[ducatijack75]

I think have a warped rotor and looking for the minimum thickness. The bike is a 1975 860GT and has 77k miles and thickness is now 5.918mm. Thanks: JB

 

[rick81mhr900ss]

My 81 MHR discs are 6.5mm thick front and back. I can't find bevel disc thickness tolerances anywhere but I have found new stainless discs being sold that are 5mm thick suggesting that from a worn pads perspective, 5mm is not an issue. Do you have an idea of how warped the disc is and how much thickness you will lose by machining it?

 

[ducatijack75]

Thanks, the outside edge is 5.35mm where the pads do not touch so there is some wear. I don't know how much the warped is so I was going to have it turned. I did find my Cagiva ,650SS, has a min thickness of 5.80mm. I dont know if there are the same type.

 

[ducatijack75]

Thanks, the outside edge is 5.35mm where the pads do not touch so there is some wear. I don't know how much the warped is so I was going to have it turned. I did find my Cagiva ,650SS, has a min thickness of 5.80mm. I dont know if there are the same type.

 

[rick81mhr900ss]

As you only have one disc, it's iron and I don't think it is drilled, it makes me wonder about the impact of loss of thickness on heat dissipation. Disc skimming is usually a pretty low cost job so I'd suggest that go ahead and do it and see where you are at. If it is over 5mm, I think you are OK.

 

[Ventodue]

5.8mm was the minimum thickness for the standard Brembo cast iron disc of the time.

FWIW, a 'warped' disc is pretty uncommon*. What is more likely is that it is bent. This normally happens when some monkey doing a tire change levers down on the disc ....

* I quote:
"Warping is, and can only be, caused by heat. There are only two ways this can happen:

1. If the disc is used in an extreme situation such as racing, track day riding or stunting. General road riding will never produce enough heat to warp a disc.

2. A sticking caliper. If the caliper sticks, keeping the pads constantly in touch with the disc, then the constant friction will send the temperature sky high. This is due either to a lack of maintenance, or caused by rider error if the rider is applying the brake without realising it - by leaving their foot on the brake pedal for instance."

 

[Ventodue]

I can't find bevel disc thickness tolerances anywhere .. <snip>

It's certainly well hidden ... But late model MHR and 900SS Workshop Manual, page I.6 for example. (Same value also given in contemporary Laverda workshop manual).

 

[rick81mhr900ss]

Ventodue: I'm glad you were able to provide this information. I only have the old MHR/900SS Owners manuals and a few reference books. One word about warping discs, same goes for cars, don't wash vehicles with hot discs as this can also result in disc warping.

 

[ducatijack75]

Thanks all for the advice. I will clean the service with 3M and see if that clears the pulsating. If that does not fix it, i will have it skimmed.
JB

 

[rick81mhr900ss]

If you carefully jack the front end up, you should be able to see/hear a warped disc as you turn the wheel. I don't have a front wheel jack stand so when I lift the front end I do so by padding the front of the engine sump with rags and a piece of wood and then jack it up from underneath with a little bottle jack.