 10-bolt with drums rather easily. You will need to do something with the braking system is all. You can try your stock system and if the rear disks grab too soon, you'll have to buy and install an adjustable prop valve (well worth the cost on any car).


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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 WS6 Trans Am T-top car
4-bolt main 350, headers, Holley 650, T-5 and 3.23's.
Daily driver and restoration

[This message has been edited by 84FTA (edited October 20, 2000).]

[This message has been edited by 84FTA (edited October 20, 2000).]

GTA91

iTrader: (1)

I understand now 84FTA... thanks! So I can make adapters or drill new holes and put those discs on my rear? If I do, why can't I just use the pv from the '88? Or can I? Thanks

8Mike9

Check the p/n's for the M/C's on the two different setups...could be your issue....the disc calipers hold more fluid than the drum wheelcylinders do...could be that the M/C's bore on the drum brake rear is smaller than the disc brake rear.

Just a guess.