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MG Midget and Sprite Technical - Disc to hub bolts

I'm in the process of rebuilding the front suspension, I got to the point of bolting up the new discs, started tightening the bolts to the 43lbs as per my manual, I tighten them in stages. The first bolt got to the final figure ok but the second gave that sickening feeling that means it's not going to tighten.

The discs are new but I used the old bolts, I inspected each bolt and the threads looked as good as new before fitting.

My questions are:- is 43lbs the correct torque setting for these bolts? Should I have used new bolts? Has anyone tried using longer bolts, as there's plenty of spare thread at the bottom of the hole.

To cover all bases I've just ordered another new disc and 8 bolts, I've also ordered some longer grade 8 / 10.9 bolts, that I can cut to make use of all the thread in the disc.

I'm using a Snap-on torque wrench that although it isn't officially certified, I check the calibration myself.

Thanks Bob
R.A Davis

interesting. I am assuming you mean the bolts that hold the diasc to the hub. They have a cross pattern on the top?
Personally I have reused mine on more than one occassion over a number of years and not come to any grief. They are pretty substantial bolts so it may be the quality of the disc??
Bob
Bob Beaumont

I use 45 ft-lbs, close enough. Where did you buy the new discs from please?
David Smith

Bob. "That sickening feeling that means it is not going to tighten" is the feel as the threads give way. Usually, it is possible to disassemble the system and inspect the various parts to see which one shows signs of the threads stripping out. Like Bob B, I have reused nuts and bolts in good condition with good success over the years. I have, however, had some significant problems with the hardness and quality of more modern parts. Disassemble, inspect, determine where the problem is, then correct it. Since you have, already, felt threads stripping, why not apply 43-45 ft-lb pounds of torque to that same bolt and give yourself an even better opportunity to see what is happening?

Les
Les Bengtson

I always try to run three different nuts (or bolts) down the full length of a thread with just fingertips to confirm the thread is good on the fixing and the test nuts or bolts as I've had new nuts, bolts and fixings that have been naff, and of course existing could be damaged.

Also a longer bolt or set screw could be tried to check for bottoming out of the original bolt when used on the new part.

Or I've found new washers thinner than previous or existing (or left off).

If possible I like to try the bolts (or nuts) without the part or component to check too. All this takes time at the start but can save lots of time and hassle as you go along.

Nigel Atkins

Thanks for all the comments. I've taken a few measurements and had a close look at the threads. I believe the old bolts I used were probably originals, I normally prefer originals if they're in good condition as generally they're better quality.

I've checked the threads on the other new disc and a bolt can be screwed in fully by hand, there's a bit of free play in the threads but not what I would deem unacceptable. I believe the cause is a combination of factors. The thickness of the hub is 6.5mm the threaded portion of the bolt is 17.5mm this means the bolt only engages with 11mm, at most, of the thread. I suspect that the material the disc is made of isn't as hard as the original. I believe that 43lbs on what is a fine thread, 3/8 UNF, with only 11mm of contact in a material that may not be as hard as it could be and a thread that isn't as tight as it could be, puts these just on the limit of their capabilities. I can't fault the quality of the machining on the disc but I have no way of checking the properties of the material.

The threaded portion of the disc is at least 20mm, if I fully utilise this I can increase the contact area of the threads by almost 100%, this would spread the load and I believe allow me to fully tighten them. I only have 1 reservation in this plan, the original bolts have an unthreaded shoulder of approximately 4mm. It's not clear if this is to aid manufacture or take the load under braking. I'll need to look at the longer bolts when they arrive and make a judgment.

David, I bought the discs from MGBhive, I've always had good service from them and never had any problems with quality in the past. I suspect due to the limited number of these sold, they're all made by the same manufacturer.


Bob
R.A Davis

Bob,
I think these bolts and their design has been discussed before with no resolution as to why they are as they are. But don't trust my poor memory as it'll all be in the Archives.

Whatever bolts you use I'd also recommend adding a spot of medium thread lock too (to clean bolts and hubs/discs). I've never had trouble removing bolts with thread lock (not that I do many) but I have been in a mate's car when a front caliper has fell off and locked up the wheel because he forgot to add thread lock that morning.

Nigel Atkins

Bob - thanks; I suspect BHive use the same supplier / manufacturer as Moss and all the other mainstream retailers. Quality is questionable to say the least. A racing pal who lives in Denmark will only use Brembo discs supplied through Peter May, due to a variety of past problems - soft material, run-out, imbalance etc etc. I have only one pair of original BMC stock left, if they don't outlast me I'll be using Brembo too.
David Smith

Bob, the material of your new disks may well be softer than the originals.

According to my ex-JLR Engineering brother-in-law, since the removal of asbestos from brake pads, to maintain braking efficiency, disks have been softer to compensate for the lower friction co-efficient of asbestos-free pads. That's why disks on moderns are even more consumable than they used to be.
C Mee

It does seem that the cast iron for discs can come from a variety of sources. Cheaper versions contain a higher level of scrap iron that reduces costs considerably but can result in inconsistent performance. Its impossible to tell from just by looking at the item. Many of these items are made abroad often in India, Turkey or China. Price is a big driver particularly as volumes of sales to the classic market is likely to be relatively small. eg How many spridgets require discs annually compared to say the average Fiesta or Golf. As Dave Smith has said NOS or quality branded items from the likes of Brembo may be the best solution
Bob Beaumont

When I inquired about Brembo about 5-6 years ago I was told, at that time at least, that the Brembo quality wasn't the best or most consistent and this was from someone that sold Brembo.

It's like Lucas NOS, it depends when the new was to its quality and after many years the stock of good quality NOS has gone leaving only NOS.
Nigel Atkins

Bob (Davies)

Just wondering - are you using copper slip? I have done on most bolts in my rebuild includng hub to disc. In which case the torque has to be reduced somewhat. I was aware on several occasions that the book torque just felt too high so I held back. These are small cars with comparatively small nuts and bolts surely?
Bill Bretherton

I removed a disc from a Spidget once and you could see a line that looked like a join in the metal, across the face of the disc, like a chord. Oddly it had seemed to work perfectly well but I didn't like the look of it so it was replaced.
GuyW

I bought a set of new Brembo Spridget standard brake discs for steel wheels last summer from Peter May Engineering (the only place I could find them in stock) and was disappointed when they arrived to see that they were made in China. Yet to fit them or use them, so perhaps the materials, fit, finish and QA is fine. Hope so.

Cheers
Mike
M Wood

Bill, no, I didn't use any form of lubricant other than what was on the bolt or disc. I do use Copperslip quite a lot but not on fasteners I really don't want to come undone. The ones I took out of the old disc were quite probably originals and they came out no problem.

The new replacement standard length bolts arrived today and they don't have the unthreaded shoulder of the originals. The fit in hub is noticeably sloppier but it does give me confidence in using longer bolts if they don't have a plain shoulder.

Bob
R.A Davis

This thread was discussed between 15/03/2021 and 16/03/2021

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