Swapping rear ends

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MikeGreer
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Swapping rear ends

Post by MikeGreer » Wed Mar 13, 2019 11:02 pm

Turned out the noise I heard last fall was the rear end. After burning through the third pig in a poke late style rear end, the consensus was get an early style and swap out the pumpkin with a new 3.9:1 pumpkin from AR. Jim Wagner had a donor I could have, so I took the bait. I ordered the new pumpkin and some other parts I wanted to change while it was apart. I picked up SS brake lines, Koni's, new axel bearings and seals, new u-joints, drive shaft bearing, an old style panhard rod, and some other stuff Csaba suggested that I would have never thought of.

The fun begins. I cleaned up the rust, pulled the axel shafts, pulled the old pumpkin, it was going well. Jim had also given me an old style drive shaft that's needed because the old style rear is 10 mm shorter than the new style. I needed to mate the two driveshaft pieces as the old one had spline damage. Done, piece of cake. I do however have a problem, I can't get the car in the garage and set up a place to swap parts to the donor rear, so the car sits outside in the driveway. It's either raining, cold as a well digger's ass, or both. But with the driveshaft out and lengthened, I'm committed.

The real fun begins after I get the old rear end out and sitting side by side withe the donor. I've had spiders over the years with both rear ends, but never realized just how different they really are. Keep in mind the donor was more or less bare and I thought I could transfer the parts I needed. So here's what's different: everything. 1)The new style has shock mounts built in, the older one has a special bracket I don't have. 2)The new style panhard rod connection to the axel is a stud welded th the axel housing at the shock location, the donor needs a bolt and is mounted 3" further outboard. That's why the panhard rod switch is necessary. 3)The late style rear uses a completely different mounting for the caliper holders, you need 5 mm shorter bolts. 4)The rear dust shields on the late style mount with 4 large bolts, the donor uses 3 small bolts. 5)The plastic spring seats are different, the step and locating pin don't match up. 6)The hard brake line is also mounted differently, haven't tried to move it yet.
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45 years with a spider, that's a long time
Restoring the '79 for the second time, you're never finished ;)
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jseabolt
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Re: Swapping rear ends

Post by jseabolt » Thu Mar 14, 2019 5:25 am

Maybe I misunderstood you but did you say you were transplanting an early style pumpkin into a later style axle? But you mention swapping all those parts so it makes me think you are doing the same thing I did ages ago.

The pinion gear broke in my 3.9:1 axle I had in my 80 model. I had a 4.1:1 axle from a 1970 model laying around and decided to use it instead of repairing my differential. Luckily I had the driveshaft, shock mounts and dust shields also from the 70 model. I seem to think I was missing the panhard rod but was able to buy a new one. The bushings in the old one would have been worn out anyway.

The pinion seal on mine was rock hard and was slinging oil everywhere, even on the rear bumper. I used the "paint reference marks and count the number of turns" method when replacing the seal.

I also had new U-joints and a new carrier bearing installed in the axle.

So far the axle seals don't leak which is a surprise. Or don't seem to be leaking. I guess if they were I'd be seeing oil on the rims. Just like the drums on my MGB were full of gear oil from bad seals.

I kind of regret using this axle. Later on when I did the turbo, I learned that turbos work better with higher final drives.

I have a 3.5:1 from a Brava I would like to use. The axle looks identical to the 124 axle but the trailing arm mounts are located in different spots and possibly the panhard rod mounts. Those could be removed from the broken 124 axle and transplanted to the 131 axle. The 131 used rear drum brakes. I don't know if the caliper brackets would bolt up to it.

Rather than breaking all these welds loose and hoping I re-weld them in the correct spots, it may just be better to transplant the 131 pumpkin to the 124 axle. I'm not sure if I feel confident doing that myself with pre-load adjustments and all.

First I would need to remove it from the car. Finding the time to do it is another thing. It's all I can do to find the time keeping what I got repaired, let alone replace something that is not broken just for the heck of it.
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Re: Swapping rear ends

Post by fp55scca » Thu Mar 14, 2019 10:32 am

--Mike, that's an excellent summary of differential change-out!

--While there were some improvements to the late style diff (the housing is stronger, and axle retention is simplified and more secure than the snap ring used on the early ones) the ease of changing the hogs head on the early style makes it the differential of choice IMHO.

--PS: Nice preservation job on your early diff! Looks like is could be POR (or equivalent)!
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 Fiat 124 Spider
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lanciahf
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Re: Swapping rear ends

Post by lanciahf » Thu Mar 14, 2019 12:18 pm

Jim Aren't the early axles lighter too? I wonder if that helps handling.
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1984 Pininfarina Spider
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cstorry
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Re: Swapping rear ends

Post by cstorry » Thu Mar 14, 2019 1:18 pm

James

The spider volumex rear end was a variant of the 132 rear end. The 132 rear end is a lot like the 131

I have a 132 diff I converted (well in the process - still need to weld the relocated brackets you mentioned) of converting for my volumex project.

One of the things I discovered is that the 132 was set up for rear drums. I had to have the end of the axles turned down as the brake discs would not fit over the end of the axle.

I also found that once I removed the brake drums I could mount the late 124 brake caliper mounts and then the calipers mounted up fine.

As noted the 132 does not have the lower shock mount built in so you need the separate bracket - I got mine from Midwest Bayless but probably available in multiple places.

... on the long road back to volumex ownership

Chuck
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Re: Swapping rear ends

Post by fp55scca » Thu Mar 14, 2019 6:03 pm

lanciahf wrote:
Thu Mar 14, 2019 12:18 pm
Jim Aren't the early axles lighter too? I wonder if that helps handling.
---------------------------------------
--Ralph, I believe that's probably true, as the later axles have a bit more cast iron in them. As for a handling advantage, I really can't say. I've only raced the early style, but I do have a "beater-1981" that I thrash around corners with the same reckless abandon as any my early Spiders! It could be that a slightly heavier diff helps balance the added weight of the 2L up front? But, I should also note that I lowered the stance otherwise stock '81 (two coils cut in the rear)...... improves handling imensely.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 Fiat 124 Spider
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jseabolt
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Re: Swapping rear ends

Post by jseabolt » Thu Mar 14, 2019 6:16 pm

cstorry wrote:
Thu Mar 14, 2019 1:18 pm
James

The spider volumex rear end was a variant of the 132 rear end. The 132 rear end is a lot like the 131

I have a 132 diff I converted (well in the process - still need to weld the relocated brackets you mentioned) of converting for my volumex project.

One of the things I discovered is that the 132 was set up for rear drums. I had to have the end of the axles turned down as the brake discs would not fit over the end of the axle.

I also found that once I removed the brake drums I could mount the late 124 brake caliper mounts and then the calipers mounted up fine.

As noted the 132 does not have the lower shock mount built in so you need the separate bracket - I got mine from Midwest Bayless but probably available in multiple places.

... on the long road back to volumex ownership

Chuck
So can the guts from the 131 be easily transplanted into a 124 Spider axle? It might be just as simple as pulling the pumpkin and as long as I keep the same parts, use the "count the number of turns" method and just stick it back in.
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MikeGreer
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Re: Swapping rear ends

Post by MikeGreer » Thu Mar 14, 2019 6:21 pm

It seems that I've often heard the 2000 sits an inch or so higher than earlier cars. The shock mounts on the late rear are about an inch higher than the mounts on the early one. Any correlation?
45 years with a spider, that's a long time
Restoring the '79 for the second time, you're never finished ;)
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jseabolt
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Re: Swapping rear ends

Post by jseabolt » Thu Mar 14, 2019 6:50 pm

MikeGreer wrote:
Thu Mar 14, 2019 6:21 pm
It seems that I've often heard the 2000 sits an inch or so higher than earlier cars. The shock mounts on the late rear are about an inch higher than the mounts on the early one. Any correlation?
I don't recall the rear end dropping any on my mine when I did the axle swap. I gave the 70 model springs to a guy and he installed them in his 79 and they did drop the rear end about 1-2".

So based on that, the ride difference is in the springs, not the mounts or the axle itself.
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Re: Swapping rear ends

Post by fp55scca » Fri Mar 15, 2019 10:42 am

MikeGreer wrote:
Thu Mar 14, 2019 6:21 pm
………. The shock mounts on the late rear are about an inch higher than the mounts on the early one. Any correlation?
----------------------------------------
--That's an interesting observation Mike. Can't say I ever really noticed that, but now that you mention it..... I can only guess that that there may have been a Federal bumper height change? I should take some bumper-height measurements of stock early bumpers vs late (79-85). Even thouhg the federal bumpers were introduced in '75, they may be a different height than the later '79-85?
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 Fiat 124 Spider
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