Dusky

Hi guys!

2 questions,
1) who is using amsoil sever gear oil as posi oil? Some 'big names' say it's all they use ( gearhead is a name that springs to mind) others say it'll cause the apocalypse. Gm additive is no option, impossible to get here, the GM tech laughed in my face.

2) what would happen if you tune the posi as per Tom's guide on YouTube, and still add the spring? A super tight diff is what I think, absolutely unpleasant on the road.


cheers

resdoggie

Are you saying you can't buy any gear oil such as Royal Purple Max Gear or similar that does not require any additive, because it's already in the formula, anywhere in the EU? Unless your going to do a dozen laps at Nurburgring, don't worry about what the "big names" recommend.

Dusky

Amsoil is the only oil wich is supposed to have the additive in that I can find
Sadly no royal purple etc (yet)
haha, would love to do a lap on the ring though! Will be going there, not sure if I ll do a lap, only takes one idiot to kill a restored car..

resdoggie

I'm sure someone will comment on the Amsoil or just give them a call or look up specs on their website. Good luck on the "ring". An opportunity I'll never have living in Canada.

76strokervette

I have amsoil severe gear in my 12 bolt conversion. I would still use the gm positraction additive.
My unit was tuned by Mike Dyer and I had him test the severe gear in assembly. He was very impressed with
the film strength and lubricity of the amsoil but still feels you need the gm additive.

stingraymax

Dusky, where about are you from ? Maybe I have some bottles left from my build.

AzMotorhead

73racevette

Not completely sure, but if the posi is tuned correctly it is preloaded already. There would be no need to add the spring. If you did add the spring in as well, the preload would increase by the spring tension amount which would in turn increase the torque needed to break the two sides of the diff free from each other. It would be tighter than it was before, but perhaps not noticeable sitting in the drivers seat. Might have a tendency to chirp around corners a bit more. No real way of knowing without doing it and seeing what the torque difference is with and without.

gkull

I just regeared my vette . The super posi kit came with more steels to put more preload and of course really locking up both rear tires. I used the Lucas Oil product with posi lube in it. When I first ran it in reverse turning tight I could hear binding. So I added an additional tube of posi lube in retrospect it probably only needed some breakin time. But I put 300 miles on it and drained the fluid refilling it with Lucas oil with posi additive in it

it always leaves two big black marks 335 wide

GTR1999

1- Can't help as I never used any oil other then a quality 90 or 85-140 gear oil with GM additive. The only time I didn't was 35 years ago and tried a well known oil that had the additive in it, it chattered badly until I added the GM additive. The additive has gone through some revisions since the old Whale oil days. I still have a case of the product used about 10-15 years ago and seems to work better then the current product when 1 bottle is used. With the new stuff, available from any dealer, summit, or jegs, use 2 bottles and it should be fine. You can use fancy names oils if you like but I never needed to. Some of the diff's out there I built have well over 100k miles on them now and going strong. Now what you can get over there I can't say or recommend.

2-A correctly tuned posi is balanced and has no lash. Springs are not required- if it is properly setup. The process is not new, it's been done for the past 40+ years that I know of. Proper spider prep and access to a surface grinder to dial in the shims is required as well, many times the standard shims will work as they come in 005 increments but to really get it right, not close - but right, I have to grind shims. A properly setup posi will also be radiused and polished more then just touching the windows. Once done the clutches are already seated since many, as in 800 revs both CW & CCW are applied to it. Tom's video covers it pretty good although we have a slight difference in assembly technique. They can be done by hand, I did 100's of them, but a machine works a lot better. Building a machine for one posi build probably is not worth it but if you know someone with a lathe that has low RPM in back gear then you can rig up a setup. I am doing one this weekend if you want to come to CT I'll show you how, yeah I know a bit more then an hours drive, lol.
Done correctly they will out perform and outlive any stock spring loaded posi, done wrong you will have an open diff.

CRN1857

Hello,

I followed the tuned posi procedure but 4,000 miles later I have way too much clutch wear, approx. 0.030" on driver side and 0.015 on pass. side. The initial setup used the preferred all steel, new clutches from Tom's and I don't think it was setup too tight. I used 100 revs both ways to seat the clutches, would that have made this much of a difference? This is in a street driven vette and that is not driven too hard. Where did I go wrong?

MelWff

The person to send a PM to about your problem is GTR1999.

GTR1999

It wasn''t set up correctly. there is more to it then leaving the springs out, it has to be balanced or it not work. I will be holding a posi tuning seminar in CT this Summer if you are near by. I can't tell you what is wrong other then it's not right, I would have to check it to see.

zwede

The reason GM put a spring in it was for winter driving. If you have one wheel on ice and one not the wheel on ice will spin without the spring. With well-tuned clutches and no spring you'd most likely still lock it even in this situation although it's pretty academic as I don't think there are many C3's driving on winter roads anymore.
Note that a tuned posi without spring will fail the old mechanics test where you jack the rear up, put it in gear and turn one wheel. The other wheel will turn in reverse indicating a bad posi but it's not bad, it's just what it does without the spring. It needs a load to lock.

jb78L-82

OP, do yourself a big favor and listen to GTR1999 who is an expert on these diffs and has not only MANY years of experience tuning them but has done 100's of builds....along the way. I will leave it at that comment............

CRN1857

Thanks for the feedback. If I was near Ct, I would definitely attend your seminar.

I used 0.055 shim on one side and 0.051 shim on the other, that allowed the crosspin to slide in freely instead of using the same shim thickness on both sides. Is that what you mean be being balanced? Also, when I set up this posi, I didn't debur the 10-17 c-clip spiders I bought from Tom's, does that make a big difference?

GTR1999

When all is said and done. The cross shaft has to slide freely out of the case with the case in any position. Your shim size sound close but every posi case is machined differently and rarely are the same size shims used in each side. Yes I have had that happen but most times I am grinding shims on my surface grinder. I always radius the spiders and ring & pinion for that matter. 100 turns is not enough by the time I am done with a posi I have put in about 800 revs cw & ccw on both LH and RH sides. There will be no lash in the spiders or endplay in them and they will not bind in rotation, any change of the procedures will result in unsatisfactory operation. Sounds like you have to start over. You are using solid steels and not fiber coat junk so that is good.

bazza77

To the O.P , are you sure you cant get the additive ? I can get it from a G.M dealer here (Holden) , Its a car yard that sells brand new cars and their spare parts counter guy looked it up and found 2 types , one you need to wear rubber gloves as it burns skin on contact ! , this was for the newer cars and another was the correct stuff for the old type diffs.

hdeyong

Hi Dusky.
It may be a little late, but I live in France and have bought #90 gear oil with the limited slip additive already in it from Direct Lub here in France. They have Dexron and most anything you might need. I also found bottles of limited slip additive on eBay, Ford and Mopar brand, but as far as I know, they're all the same.