Rear Monoball Install Questions

[GAJon]

I am trying to install a set of Rennline rear mono balls on my ‘76 Targa and have hit a bit of an issue. I had the arms out and got the cartridges installed without any issues. Once those were in, I used a clamp to press the two bushing into either side.

I got the arms back in the car and pretty well aligned within the trailing arm mount. The issue I’ve run into is the bolt seems extremely tight in the monoball bushings. I measured the bolt at about 13.8mm and the bushing on the monoball seems to be about 13.5mm. Did confirm these are M14x1.5 75mm 10.9 bolts, which is the OEM spec.

I tried dry fitting a bolt on to the arm not on the car and it wouldn’t go in past a few threads without some mechanical help. Again, worried about damaging the monoball or the bolt itself..

If I try to force the bolt on, it will sort of go, but it seems to be deforming the threads some. When I removed the OEM bushings, the bolt didn’t seem nearly this tight.

Is this normal? Should the bolt be that tight in the bushing?

Edit: I removed one of the bushings and took a hammer to it and the bolt. I was able to drive the bushing down over the threaded portion of the bolt, but could not get the bushing over the unthreaded portion. It seems too tight or undersized for the bolt. For reference, Pelican sent me a MB12-Z, though the Rennline site shows a MB12-30. Maybe there is a difference there? I already sent an email to Rennline..

Are there different bolt sizes over the years? All the DIY’s and articles on install make it seem pretty straight forward.

[Dan J]

Should slide right in with very little force The bolt has always been 14mm
I wonder if the spacer is american 9/16 ?

[356911930]

Hey Jon,

I just measured the Elephant Racing rear trailing arm monoball and the ID is 14.02mm. This is just for reference as I do not know the design intent of the Rennline porduct.

Best of luck,

Rahl

[GAJon]

Thanks for validating the measurement. I took each of the bushings out and none of them are going in with either bolts without force. Sometimes a sanity check is needed when working on these old cars.

Also the MB12-Z part that Pelican sent me is listed as NLA (ordered back in November), maybe this has something to do with it...Currently shows MB12-30 as the part I need.

I guess I’ll wait on Rennline to respond directly.

I’m wondering if I got bushings intended for a 944/964/993 model. Those appear to have M12 bolts. Seems like a 1mm over tolerance is a bit wide, but maybe something got swapped out.

[GAJon]

Originally Posted by 356911930

Hey Jon,

I just measured the Elephant Racing rear trailing arm monoball and the ID is 14.02mm. This is just for reference as I do not know the design intent of the Rennline porduct.

Best of luck,

Rahl

Just talked to Rennline support, they confirmed ID of bushings is specced at 13.9mm on their drawings. So the parts I'm measuring are correct for my car as delivered from Rennline.

So either their design is too tight or my bolts are too big or both. Going to try and return and also get some new bolts.

[Eagledriver]

I have the same problem. Bushings are too small. I was planning to ream them to size, but it’s very annoying to get defective parts.

[GAJon]

@Eagledriver, I just received a new set of M14x1.5x80mm bolts from Pelican (Porsche brand). I tried both bolts on all four bushing and I can't get the bolts through any of the bushings. Talked to Rennline again and they are going to check their stock.

Sounds like they have a batch that were under tolerance if you're seeing the same issue. I don't have a way to ream these accurately, so I requested a new set of 4 from them to replace the ones I have.

Will let you know how they respond, but I'd recommend reaching out to them too.

[Mahler9th]

I am not familiar with the Rennline version of this type of part. And I do not know how many sets they have in the field, et cetera.

However I have been advised that it is a good idea to keep an eye on these types of monoballs-- from folks with decades of professional racing experience.

So whatever version you use, I suggest keeping a close eye on them as part of your regular inspection routines.

[Eagledriver]

Originally Posted by GAJon

@Eagledriver, I just received a new set of M14x1.5x80mm bolts from Pelican (Porsche brand). I tried both bolts on all four bushing and I can't get the bolts through any of the bushings. Talked to Rennline again and they are going to check their stock.

Sounds like they have a batch that were under tolerance if you're seeing the same issue. I don't have a way to ream these accurately, so I requested a new set of 4 from them to replace the ones I have.

Will let you know how they respond, but I'd recommend reaching out to them too.

Thanks. I’ll give them a call.

[wjdunham]

Originally Posted by Mahler9th

I am not familiar with the Rennline version of this type of part. And I do not know how many sets they have in the field, et cetera.

However I have been advised that it is a good idea to keep an eye on these types of monoballs-- from folks with decades of professional racing experience.

So whatever version you use, I suggest keeping a close eye on them as part of your regular inspection routines.

Why so - do they tend to bind up? I have Elephant Racing version in my street car, I don't believe there is a grease fitting for this bushing, but I could be wrong. Better check...

[Mahler9th]

My application is racing. I raced my 911 with factory bushings for over a decade... no problem. I converted to monoballs as a "while I was in there" project.

It has been a while since I have done research. About five years. At the time, a friend/expert advised caution regarding longevity of parts. He has a lot of experience with racing and working on Porsche factory race cars as well as cars converted from street versions used at pro level.

I think the monoballs from Eisenlohr Racing Products (ERP) have a lot of real world use... not sure about others. Not sure whether they are available any more.

I think it makes sense to keep an eye on whatever monoball product is used in that location.

[KTL]

The durability issue with monoballs is that over time the fit of the ball in the outer race/housing becomes sloppy. It depends on which type of monoball you're using and how much slop you'll experience. These rear monoball cartridge inserts typically use a teflon-lined spherical bearing. Over time the teflon liner (it looks like a piece of fabric if you look closely from the side) gets pounded and squished and that allows for some slop. So it's good to take a close look at these inner pivot points every so often to check for excessive play. Put the car on stands and place a floor jack up against the trailing arm near the pivot point. Should be easy to see if there's any significant movement of the ball.

The trailing arm spherical bearings seem to have OK longevity because they're loaded in a proper fashion- they take load in the radial direction, which is mostly perpendicular to the bore. Other spherical bearings like those used in the front upper strut mounts tend to wear out sooner because they are loaded in an axial direction. The axial load is attempting to push the ball out of the housing through its opening.

[TimT]

The durability issue with monoballs is that over time the fit of the ball in the outer race/housing becomes sloppy.

Yes, The Teflon liner basically gets hammered out by the loads it sees.. and disintegrates.. This allows the joint to move much more and progressively the tolerances and alignment go out the window...

It seems the failures increase as horsepower goes up (go figure), on big power cars we got into the habit of changing the insert once a season or more.. just check the joint for excess movement every time you nut and bolt the car.


Oddly the monoball on the spring plates (if you have 935 style) seem to last a good long time...

[GAJon]

Originally Posted by TimT

Yes, The Teflon liner basically gets hammered out by the loads it sees.. and disintegrates.. This allows the joint to move much more and progressively the tolerances and alignment go out the window...

This is partly why I got the Rennline version. They are rebuild-able with new spherical bearings. Don't have to take the cartridge out of the arm and could probably do it with the arm still hanging on the car.

Now if only the bolt would fit through the bushings... @Rennline did reach out and said they are working on a solution, just slow due to staffing levels.

Eventually will have this car off of jack stands and out of the driveway.

[GAJon]

Quick Update, @Rennline did check their stock and found that some were undersized. They are overnighting me a new set of bushings as well as updating their stock.

Frustrating issue, but great support from them in getting it fixed!

[Mahler9th]

Outstanding. Rebuildable-- that is great. And great to hear about the customer service.

Thanks for posting.