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cracked discs
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Author:  sanzomat [ Tue Dec 01, 2020 5:21 pm ]
Post subject:  cracked discs

Just a quick warning on something to look out for if you have drilled discs. Mine does, were on when I bought the car six years ago. If they have been on since the build then I guess they've done okay - 16 years/32,000 miles. I had mine MOT'd today and the tester called me under the ramps and showed me two cracks in the offside front disc, both started at a drilled hole and went all the way to the outside of the disc. They are abut 60 degrees apart and both on one side of the vented disc only. They are on the inside so I don't often look at that side. The outside is fine. Not sure how long they've been like that but I think not long as I've noticed a slight vibration under braking just recently, since the last track day I did in October. I guess mine get some fairly extreme use - I get through a set of EBC Bluestuff in 6 track days!

Author:  sanzomat [ Tue Dec 01, 2020 8:30 pm ]
Post subject:  Re: cracked discs

Hmm... Anyone know what car my discs are from??? They are 280mm diameter. The calipers are MG Maestro single pot/sliders but MG Maestro uses 240mm discs so a previous owner/builder must have found a suitable 280mm disc. Is there something commonly used at this size on Libra/Spyders? Trouble is the 95.25 PCD really limits the choice but I suppose something else could always be re-drilled.

My other option is to mount the AP 4 pots and 304mm discs that I have in my shed but then I won't be able to use most of my rims.

I might also see if I can alter the caliper mountings to use the Maestro calipers with the 304mm discs.

Author:  mark h [ Tue Dec 01, 2020 11:41 pm ]
Post subject:  Re: cracked discs

i have 280mm discs i got them from hi spec with calipers from them they may be ok for for you if the centre of the disc is in centre of caliper but may be able shim out if needed i can face wheel fits to face out pad touches if needed

Author:  bob snooks (835) [ Wed Dec 02, 2020 12:17 am ]
Post subject:  Re: cracked discs

I have MG Maestro calipers with re-drilled Rover 820 280mm discs, so would guess yours will be the same.

Author:  kaighn80 [ Wed Dec 02, 2020 4:18 pm ]
Post subject:  Re: cracked discs

the shape of the spots or grooves can make this worse, did you notice if the spots go all the way through and if they have a point, when i groove my disks i use a ball nose cutter to make the channel radius-ed to reduce the chance of cracking, but its always a risk.

Author:  sanzomat [ Wed Dec 02, 2020 6:32 pm ]
Post subject:  Re: cracked discs

Had the old ones off today.

Thanks for the suggestions. I looked up the Rover 800 ones, looks like they come in 263mm or 285mm depending on the model, the 285mm being from the 820 Vitesse turbo. That car uses 4.5" PCD so would definitely need re drilling for the 3.75" we have.

My old ones are definitely 280, not 285 and seem to have been altered from 4x100 PCD as you can see the slotting of the holes but only by around 2.5mm each hole. The offset/bell depth is different from Rover 800 too.

As I had a pair of AP 4 pots and 304 discs from a TF160 sat doing nothing in my shed I decided to bring those into service. Will mean half the rims I have with various track tyres/slicks won't fit over them but I can keep the rims that do fit for the fronts and can still use any of them on the back. I have one matching set of TF 11 spokes I can use for road use.

I mounted these today, all physically fitted with no troubles but whilst the pedal seemed to firm up nicely when bleeding them when I went for a slow drive up the lane the pedal sank so I'm guessing some trapped air or a stuck piston that has now freed. I've used up all the brake fluid I had so will order some more and have another go at bleeding them.

Here are the cracks:
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cracks1.jpg
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Attachment:
cracks2.jpg
cracks2.jpg [ 293.11 KiB | Viewed 632 times ]

Author:  sanzomat [ Wed Dec 02, 2020 6:36 pm ]
Post subject:  Re: cracked discs

kaighn80 wrote:
the shape of the spots or grooves can make this worse, did you notice if the spots go all the way through and if they have a point, when i groove my disks i use a ball nose cutter to make the channel radius-ed to reduce the chance of cracking, but its always a risk.

They go right through both parts of the vented disc. There is no doubt that the cracks start at the holes as there are small cracks starting at many of the other holes and the larger cracks go right through holes.

I agree that radiused grooves are a much better way to let the hot gasses/dust escape. These drilled holes were on the car when I bought it. I think I'll stick with plain for now.

Author:  madmazturbo [ Wed Dec 02, 2020 7:31 pm ]
Post subject:  Re: cracked discs

Those track days have finally killed them it looks
Think you’ve done really well to have lasted this long :lol:

Author:  Elansprint [ Wed Dec 02, 2020 9:11 pm ]
Post subject:  Re: cracked discs

Dave it is a common problem on all drilled discs seen it on TVR's
Ian

Author:  sidewinder [ Thu Dec 03, 2020 12:59 pm ]
Post subject:  Re: cracked discs

Probably you have the MGF front upgrade kit as produced by some folks? Uses same calipers but with different bracketry to allow for bigger discs.

My 304s are grooved/dimpled, i'd never want fully drilled discs precisely for the reasons you've shown.

You can get two pot AP calipers for the 304 discs as well

Author:  kano nordie [ Fri Dec 04, 2020 6:14 am ]
Post subject:  Re: cracked discs

We had drilled discs on Jedi race cars, but after cracking have now changed to grooved
John

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