57 replies to this topic

[Spider]

Correct me if I'm wrong, but the only time this primary gear is spinning on its bushes is when your foot is on the clutch ? In which case how can a more powerful mpi engine do more damage than a standard one ?

 

And during such times, the bush is un-loaded, so HPs / Torque isn't a factor.

 

Where they supposed can go though is from the pressure on them, though, again, the factory found this back in the mid-60's and addressed it. Standby and I'll post up the info.

[Spider]

 

<EDIT: In respect of what AC has suggested in regards to this mode of failure, I am keeping an open mind on all this and certainly am not discounting it. It is a possibility that is included in our list.>

 

I'll also add that I'm yet to see a damaged crank from a Deva bush wearing or failing >

 

<2nd Edit: AC, many thanks for your additional info on clearances and your experience, cheers >

Edited by Moke Spider, 23 May 2016 - 10:51 PM.

[pete l]

Any idea what my failed bush is made of ? 

 

 WP_20160517_07_39_40_Pro.jpg   26.59K   26 downloads

 

 

[ACDodd]

That's a steel back factory bearing (FRONT BUSH).

Torque and gearing is a factor. When pulling away and the clutch is slipping. Heat is transmitted through the clutch to the primary gear. It picks up on the front bush.

Ac

[pete l]

OK AC, thanks for all the details etc. 

 

What about MG metro TURBOS ? 

 

I had one from new in 1987, I sold it with 125000 miles on the clock, Never had a problem with the primary gear, clutch yes, gearbox bearings yes, engine and turbo gave no problems at all. Surely the metro turbo has a lot more torque than an MPI engine ?

[ACDodd]

It also has a lot lower gearing and different oil spec. It also had a much stronger clutch. Factory turbos only had 85lbft of torque.

To supply and fit bushes to a seviceable primary gear usually around £95plus shipping. The problem here is the crank tail needs to be inspected polished or ground depending on the condition. Then the gear bushes need to be sized to suit the crank. Ideally the crank should sent to the person machining the bushes so there can be no argument regarding the running clearance.

Ac

Edited by ACDodd, 24 May 2016 - 11:30 AM.

[pete l]

I'm up for that, I will remove the crank and let you do your magic on it and the primary gear..

 

Pete

[Spider]

I've had some correspondence back from our bearing designers on this.

 

They have said that the bronze based materials should not be used for this application, the surface speeds (RPMs) are way too high for dry running. Some Bronze Bush materials can withstand dry running up to (what works out to be) 410 RPM on a continuous basis ramping up to a max of (what works out to be) an absolute upper limit of 2940 RPM for very short times and allowing for a cooling period between. They did say that increasing the clearances will reduce the likelyhood of siezure, however it does not remove the high wear rate that our crankshafts would see as they only have a hardness of 240 - 300 HB. If that were increased to around 800 HB or higher, then this would be less of an issue (off hand, I think that's getting in to Nitrided territory).

 

For the most part, this confirms my own findings and what we are all seeing.

 

While they didn't spend time on it, they did say that if pressure fed with Oil, it would be highly likely satisfactory.

 

They did suggest a couple of materials, however, curiously, they suggested Deva as being the best option.

 

AC - based on this, I would express caution at what you are doing, though I'll leave that to you. I feel you've had a run of good luck here.

 

I've also had some correspondence back from Deva. Due to the high ranging applications and grades of the material as well as their (usual) short lead times for manufacture, they don't hold it in stock. They are coming back to me on this.

[ACDodd]

Thanks for the update Moke. I will ride my luck till it ends.

Ac

[Spider]

Any idea what my failed bush is made of ? 

 

WP_20160517_07_39_40_Pro.jpg

 

 

 

What's happened here is the Rear Bush (the other one) has worn (wrong bush material) and allowed the Primary Gear to skew over. This then wore this bush on it's Rear side (flywheel side), you can see that because the is still some bearing material left in it on one side but none on the other. The Bush is steel backed and after all the bearing material had gone, it's been metal on metal contact resulting in a damaged crankshaft. The bearing material on these bushes, while excellent, is only thin, it doesn't take much to wear through them.

Edited by Moke Spider, 28 May 2016 - 06:29 AM.

[pete l]

No, it's an original primary gear with deva bushes.

[pete l]

Look what I've just bought.

 

 primary.jpg   34.85K   39 downloads

[Spider]

We have another 'winner'. Total of less than 2000 km, and clearance was set (so I'm told) at 0.005"

 

[pete l]

That looks better than my crank !

[RobbieP]

Sorry to bring up an old post here but I'm having this problem. I found this video on Youtube and I'm wondering if this solution will help with the  bushing issue or not.. 

 

 

and just to be clear, the best option for replacement is the Deva bushings front and rear? 

 

thanks