CNC Diff mount

[MrGumby]

I finally got a chance to try out the CnC rear diff mount of James' (jimjim69) that was the subject of so much controversy a few weeks back. Let me first start off saying that, in spite of his online shenanigans, Jim is actually a pretty decent guy that really knows his stuff when it comes to E21s so when he offered me a dogbone to test out & give my opinions, I jumped at the chance.

First impression of it was "damn, this thing is no joke"! Seriously, it makes the stock piece look like a bargain basement Chinese knock off. And it's not just a block of aluminum with a couple holes cut into it for some bushings, it's really a beautiful piece. If you were so inclined, you could buy it to keep on your desk as a paperweight.

To be honest, I was a bit skeptic as to if it was even remotely necessary so I decided to conduct a quick test. After I lifted my 83 up in the air, ahh the beauty of having a lift to my disposal, I grabbed a hold of the diff and gave it a wiggle up & down to see what kind of play I had. There was more than I really had anticipated, it moved at least 3/4 of an inch and with the dogbone completely removed, 2-3 inches easy. Since the front part of the diff is bolted directly to the rear subframe, that translates into unwanted trailing arm movement as well and nobody likes that.

Fitment is spot on, with the only speedbump being that the bore for the mounting bolts were 10mm instead of 12mm so I ended up having to source some proper size hardware. I found that 2NZ headbolts have the perfect size shoulder to fit the bore of the diff cover and are grade 8 so shear factor is no worry. I brought this to James' attention and he will be sourcing some larger bushing sleeves this week and the "production" parts will have the correct size installed.

With the jimjim unit installed, the vertical movement is completely gone. No matter how much force I put on the case, it just wouldn't budge. Given this and the urethane bushings, I was expecting all sorts of new noise and harshness to come through when driving. Much to my surprise, there was none. In fact, it cured the *thunk* I got when popping the clutch a bit too sharply as well as the *growling* when I accelerated from too low RPM in too high a gear. "Right on", I thought, "improvements felt and I'm not even out of the parking lot yet"! No more wheel hop when I "drift" the U-turns and the rear is more firmly planted when cornering hard. Theoretically, I could see the end of wheel hop with straight line racing, but unfortunately I'm lacking enough power for that to be an issue though you M20 and turbo guys would likely see an improvement there. Even my wife noticed an improvement in the way it handled.

A great overall impression of the part. Excellent craftsmanship, no-brainer installation and worthwhile improvements to ride and handling as well as heavy bling factor. For those that are interested and don't already have jimjim's off-forums contact info, feel free to PM me and I'd be happy to pass your information on to James.

My camera decided to go south on me when I was installing the part so I'll have to update the thread when I get pictures of it installed. Here's a couple of teaser pics in the meantime.

[epmedia]

James delivers, again
I have some wheel hop, on the track (once) and reverse up the driveway.
I'm a "broke-ass" otherwise I'd be in for the piece too.
Right on, James

[fullswing]

Awesome news! Looking forward to more info.

[MrGumby]

Seems I can't edit my post from my iPhone. Price is $150 shipped

[TheNeek]

I think the important point here is rubber bushings vs. poly. Not steel vs. aluminum.

[Tom D]

i would also question the condition of your old rubber mounts. mine are brand new and i have no play (movement) let alone 3/4".

[Jester323]

That looks like a pretty amazing piece there James.

agreed on the urethane vs aluminum (I'm kind of a fan of urethane though)

Also, urethane modified rear subframe mounts would really help you if you are getting that much play in the subframe with the dogbone disconnected.

[blitzed310]

3/4" movement is non existent on my stock piece, 3/4" is very . Sounds like yours was completely shot.

[fullswing]

and there's the comparison we usually forget to make.

old rubber is always worse than new rubber, let alone worse than urethane.

where urethane wins (holding design constant) is longevity from resistance to breakdown from heat and oil.

when the design is better (for example, jester's control arm bushings, etc), urethane wins out big time.

[77 320i]

That looks like a pretty amazing piece there James.

agreed on the urethane vs aluminum (I'm kind of a fan of urethane though)

Also, urethane modified rear subframe mounts would really help you if you are getting that much play in the subframe with the dogbone disconnected.

Jester...are you pouring urethane replacements for the rear differential hanger? thought i would ask before ordering, or trying to hunt down a quart myself..PM me and let me know thanks!

[blitzed310]

Jester...are you pouring urethane replacements for the rear differential hanger? thought i would ask before ordering, or trying to hunt down a quart myself..PM me and let me know thanks!

I'm not sure if he is, but you can use the urethane from McMaster-Carr, I made 5 rear subframe set-ups already. The rear carriers seems to be very simple enough to make an in-place mold.

http://www.mcmaster.com/#liquid-urethane/=f18deh Use either the 80A or 94A, I used the 94A.

[77 320i]

blitzed310...thanks, yeah...i think the forsch 60a (medium/hard) is actually the liquid urethane of preference...i can do it and i will if necessary...BUT...ordering the product, setting up the form, blah, blah, blah...i'm just hoping for a quick exchange because i have the transmission out and i'm about to lift it back when the throw out bearing and sleeve are delivered...now that the subframe is finally buttoned back up...time is a big consideration...i STILL have a ton of work before this thing back out on the streets...and i'm not using 'house money'...so!

[jjgbmw323]

Does the ALPINA finned Diff work with this? Can I see pics of the install?

[TheNeek]

can i see pics of the install?

lol

[jrcook320]

Does the ALPINA finned Diff work with this? Can I see pics of the install?

No. The alpina diff cover doesn't use a dogbone, the bushing is pressed directly into the cover.

[blitzed310]

No. The alpina diff cover doesn't use a dogbone, the bushing is pressed directly into the cover.

And there is an additional mount, one on each side.

[TheNeek]

I need to find an alpina cover

[hakhawk]

I need to find an alpina cover

you could just shell out 800 big ones for the vintage parts replica!!

lol, i kid i kid.

[LJ851]

I need to find an alpina cover

There is a possibility i could be persuaded to part with mine, how badly do you need one?

[77 320i]

wow...this is amazing...we have a member that doesn't type english sentences in such a way the backyard average mechanic can understand,...he then goes on to get grief for it continuously cause no one can understand a SINGLE post he's EVER made...why, he keeps drinking the bong water...(meanwhile its rumored he is a near mechanical genius when it comes to e21's)...the sage continues when he produces a somewhat unnesessary "racing" part that is actually filed tested by an actual credible member...then the part won't fit on a real racing differential for our cars...that same poster causes enough turmoil to get bounced...and now chaos and confusion persists despite he's gone...lol...then said racing parts begin near an auction process that competes with the costs of my entire car...why? "cause racing parts are cool"...i think that if we could get someone to type this up (preferrable someone with a southern california connection!!)...in a somewhat sensible fashion we could make enough money off the screen play alone to pay for EVERYONE's car to be featured on the calendars for 2012!!!! its just an idea...lol...

[MrGumby]

Wow, sure do miss a lot when you don't check in for a day! So for all fairness I probably should have compared the urethane mount to a factory fresh one, but that wasn't an option. I guess the key element to the added performance would be more the urethane as opposed to the aluminum bit, but I could see a need for the CnC portion. When I had my Volvo 850 T5-R, I broke the metal frame of the upper dogbone mount launching too hard racing an Audi so for me, the added insurance of the billet piece compared to the flat steel unit is worth it. Not that I have the power to snap even the stock piece...yet. If nothing else, it helps with the bling factor.

[Jester323]

blitzed310...thanks, yeah...i think the forsch 60a (medium/hard) is actually the liquid urethane of preference...i can do it and i will if necessary...BUT...ordering the product, setting up the form, blah, blah, blah...i'm just hoping for a quick exchange because i have the transmission out and i'm about to lift it back when the throw out bearing and sleeve are delivered...now that the subframe is finally buttoned back up...time is a big consideration...i STILL have a ton of work before this thing back out on the streets...and i'm not using 'house money'...so!

Actually, the new OEM rubber is about shore 75-80A, 60A would be very soft. I've spoken rather extensively to the folks over at Forsch Polymer, and not all hardness grades will have the same longevity, in fact, there are some that they have told me that they would approve the use of for a mock-up, but not for a permanent automotive part, 80A is one that I wouldn't use.

I've made in-place bushings for the dogbones in the past, and honestly I won't do those the same way again. I'm actually half way done with my mold for these. Out of respect for James, that will be all that I am going to say about that in this thread. I will however state that James' product here looks like a vast improvement over the OEM part for those who are more performance minded.

[77 320i]

I've made in-place bushings for the dogbones in the past, and honestly I won't do those the same way again. I'm actually half way done with my mold for these. Out of respect for James, that will be all that I am going to say about that in this thread. I will however state that James' product here looks like a vast improvement over the OEM part for those who are more performance minded.

Jess, i didn't quite understand everything you said here regarding your ability/readiness in your post...are you completing the forms for OEM/steel differential supports filled with urethane in a time frame that will permit accepting an order? if so, i'm ready to order them and will need your address again to begin the exchange process...i didn't keep your contact information from my last purchase...

in regards to james...i didn't see the posts that were removed by the admin so i can't comment...however, let me clarify that my comments above were NOT intended to be demeaning (just for the record) but rather it is quite amusing to watch (virtually) all of the back-and-forth that has happened and THEN he fabricates something some seriously consider useful...i run an m10 so i'm not thinking its an application for me necessarily...but the motivation for my comments were there are a lot of diverse personalities in this forum and watching those dynamics interact are to say the least...interesting. let me know.

[jjgbmw323]

you could just shell out 800 big ones for the vintage parts replica!!

lol, i kid i kid.

Yes I have the replica part and I paid nothing like that. Thanks for telling me that the cnc part is not the right one for the cover. The cnc part does seem to me to be a good part with excellent workman ship.

[Jester323]

This thread is about James' piece, not mine is all.