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Discussion Starter #1 (Edited)
Well I started gathering parts to build a 12-bolt, 373 diff for a fellow corvette buddy running 650+ hp. These can be used in the 63-79 vettes or the 80-82 but you have to use an iron cover and fab the mounts for those.

I really like what I see so far, take a look at these baby's. You like to put your foot in it with a lot of HP, you may want to get one of these.


Here we have from L-R a 4 series case, new 12 bolt case and new 10 bolt case


Here are the cross shafts from the 10 & 12 bolts





Going to use 10-17 spider, heat treated, 30 spline.



I'm also going to start machining the 10 bolt cases and fit the 12 bolt spider and cross shafts. For more strength for the 10 bolt blueprinted builds.:thumbsup:
 

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Discussion Starter #3
A lot of grinding so far. I have the case just about fit but not yet. I can see why a vendor wouldn't want to do these- too much labor per profit.
 

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Get one the way you want it and have it CNC copied. The following units would be much less labor intensive. Even if you farm them out, do 5 or 10 at a time and everyone wins?
 

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A lot of grinding so far. I have the case just about fit but not yet. I can see why a vendor wouldn't want to do these- too much labor per profit.
Does the stock case have to be machined too (shoulders and bearing locations), or just clearance ground?
 

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Discussion Starter #6
Clearance ground on a radius then polished. CNC is an idea I have a couple of contacts. This is taking longer due to making some tools up as I go along. Once I have that done then the next one will go faster.
 

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Gary,

Maybe you can answer this....what would you say the limit is for a toughly built 10-bolt? It would be going in a car that sees street and strip duties and I currently have drag radials in the rear. I don't childishly abuse the car but I do drive it hard when I'm at the track. Also, does a 12-bolt cost significantly more? I know a 12-bolt will hold up to my requirements but if I can get away with beefing up my 10-bolt and save a bunch of cash then that's probably my best plan of action. BTW, to my knowledge my current rear-end is the stock '69. I was told it was rebuilt once from the previous owner.
Thanks!
 

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Discussion Starter #8
Gary,
That makes sense. When I get closer to being able to pull the trigger on one I'll contact you.
Maybe you can answer this....what would you say the limit is for a toughly built 10-bolt? It would be going in a car that sees street and strip duties and I currently have drag radials in the rear. I don't childishly abuse the car but I do drive it hard when I'm at the track. Also, does a 12-bolt cost significantly more? I know a 12-bolt will hold up to my requirements but if I can get away with beefing up my 10-bolt and save a bunch of cash then that's probably my best plan of action. BTW, to my knowledge my current rear-end is the stock '69. I was told it was rebuilt once from the previous owner.
Thanks!
Well I'll give my opinion and I'm sure there are more. A stock iron unit comes in couple of versions depending on the year. In your case, 1969, they had the weaker 10-18 spiders and better posi cases then the 63-68 run. Probably has the 1310 pinion yoke as well. Add to that, if still bascially stock, 2.5" - 1/2 shafts, stock spindles & side yokes. How much will it stand, hard to say. I've seen some places quote up to 600hp for 10 bolts- stock built but I'd wouldn't trust that myself.

I've seen 375 HP SB's crack a case in 1/2 with enough abuse and 400-450 cars keep a stock unit in tact, so a lot depends on power, tires and driver.

If I had to give a borderline guess for a built 10 bolt I think 500hp would be a good nominal range but again look how it's going to be used. Tom told me he has 12 bolts in 7 sec vettes. How many passes before they're checked I couldn't say. The one I'm building now will have all the HD parts in it along with Tom's 1350 axles kit which has 3.5 shaft with .134 walls, 31 spline spindles and 30 spline axles. I've already put about 3/4 of the time into mocking up this one compared to a beefed up 10 bolt build. That time will go down a little but I think I will still hand build them. CNC cost would just push up the price, as I have little volume on housings.
 

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Discussion Starter #9
I got the 1350 axle kit in and it's pretty impressive. 31 spline spindles,30 spline axles, heavy duty flanges. larger bearings. Of course they're not a direct bolt on, but will be fit and setup.
 

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I saw one of the 12 bolt posi carriers for a chevelle when I was in Texas this summer - I think it was a 3 series, not a 4 series, but either way, it was amazing how thick that ring gear flange was!
 

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Did you hand "carve" per-say the side of the housing to get the parts to clear, or did you use your mill? What other machining is required to build a 12 bolt?
 

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Discussion Starter #14
All hand fit. All the parts are special machined and expensive. I use the surface grinder and Mill as well. Side shims will have to be machined and I had to make some tools up.

I heard some rebuilders have used some of my previous posts, not sure if this is true but it certainly could be. I'll only show general pictures of this job,like those in the Hot Rod article. I wouldn't attempt it without a mill,lathe, and grinder.
 

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Discussion Starter #15
This one is taking me some time to do but I'm going over every step, fitting things. Some parts I expected to bolt in are the ones I have to machine. I know how to machine the steel caps and the tricks with fitting them,but I haven't even got there yet.I'm not rushing this job and will only ship it once I'm 100% satisfied with it, no matter how much time I put into it. Once I get all the tools made it will help on the next one. I really like the parts, so much so I'm thinking the 373 10 bolt I built in my 69 will be coming out for one of these when ever I get back to building that car. That 10 bolt only has 1200 miles on it too!:crazy: :laughing:
 

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Discussion Starter #17
Continuing on, I set up the housing in the Bridgeport and dialed it in. Then drilled and tapped the LH side for the steel caps larger bolts. Next the cap will have to be fit to the new race in the housing. This is the same procedure as I outlined in my steel cap thread. Some use a drill bushing fixture for this, I like the mill better.



 

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Discussion Starter #18
Well put some time into this over the Holiday. I have it just about done now, I have to fit the HD rear cover. Let me say this, I do not recommend this job for the DIY'er. I had to machine just about everything with this job to get it done, there is no way I would have been able to do this 12 bolt conversion without the machine tools and fixtures I had to make up. I put a LOT of time into this job and revised it a little bit to the way I build differentials. I use a solid sleeve where the others don't. It takes longer but I feel better about using it rather then nothing.

Solid sleeve fitting






Pattern





axles polished and painted






C Clip used instead of snap Rings



C clip had to be ground to size to setup 005 endplay in the axles.





Now I have to fit the new HD rear cover and it should be done
 

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Discussion Starter #19
I use 500hp as the borderline between a 10 and 12 bolt but there are some things to consider. The more power you have doesn't mean you need a 12 bolt. If you have st tires that will spin they will act like a relief valve to a point. given enough abuse either will break, more so with the 10 bolt then the 12 bolt.
Looking over the parts I'm working on now they are impressive and I would strongly consider them with 600+ HP and hard driving. If you are just going to push at the track very little and no hard street use then a blueprinted 10 bolt would work in most cases, keep in mind everything will break so be honest with yourself. The 12 bolt ratios are limited to 373 411 or 456. They are also a LOT more money.
I haven't used a Tremec so I can't say how they'll hold up. The OD is nice but not much good if the tranny comes apart at the track. Others here and on the other forums can be of more help with this. A lot of the track cars are using built TH400's.
Look over the threads above in the tech index, I cover the 10 bolt builds in detail.
 

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Thanks, Gary. With street tires, I think a strong 10 bolt will do nicely.

You are a huge help on this forum.
 
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