8 3/4 sbp upgrade strength

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downundertdart

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hi all. I’ve bought an pirate jack front and rear kit disc brake kit for my 68 gts, thinking that (or when my father ordered it for me - and talked to them) that it would upgrade the rear bolt pattern to the bbp. So looking at the instructions now, it’s looks to be that it can be both for bbp and sbp so if that’s the case they don’t actually provide the solution.. so anyway what I’m trying to ask is, drilling out the axels to bbp, how much power have people put through them or 1/4 mile times ...

just trying to get out of new moser or dr diff axels after already spending prob $500 on the rear for the disc brakes there ... thanks for the help...
I head to Hawaii (from Australia and where the dart is) in like 4 weeks so trying to see about shipping something there (if needed ) compared to the cost of shipping to Australia... or hopefully not have to buy any yet...
Thanks again
Luke
 
All my cars are small bolt pattern never had a problem i run stock steelies, but if you want other styles of rims then BBP is the way to go. that would be the only benefit i would see over sbp
 
.. ah sorry yeah the kit I bought already converts the fronts to bbp, just after I suppose the answer to how much power/abuse drilling out the rears to bbp will they take before snapping?/breaking?
 
My axles were drilled and tapped for 1/2" studs 5x4.5 at a machine shop. 375hp and at least 1k passes with drag radials. I've found the ring gear is the weak link on my car, cracked the base of ring gear teeth twice after many passes.
 
People run 8s with the small bolt pattern. It's not about strength with the big bolt pattern, it's about a larger wheel selection. That's all. You're only talking about a 1/2" bigger bolt circle. That's 1/4" per side. Sure it will be some stronger. I've been messin around with these cars since 1974 I have yet to see one shear off all five wheel studs because the bolt pattern was too small.
 
Big or small its keeping the lugs tight that prevents breakage.

If your racing the lugs should be checked before every race. If I swap tires I also check them during. Street cars should also be checked periodically.
 
.... so pretty much keep lugs tight, and then once redrilled, pretty much gonna be splines etc that will strip/brake -not the redrill part of it...thanks very much..makes me feel a bit better for buying the rear brake kit !
 
If you're doing the stock axle re-drill, I believe you'll need screw in studs.
Not much left out there for press in...……...
upload_2019-8-15_9-16-36.png


My old 7/16" - 4" pattern with Cragars were constantly loosening. IMO, Their washers were junk in most cases and the area they have under the lug hex was just too small. It was a great day to get rid of the 7/16" - 4" pattern front and rear for me.

A "re-drill your drums" Moser for comparison to a standard. A thicker and larger flange diameter using press in studs...

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My 408 stroker Dart with 540 ft/lbs or torque and 28x12.5 MT's with a 4 speed. No problems with the small bolt pattern. I'm running Moser axles. Rims are steel 15x10 in the 5x4 bolt pattern.
 
I think about half the people commenting missed the OP’s point. He’s ditching the SBP and going to BBP, that’s not what he’s asking. He wanted to know if re-drilling the factory SBP axles to BBP creates a strength issue, ie, is the axle flange strength an issue with the re-drilled axles.

I think all you have to do is look at the re-drilled axle flange Daves69 posted to see that strength is definitely lost at the flange. Obviously it still works as plenty of people have done it, but that isn’t much material on the outside of those lug studs. I for one would just buy new axles. You’re increasing traction, you’re increasing braking forces, you’re increasing horsepower. All of that you’re going to put through a 50 year old axle that’s been made weaker than it was originally.
 
Metal fatigue is a funny thing, there is no predicting it. It is possible that you will redrill to BBP and never have an issue, it is equally possible that you will do it and something will let go on your first run. It's a Mopar 8.75 so it's one of the strongest OEM rears ever made, with that said.... once you start changing things, you've changed things. The rear quarter panel you save could be your own. I agree with 72blu = = word for word!
 
Other thing to remember is that the existing holes need to be properly welded up. The mounting flange face might get warped, or otherwise no longer be flat. Which requires re-machining of the face.
 
Unless it's been mentioned and I missed it.....there is a strength difference between the small and big bolt patterns because aren't the studs 7/16" compared to 1/2"?

If you get aftermarket axles with the big bolt pattern, you can upgrade to the more readily available E & B body brakes as well. The correct offset drums for the small pattern combo are rare. There is the brake drum register you have to consider when changing things around as well as using the proper machined axle bearing location vs brake assembly.
 
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69_340_GTS said:
Other thing to remember is that the existing holes need to be properly welded up. The mounting flange face might get warped, or otherwise no longer be flat. Which requires re-machining of the face.
Click to expand...
Best to get new axles.
 
Considering the carnage that WILL HAPPEN if an axle fails, new axles are the best bet.
 
i had a set done many, many years ago by a place that made axles, and had done a lot of these over the years. they told me good for about 400hp at the crank on a 9" slick.
 
... currently engine is a 360 with a mopar purple cam and 3200 stall, but I’m inheriting my fathers twin turbo 360 engine made 540hp at the tyres(he’s going to an hellcat engine ) ... so then if I did get new axels who’s best or cheapest but good ? Dr diff seems cheaper than moser and haven’t looked anywhere else yet... they’re too dear to get custom made here $900 -1300 aud.....while I think online with shipping to Hawaii which I can pickup up in 4 weeks time will be $650 Australian
 
... so just had a look and talked to dr diff, the rotors that come with the pirate jack kit have the bigger axle centre (is it a flange(on the axle face?) .. so can I get a spacer made up to take up the distance between that inner flange and the rotor centre hole? (That’s if I don’t get new axles made up)
 
Locomotion said:
Unless it's been mentioned and I missed it.....there is a strength difference between the small and big bolt patterns because aren't the studs 7/16" compared to 1/2"?

If you get aftermarket axles with the big bolt pattern, you can upgrade to the more readily available E & B body brakes as well. The correct offset drums for the small pattern combo are rare. There is the brake drum register you have to consider when changing things around as well as using the proper machined axle bearing location vs brake assembly.
Click to expand...

Correct, the factory 5x4” axles used 7/16” studs, 5x4.5” axles use 1/2” studs. But again this is moot, the OP is using the BBP to match his front disks.

He also already has a rear disk conversion.

mbaird said:
I ordered my Dutchman axles with both bolt patterns.
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Why? The axle flange standoff can only be right for one of those bolt patterns.

Which means if you switch the pattern, you have to re-drill the drums for whatever brakes match the axle flange standoff. More than likely those are small bolt pattern axles (have the small bolt pattern axle flange stand off), which means you’re still stuck using SBP brakes and re-drilling the drums if you go BBP.

downundertdart said:
... so just had a look and talked to dr diff, the rotors that come with the pirate jack kit have the bigger axle centre (is it a flange(on the axle face?) .. so can I get a spacer made up to take up the distance between that inner flange and the rotor centre hole? (That’s if I don’t get new axles made up)
Click to expand...

The hub register on the small bolt pattern axles is a smaller diameter than it is on the big bolt pattern axles. You can see the hub register on this set of Dr. Diff axles and how it fits tightly to the center bore of the drum. I also took a picture of the bare axle flange so you see how much material is outside of the studs for a true BBP axle.

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The other thing is, I don’t know how wheel spacers would be illegal there but a spacer between the axle register and the rotor wouldn’t be. It wouldn’t be hard to make, it would just have to make up the distance between the outer diameter of the axle register and the inner diameter of the rotor. But I don’t see that getting approval there if a plain old wheel spacer doesn’t.
 
Correct. I will be drilling my drums for the large bolt pattern as well.
If in the future I decide I need more brake I will switch to discs and shim accordingly.
 
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