Master cylinder

[Blaketon]

I have the engine and box out at present and as part of the jobs I am doing, need to remove the brake master cylinder. Would I be right to say that if the bolts are put in, so that the nuts go on the outside of the chassis leg, they'd not come back out with the 5 speed gearbox in place? Does having a hydraulic clutch make any difference to this (I can only see the remains of the mechanical clutch, such as the bushes in the chassis legs, so don't know exactly what went where)? I don't really want to have to remove the torsion bar right now but if needs must I suppose I will have to. One job leads to another!!

[philthehill]

The nuts have to go to the inside of the chassis rail. The master cylinder bolts have special thin/reduced height heads so as not to foul the torsion bar when fitted.
If the bolts are fitted from the inside of the chassis rail there is a good chance that the end of the bolt will rub against the torsion bar as the torsion bar does not remain straight when flexed.
What I have done make special studs so that I can use half nuts on the outside of the chassis rail and normal nuts on the inside of the chassis rail. The bolts can then be easily removed to facilitate extraction of the master cylinder from the inside of the chassis rail.
The torsion bar should flex downwards sufficiently to allow removal of the master cylinder bolts. That can be made easier by grinding off one flat of the master cylinder retaining bolt.

[Blaketon]

Have you a picture of what you did? Would a bolt, of the right length, work with a half nut and shakeproof washer? I know that normally a thread is expected to engage by the same as it's diameter but these bolts are taking sideward thrust (Besides the heads of the bolts are thinned). I had assumed this was to make it easier to fit them (I was planning to just cut them off, is there is an alternative (As you say there is ). I wonder whether this feature was a mistake in the design, that was overlooked (For the sake of a bit more clearance)?

[geoberni]

I know that normally a thread is expected to engage by the same as it's diameter but these bolts are taking sideward thrust (Besides the heads of the bolts are thinned).

I'm curious as to what industry you got that definition from .
As a former aircraft engineer, I can tell you that unless detailed otherwise, the Aviation industry works to 2 threads protrusion beyond a nut.
In my 37 years in the RAF I only encountered one instance where BAE granted a concession for some bolts included in a modification to only protrude 'beyond the nut' rather than 2 or more threads.
We were doing the modification and raised a technical query with them about the bolts being too short. Longer bolts were readily available and used elsewhere on the aircraft; we could see no reason why longer bolts couldn't be used.
BAE were the design authority, it was their modification, they noted our concern but confirmed the bolts supplied were suitable so long as they did clear the captive nut.

[Blaketon]

From one of the textbooks when I was in college, undertaking training in "Engineering manufacture" (Turning, milling, grinding, bench fitting). I can see that the extra two threads would be a safety measure and save time but once they are in thin air, they don't actually do anything.

[olonas]

As Phil has written, I have found that the torsion bar will flex sufficiently to remove the bolts. I use linked small "D" shackles, a steel bar and a piece of wood between the bar and chassis rail. The bottom of the chassis rail/wood make a perfect prying point.
I did consider fitting them the other way but, again, as Phil writes the nuts and end of the bolts are very close to the torsion bar.

[oliver90owner]

From one of the textbooks when I was in college, undertaking training in "Engineering manufacture" (Turning, milling, grinding, bench fitting). I can see that the extra two threads would be a safety measure and save time but once they are in thin air, they don't actually do anything.

It seems you were both at cross purposes? Thread engagement length is reference to the thickness of the nut. Thread protrusion is an entirely different issue.

[Blaketon]

I was making the point that half nuts are thinner than the diameter but since the thrust is sideways, on these bolts, a half nut may not be any disadvantage. Afterall, the bolts have thinned heads.

Bolts are supposed to engage fully with nuts, that normally are as thick as their nominal diameter but the limit would be unilaterally plus; how much depending on what room there is and unless the bolts are made for the job, you would normally go for the shortest standard size, that allows full engagement, plus a little for error.

[Bill_qaz]

But surely a half thickness bolt head part of the actual bolt material is stronger than the reduced number of threads in a half nut.
I'm sure if enough load was applied the thinned nut would fail before the thinned bolt head. Although in this application the maximum load is during brake application and most of the load is from bolt to chassis rail.rather than stretch on the bolt.

[oliver90owner]

But surely a half thickness bolt head part of the actual bolt material is stronger than the reduced number of threads in a half nut.
I'm sure if enough load was applied the thinned nut would fail before the thinned bolt head. Although in this application the maximum load is during brake application and most of the load is from bolt to chassis rail.rather than stretch on the bolt.

Bill, general engineering design is for the bolt shank to fail before the threaded part.

Not only thread length, as a ratio of nut thickness to bolt diameter, but also the component materials and also the percentage thread engagement would be taken into account for proper designs.

I would expect both these parameters to be tightly specified for aeronautical parts - thus avoiding cheap chinese nuts and bolts - with minimal tensile strength and sloppy thread engagement.

Most steel bolts would also specify rolled threads, not machined from the round. Normal engineering thread engagement is around 70- 75% but more highly stressed parts might spec 80% or more?

I don’t think this application is under-specced. As you point out, the bolt is likely far less stressed than the chassis and diameter was likely selected on that criterion.

[Bill_qaz]

I disagree a half nut with two or three threads of engagement would fail and pull the thread before a reduced thickness bolt head would shear. The discussion was that the original reduced thickness head bolt is stronger than using half nuts. I didn't make any reference to aeronautical. My only experience is 50 years in commercial vehicle engineering.
But in the application of the master cylinder the tensile load is low

[Blaketon]

What I have done make special studs so that I can use half nuts on the outside of the chassis rail and normal nuts on the inside of the chassis rail. The bolts can then be easily removed to facilitate extraction of the master cylinder from the inside of the chassis rail.

Was there any reason for using studs, rather than shortening a bolt to length, other than being able to fine tune the right hand side protrusion? Did you use a shakeproof washer each end?

[philthehill]

I just used what came to hand.
The studs if I remember are Minor cylinder head studs cut and threaded to suit.
The nuts between the chassis rail and the torsion bar appear to be nylock half nuts.

[Blaketon]

I got the old bolts out and I don't think it was feasible to bend the torsion bar (It would have needed to move rather a lot and it mentions, at https://web.archive.org/web/20160516013 ... r-removal/, that some cars don't lend themselves to this). I cut the heads off but think I'll be OK with the bolts in the other way (I have some HT 3/8 X 3" UNF) and a half lock nut. I can use washers, on the inner edge, to ensure the threads don't protrude more than necessary. I noticed that the housing, for the pedal spindle bushes, is about as close as the master cyclinder bolts will be to the torsion bar and there was no evidence of contact there. Using a couple of all over ground lathe tools and feeler gauges, I found the old bolt heads were around 0.505" from the torsion bar and the pedal bush housing about 0.410" from the torsion bar. With a thin washer, under a non lock half nut, the gap, from bolt to torsion bar (Inserted from inside) was 0.395", so I think it's worth a try.