Trailing Arm Odd Question

[KMoore68]

Hi,

I was hoping for some clarification the problem I am having with my 1977 911 S Targa.

Earlier this year, the threads pulled on the trailing arm, and I ended up sourcing a replacement locally.

In looking at the shock bolt length and comparing the old and new trailing arm, it seems that the bolt I have is the wrong length but it worked in my original trailing arm.

For clarification, original trailing arm part number 911 331 513 0R
replacement trailing arm part number 911 331 513 0R

The bolt length on the official parts PDF that I have lists it as M14 x 1.5 x 75MM however my bolt is M14 x 1.5 x 85MM.

I'm a bit stumped why my original trailing arm bolt length is different even though the part number hasn't changed.

I'll probably end up ordering the 75 MM bolt from my local Porsche dealership but was hoping someone here could explain why.

Thanks,
Kevin

[dtwa]

The 84-86 PET lists almost the same part number (haven’t seen a Porsche number end with 0R before)):

911 331 513 06 Rear-axle trailing arm /L 1 CARRERA
911 331 514 06 Rear-axle trailing arm /R 1 CARRERA

And specifies an 85mm bolt.

900 082 054 02 Hexagon-head bolt M 14 X 1,5 X 85

Just another data point.


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[dtwa]

I would run the longer bolt to get the extra thread engagement. —Dave


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[KMoore68]

Originally Posted by dtwa

I would run the longer bolt to get the extra thread engagement. —Dave


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I actually can't run the longer bolt, it bottoms out and there is a gap between the end of the shock and the bolt.

I need to run the shorter bolt.

[nickelplated5s]

That's what washers are for.

[phelix]

Any chance there’s something in the new-to-you trailing arm’s shock absorber mounting hole?

[Dpmulvan]

There’s 3 different length bolts.

[Dpmulvan]

An 85mm, 90mm and a 130mm.

[Dpmulvan]

76 and 77 used the longer 130mm bolt.

[Dpmulvan]

Originally Posted by KMoore68

I actually can't run the longer bolt, it bottoms out and there is a gap between the end of the shock and the bolt.

I need to run the shorter bolt.

Shouldn’t bottom out with the correct washers.

[KMoore68]

Originally Posted by Dpmulvan

An 85mm, 90mm and a 130mm.

The Porsche PDF I have calls out a 75 MM bolt, for a 74-77 911 Trailing Arm,

Does that mean there are 4 lengths?

I'm assuming the part numbers aren't the same for the trailing arm.

[KMoore68]

Originally Posted by Dpmulvan

Shouldn’t bottom out with the correct washers.

10MM is quite a difference to make up with washers.

[Dpmulvan]

Originally Posted by KMoore68

The Porsche PDF I have calls out a 75 MM bolt, for a 74-77 911 Trailing Arm,

Does that mean there are 4 lengths?

I'm assuming the part numbers aren't the same for the trailing arm.

[KMoore68]

I saw that image in the PDF but it doesn't mimic the connection to my trailing arm, that image has a bolt and nut where my trailing arm only has a bolt.

[Dpmulvan]

Originally Posted by KMoore68

I saw that image in the PDF but it doesn't mimic the connection to my trailing arm, that image has a bolt and nut where my trailing arm only has a bolt.

Those are washers not nuts.

[KMoore68]

Sorry for the confusion, the pic you posted matches the 78-83 PDF that I have, in the 74-77 PDF, it shows the bolt, two washers and a nut.

My original trailing arm only had a washer on the outside of the bolt, and nothing on the inside.

None of the images I have seen online also have a nut on the inside of the bolt connecting to the Trailing Arm, so i'm a bit confused.

[Dpmulvan]

Originally Posted by KMoore68

Sorry for the confusion, the pic you posted matches the 78-83 PDF that I have, in the 74-77 PDF, it shows the bolt, two washers and a nut.

My original trailing arm only had a washer on the outside of the bolt, and nothing on the inside.

None of the images I have seen online also have a nut on the inside of the bolt connecting to the Trailing Arm, so i'm a bit confused.

My mid year had 75mm My SC 85mm. Perhaps your original 77’s were a transition to the next year 78 when the SC came out and your new used ones are the earlier shorter version. That’s all I can think of, I’m sure someone here has a lot more info than I do.