Which gearbox/diff

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MBlades
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Which gearbox/diff

Post by MBlades » Mon Apr 22, 2019 9:55 pm

Hi guys

Got a 1973 240z auto, planing a gearbox change, after some advice. I’m thinking a 5 speed box from 280zx as I hear straight bolt in replacement ? Would I be wise to change diff too ?

Again is this straight bolt in from the 280zx ?

What else will I need as I’m going to need to get all parts b4 starting, total novice at this so all advice appreciated. Or any other suggestions
Cheers
Mark

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bernjean
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Re: Which gearbox/diff

Post by bernjean » Tue Apr 23, 2019 11:15 am

If you want something really good an easy change is to use an SR20DET box out of an S13 Silvia. If you go to the technical forum there is a post on what is required to make the change. You will need an original Z bell housing and the Silvia box will bolt up to that. These boxes have been used for Track work. I did this conversion to my car and has been one of the better changes I have made. Gives the car a lot more modern feel. In regards to diff, it depends on what you currently have fitted. Auto should have a 3.5. Google will tell you what came out new with your car, Others can kick in here but you could go to 3.9. The lower you go the quicker your car will be of the line but will rev more at higher speeds. Remember if you change your diff ration you will also have to change your speedo gear cog.
All the best with the change. keep us posted on what you do.
Bernie Kant

MBlades
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Re: Which gearbox/diff

Post by MBlades » Tue Apr 23, 2019 1:05 pm

Superb Bernie, great info,I knew this was the place to ask
Thanks for your input
Regards
Mark

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stevenewenham
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Re: Which gearbox/diff

Post by stevenewenham » Tue Apr 23, 2019 1:58 pm

If memory serves,that same C gearbox referred to by Bernie was also fitted to RB20det Skylines, but for that
application was some 50mms longer .This means stock driveshaft will work,whereas S13 version requires
a longer driveshaft to be made.
Stock diff for manual 240 was 3.9.
In my case,late 260 2 seater, Skyline C box ratios are pretty much identical to stock box except for 5th,
which is way taller.So if you want to have stock cruising revs in 5th you could potentially go to 4.1 or 4.4 diff,
with lower intermediate gears making your car more lively.

MBlades
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Re: Which gearbox/diff

Post by MBlades » Tue Apr 23, 2019 5:10 pm

With 240z bell housing, I’m guessing as it’s an auto ? Yes really am a novice 🤣 also was wondering if I’d need yo alter tunnel for stick ? Reason being it’s a one owner totally original, that’s reason I’m wanting bolt on where poss. Thanks in advance

Mark

MBlades
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Re: Which gearbox/diff

Post by MBlades » Tue Apr 23, 2019 5:12 pm

stevenewenham wrote:
Tue Apr 23, 2019 1:58 pm
If memory serves,that same C gearbox referred to by Bernie was also fitted to RB20det Skylines, but for that
application was some 50mms longer .This means stock driveshaft will work,whereas S13 version requires
a longer driveshaft to be made.
Stock diff for manual 240 was 3.9.
In my case,late 260 2 seater, Skyline C box ratios are pretty much identical to stock box except for 5th,
which is way taller.So if you want to have stock cruising revs in 5th you could potentially go to 4.1 or 4.4 diff,
with lower intermediate gears making your car more lively.
Apologies Steve I’d missed your reply again really appreciate appreciate your input too thanks
Mark

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bernjean
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Re: Which gearbox/diff

Post by bernjean » Wed Apr 24, 2019 10:13 am

No need to alter the tunnel. But the shifter does sit a little back from the standard. I never fitted the gearbox myself but had it done. I asked that they retain the standard position for the lever for the look inside the car. I believe they cut welded or bent it to work. With this converstion, you do need to make a few changes to the cross member. The guys that did mine found another cross member they could alter without it looking like changes had been made. The advantage of doing my conversion is that I don't need to have any certification done. The conversation looks completely standard so no chance of anything being picked up WOF time.
My car is a 260Z 2+2 and we were able to retain the standard driveshaft. Sounds like maybe the Skyline box is a better application because the lower ratio diffs are better of the line. When I change mine I might have mine available, my car was originally a 2.0 litre so I have the I believe the 4.4 diff. It will need some work as it does wine a bit at higher speeds.
Bernie Kant

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old_datto_fan
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Re: Which gearbox/diff

Post by old_datto_fan » Wed Apr 24, 2019 5:53 pm

I've gone with the RB20DET box and it works great for me. Much shorter throw then the standard Z boxes. Driveshaft didn't need to be changed. We had to slot the holes in the gearbox crossmember just slightly. Speedo cable worked fine.
Don't forget you will need a pedal box too and they may be hard to find. I wouldn't do anything until you have that sorted as the rest of it is fairly easy to locate.

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stevenewenham
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Re: Which gearbox/diff

Post by stevenewenham » Fri Apr 26, 2019 8:59 am

In my case we did not bend the gear stick, so it does sit slightly forward of the stock position, but not enough to require any change to the hole in the tunnel.Found a cross member from the Mount Shop that worked well.It did require maybe 5mm trimmed off the front of the hole in the console,but done in a way that can't be noticed.Bending the stick would probably avoid the necessity for that.I wasn't big on the bending thing as I have heard of them breaking after being heated for bending.

MBlades
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Re: Which gearbox/diff

Post by MBlades » Sat Apr 27, 2019 6:56 am

Thanks again guys, what else or problems can I expect changing to Auto to Manual ?? Got the chance of a standard 240 box, but not sure if I should just go with the Rb20DET, if I went 240, what other parts I’m I going to need to get ? Cheers Mark

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bernjean
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Re: Which gearbox/diff

Post by bernjean » Mon Apr 29, 2019 8:43 am

Clutch master and slave cylinder and makeup lines. Should not be too hard to obtain. Also, it will need engineer certification. They may require a driveshaft loop.
Bernie Kant

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Andy
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Re: Which gearbox/diff

Post by Andy » Fri May 03, 2019 7:02 pm

We "eventually" went to the Silvia version of the C box.

Shortened the driveshaft, well actually remade a new one as the originals twist the tube and break UJ's once horsepower climbs, so wanting to keep the original driveshaft shouldn't be a reason not to choose the Silvia version.

The longer box puts the gearshift in a better location with no need to crank the gear lever.

We also chose the Silvia version as the Quaife internals (and most other manufacturers) are designed for it.

Gearbox change is a good mod but the best part when we changed was the new ratios and shafts available aftermarket for the Silvia version. The factory ratios are rubbish for spirited driving.

S13 or S14 are ok to use, they vary only in the holes in the sandwich plate, block them up on the S13 version to prevent oil surge.

You will work out pretty fast if you need to change the diff ratio, there are calculators online that will help you with tyre size, final drive, rev range and gear ratios input.

Best diff centre for me has been helical/torsen type. Fits in the R200 casing, seems un-breakable and good characteristics. Quaife give theirs a life time guarantee. Never had a Nissan R200 LSD plate diff last very long.

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