rear diff pin mod for high hp
 

i am calling this the rohosnyper mod

I was about to buy a 9" kit for $6k and had several people tell me to give the stock one a chance. rohosnyper currently has the best 60' on the list at 1.33 and is running a stock rear diff other than being pinned. he told me he hasn't had any issues since pinning. you can see in the pics the small weld that holds the ring gear to the carrier. i am told this is the weak link.

i purchased a spare rear diff off of ebay for $500. i also had to get a e22 torx female socket and a slide hammer. the e22 socket was too fat and i had to grind it down a little you can see it in the pics.

with the slide hammer the axle stubs came right out. with the e22 socket the side cover came right off. after that the carrier just lifts out. after taking a look at things i am going with 8 pins 1/4" x 3/4". i got the alloy steel ones from McMaster-Carr. i also ordered some .248" drill bits for a snug fit. i am going to put the bmr bushings in while i am at it.

right now i am waiting for the pins but probably wont get back to it for a couple of weeks.

I use dowels daily and if you try to press a .250 dowel in a .248 hole you will probably break the dowel as they are hardened. I'm my trade we drill a 15/64 hole and ream 1/4" the drill may work because the hole wont be round. PM me your address and I'll send you a reamer dowels and some locktite if you want.

Hey a pro. Great! I don’t know what you just said.

Start with a 15/64 hole, ok

Ream 1/4”? What does that mean and how is it different than drilling a hole?

What is a reamer dowl?

I believe rohosnyper welded the pins in after pressing them in. See any issue with that?

I planned on doing a test on a chunk of metal before I started drilling holes in the diff. Thanks for your help.

I just looked up a reamer. Looks to be a fancy drill bit that gives a rounder hole and is .2495”. That makes some sense to me now. Your saying my holes are too small but might work because they won’t be all that round?

Any suggestion on number of pins or pin material?

Ok - before you start drilling holes here, let me interject.

I work as technical support in a large industrial supply company for machining customers, and have been making parts since the early 90s.

First thing you should consider it getting a way to hold the surface you want to drill square and locked solid so it doesn't move because you will need three tools to make the hole. A good spot drill, a good drill, and then the reamer. Don't just throw a drill in a DeWalt and go to town, I would suggest a drill press at least and some kind of jig / fixate to hold it solid and still while you change tools.

Second, when you drill thru the carrier and into the ring gear, you might have issues. Ring gears are heat treated high alloy steel, usually in the upper 50s on a Rockwell C Hardness scale. Hardware store drill bits are not going to do this, they are around 60-63HRC. Your gear is probably 57-59HPC, so basically the drill and gear are the same hardness. You will not make one hole.

You will need cobalt at least, a way to cool and lube everything while cutting, and you will need to go slow.

I would highly suggest a spot drill - this will ensure a strait hole. Cobalt, 5/16" or larger - https://www.mscdirect.com/product/details/58816141

If you have a good solid setup that won't let the part move, you can use a 6.1mm drill - which is only about .009" smaller than .250". However if things are moving you will end up with a "tri-lobe" hole that will be bigger than .250"

For a drill - https://www.mscdirect.com/product/details/80994221
Run the drill no faster than 500rpm, or you will burn it up in steel that hard. Use oil.


For a reamer - https://www.mscdirect.com/product/details/30145577
Run the reamer no faster than 380rpm. Use oil. Any faster and you will end up welding the reamer in the hole.

In my god honest opinion - dowling the gear to the carrier is not going to do much if anything, with the surface area and I assume a press fit (the ring gear was probably heated to go on the carrier), it should be the last thing in that differential that moves. It would be as if both pieces were made of one.

Wow! Great info and thanks for all the links! I have a drill press and can make a jig to hold the carrier solid. What is the purpose of the 5/16” spot drill? Should that be 3/16 and you are suggesting a 3 step drilling process?
Edit: I just looked up spot drills. It just helps to locate the hole. I need to get better at looking things up before asking questions.

What kind of oil should I buy?

Do you see any issue with tigging that pin to the carrier to ensure it stays put?

I don’t know about how much it helps other than one of the quickest guys out there has done this mod. I was told he ripped the ring gear right off the carrier on a stock diff. It isn’t a ton of money vs $6k for a 9” so I am going to give it a try.

Wow great info on drilling. I used to dowel my flywheels on my Toyota 3TG as it would just sheer off any type of flywheel bolts.

I have not seen the ring vs carrier, would it make more sense to weld more area or is the ring fully welded on the outer and inner circumference?

It is fully welded on the outside. No weld at all on the inside that I can see. It looks like I could weld about 3” sections in a couple of spots, maybe more. Hmm, might be worth a try. I would need another expert to give me some pointers.

You want to create a spot larger than the drill itself. Drills are meant to make a chip from the center out, not from the outside in. If the spot is smaller than the drill, when the drill first contacts the surface, it will be touching at the top of the spot before the center.

Cutting Fluid - hands down - Castrol Moly-Dee. Its not cheap, but a little goes a LONG ways and there is nothing out there that works as good, except for aluminum. Use Crisco for that.

It looks to me like the entire outside diameter where the carrier meats the ring gear is induction welded. Induction welding is where they would heat a very specific, small area on one or both parts with a electric coil. It can be done fast and quick enough where it won't screw up the hardness of the base metals.

As far as welding the dowel - if there is a good press fit it would be stronger without.

Yea - adding a dowel could help, but it is just going to keep the potential of the ring from wanting to "turn" on the carrier. My concern is that dowel pins are hard. You can take a 1/4" dowel that is 2" long, put it in a vice, and hit it with a hammer and bend or break it. Ive broken plenty in my life LOL

Reading into it - the standard SS housing suffers from a problem that the case halves want to split apart from each other on hard launches. The pinion wants to "walk" on the gear, and it is easier to actually have it just pry the case apart.

I own a 1LE, which has the ZL1 rear diff so I guess I haven't paid attention to the fact how much different the two look.I have no idea what it would take to put a ZL1 center section into a standard SS, but that may be the right way to go here IMO. The case is way different. I see used center sections are $800 on ebay. However I wonder if things like the axles, rear cradle, etc... are all the same? That and the 1LE has a e-diff, so no idea if any of that is retrofittable or can be swapped with a standard clutch setup.

Ring gear size isn't the only thing that adds strength to a rear end. I see there is a company that makes a 9.9" ring gear center section for the 6th Gen IRS. However the location above / below centerline of the pinion in relationship to the center to the axle is actually just as important. The farther offset they are, the more strength there is. It looks like the SS diff is pretty much on center, where as the 1LE/ZL1 diff is maybe 2"?

Something like this might be something to consider? This is a 1LE 3.73 diff, axles, arms, cradle, etc.... hell it even has the rear mag shocks

https://thepartsfarm.com/products/20...l-assembly-lt1

I took the liberty here to add some text to your picture - the arrow to me indicates the "weld". If that diameter is 8" in diameter, we are talking what, 25" of linear weld? Not sure if a couple of inches of MIG spot weld will be much help?

I don't want to open up that can of worm.

thanks for the help. I am going to use the tools that you suggested and see how it goes. worse case it blows up on the line and I am out ~$600 and still have to get a 9". if I can get into the 1.3x 60's I will be thrilled.

Parish the mad scientist is at it again. Good luck

I am just copying rohosnyper. Thing is he couldn’t remember exactly what they did. Lucky for me I got some experts chiming in.