Morris Minor Owners Club

Don't agree with you there Cam, my pedal is rock hard with
a servo. I always understood that disc brakes were developed
purely to overcome the problem of brake fade anyway(on aircraft)?

Willie,

Disc brakes were developed to overcome the problem of brake fade, which they do.

A properly set up drum brake will stop a car just as effectively (if not more) than a disc of the same size due to the brake lining area in contact with the disc/drum (all other things equal).

BUT jam on a couple of times and the drum friction lining can't cool down to it's optimal operating temperature, and so fade occurs. This does not happen anywhere near as often with discs due to the more efficient cooling of the external disc.

The differences nowadays lies in the materials for discs being better (more developed) than those for drums.

Makes sense, if drums were that good, they would fit them to modern cars, but a lot of them now have discs all round, even non-sporting saloons (e.g. my dad's 1994 Honda Civic 1.6).

For a while, I ran 2 minis together, a 1985 (disc) 1000 and a 1979 (drum) 1000. The difference in braking was phenomenal. The '79 mini used to fade quite badly, and you always had to bear it in mind, whereas the '85 mini you could tear about all day long and still have effective brakes.

With regard to the servo, I suppose it's all about personal preference.

At the end of the day, regardless whether it is foot pressure or foot pressure and manifold vacuum that is forcing the fluid into the slave cylinders, they are operating from the same point (pressurised hydraulic fluid in their supply lines) so if you stand on the pedal harder, it has the same effect as a servo (i.e. more hydraulic pressure).

I'm sure that your pedal is rock hard (good bleeding skills!!), but have you tried it without? it may be harder!

The vast majority of the cars that I have driven have had servos and all have that characteristic 'servo' feel to them, but I freely admit that I have not driven a servo'd Minor, but logically it must have similar characteristics.

Personal preference again! But I feel a lot safer in a non-servo'd disc braked car than in a servo'd drum braked car!

One thing I'd like to mention, talking about brakes etc, I am wanting to fit discs some time in the future but it seems on most, if not all conversions the holes securing the original backplates need to be enlarged and consequently there is not a great deal of metal left around them. I know there are plenty of Minors with discs, but this point has always made me wonder, as there must be a fair amount of force at these points when braking. Does anyone else let these sort of things worry them?

Not really,

You are only enlarging the holes up to 10mm which is not all that much really.

The forces when braking are directed toward trying to move the caliper downwards, thus trying to spin the caliper mounting bracket and backplate about the stub axle itself, of which the backplate is touching in the centre.

It is more likely that the backplate / caliper mounting bracket would twist or the bolts would shear before being ripped out of the stub axle assembly.

But this in itself is nothing to worry about because the tyres would have lost traction a long while ago, which reduces the forces on the braking components anyway.

So, if you think of skidding due to locking up as a safety stress 'limit' on the brake mounting components, I think you won't be far off!

Hope this helps.

Thanks for the reply cam, I like your reasoning behind the tyres loosing traction that could be looked upon as a sort of safety valve I suppose, The castings that I will be using (swivel pins) have been drilled off centre to the surrounding metal which prompted me to bring the issue up. I've noticed this on several swivel pins. They are obviously jig drilled and if the casting is a bit light then tough luck!

Hi... backing up a couple of posts, yes I'm aware of the effects of a servo plus the limitations of drum brakes vs disks.
A few years ago I'd have been able to show the calcs off the top of my head - the difference is easy to show just with dimensional analysis, you don't even need to stick numbers in. The cooling factor is higher by at least one power or maybe a square.
These days I'm rather rusty on the maths, and my engineers prefer to do it all on computer modelling rather than by thinking but at least I knew how once upon a time.

.

the tyres would have lost traction a long while ago, which reduces the forces

Very true. The one area that can increase forces is where there is variable road grip. If a wheel locks on low grip (greasy road) then you hit high grip (like the patches put down before pedestrial crossings) there can be a nasty shock loading passed through, as the locked brake has a higher grip (changes in brake lining 'mu' relative to slip speed).
I'd still think that the oversize holes would cope, but would like to look at it to see if it 'looks' sensible - if in doubt, compare the cross section of the metal left (across the region it would have to shear) to the cross section of the bolt.

Anyway, back to drum brakes. I drive all kinds of cars, and sometimes I even get to drive my own, on which the brakes vary from non servo 8" drums to 12" cross drilled vented disks. There's a huge difference there.

The Minor taught me all about good driving style such as using engine braking as much as possible and why it is a good idea to give a good braking distance. Brake fade is made worse if the front drums are worn or badly rusted, or both (like my first Minor). With good drums, brake fade occurs much less often but will still happen, so you need to adjust your driving style to suit the car.
My interest with the servo is to reduce the pedal force, because Gayle is still not keen on driving the Minor and I'm hoping this will help. Also for those off-days when I forget which car I'm driving.

Back to the 'all round disk brakes'. Often these are a marketing gimick unless the car is bigger than the average saloon. On a medium sized car well designed front disks will give so much weight transfer that drums are more than adequate on the rear. You'll find many cars with rear disks have incredibly small rear ones, as they don't do a lot of work.

Personal preference again! But I feel a lot safer in a non-servo'd disc braked car than in a servo'd drum braked car!

Me too, but I was trying to stick to a small budget and the servo was about £25 second hand (less than I paid for the remote reservoir kit !).

(swivel pins) have been drilled off centre to the surrounding metal

I've seen that too! I though it was a one-off Friday afternoon job, but maybe it is more common. That would not look nice when drilled oversize as there was not much left on the thin side to start with.

If a wheel locks on low grip (greasy road) then you hit high grip (like the patches put down before pedestrial crossings) there can be a nasty shock loading passed through, as the locked brake has a higher grip (changes in brake lining 'mu' relative to slip speed).

That's a very good point, and one that I did not think of. Even so, as you say, the holes would probably be OK.

I would think that with the section of the standard Moggy tyre and tyre compound, the coefficient of friction between the tyre and road surface would not be that great even on dry roads on a hot day. (I have locked my 145s up before now on a dry road and come squealing to a halt).

The problem may lie in having large section front tyres (185+) with a very sticky compound (such as the Yokahama A008s).

It would be interesting to see (from a distance ) at what point things would start to collapse.

There is really no excuse for the off-centre holes. That's just shoddy workmanship. When I did mine (using a hand held power drill) I took LOTS of time and was VERY careful to line everything up, the result of which was evenly enlarged (and centred holes).

Yes CAM My brake pedal 'hardness' is as good with the servo as
it was before I fitted it. My Modern servoed Daewoo feels more
spongy than my Minor! An interestiing point about servoing a
car which was not designed for it is that you you could get far
more forward weight transfer during heavy applications...this
could lighten up the rear so much that the rear wheels lock FIRST.
Of course ,if the rear wheels lock up then you lose any control over
which way the car will go,hence the introduction of pressure limiting
valves in the rear of later vehicles. Have any of you any knowledge
of pressure limiting valves which have been successfully fitted to the
Minor?

Willie,

I have not any personal knowledge of pressure limiting valves which have been successfully fitted to the Minor.

BUT, being as the early Minis were similar (and a bit front heavy), then one would assume that a later mini pressure limiting valve could be used. Although the Minor may not need it except under extreme racing conditions. I have not had any problems so far !

Of course if you want to be on the safe side, then convert to a split braking system, with variable bias valves (available from Demon Tweeks) and tune them accordingly.

But I think that that is totally over the top for a Minor.

At the end of the day, if the weight is the same and the maximum speed you are stopping from is roughly the same, then the standard system should suffice. The difference with a tuned engine is - how often you are braking from you maximum speed in short succession!

Hey Chris

Now I would cough up a few hundred quid to get that extra 10 bhp

I'm not an expert on heads, but a 1275 midget head sounds right to me (though I'm sure someone will know) There's one in MM for £50 - 07905 174840

ps - the red cars are not at the front anymore - hope it lasts.. ;-)

Sounds good, but just make sure that the valves do not hit the top of the block, because you would be fitting a large block head onto a small block. Also make sure that it has the correct number of studs (was a problem on minis - not too sure about Midgets as mine was a 1500).

Having just had a look at David Vizzard's book, apparently the mini 850 racers used to do this mod, but they had to make a cut out in the cylinder wall, as the exhaust valve used to overlap the bore.

The diameter of the mini 850 bore is 62.94mm whereas the bore of the 1098 minor is 64.58mm - a difference of 1.64mm which is 0.82mm either side!!!

The bore of the 1275 midget engine is 70.61mm, plus the cylinders on the 1275 were arranged more uniformly, and so are in a slightly different place to the 1098.

I know that if you were trying this with a 948cc minor, then you would definately have to make the cut outs as the bore is the same as the 850 mini.

It seems likely that you will have to make the cut outs (engine out and block down to engineering firm), as with only a 0.82mm difference at the side of the bore between the 1098 and mini 850, the valve would probably still overhang.

Cut-out's in the block would make it a bit more complicated (far too complicated?)! I wondered if someone would find a decisivee yes/no for the Midget item.
I had a 1300 'bigger' valve head on a 1098 without that problem, but I wouldn't know the best way to repeat it, as it was done by the previous owner.

The 1098 midget head - 12G295 is a better bet as it has the same bore offsets, a 1275 mk 4 midget head 22G ( I think) 940 will definitely need cutouts on a small bore block as I fitted one to a 1060 cc 948 race engine ( + 060 Metro pistons) a couple of years ago and had to have the block machined by about 3/8 " even at that size.

I think that you will find that the Midgets '61-'64 did Not have
12G295 heads? The cars which had 12G295 heads fitted were:
Riley,Wolesley,M.G,. and Midget '64-66. (all '1100' models of course). Moral...always check
for the number cast in the head.

Cheers Cam.
I got the 266 suggestion from Dave Vizard. You really don't want to disagree with him
How easy is it to change a cam, Cam?

How easy is it to change a cam, Cam?

Very funny....

No, actually it's a lot easier to change in your engine than it is in mine!!

This is due to you having tappet covers which will allow you to fish out the cam followers (tappets) out without turning the engine upside down (like I have to).

It's generally quite straightforward as long as you use common sense and new gaskets!!

If you are feeling adventurous then why not fit a duplex timing chain / sprockets while you are at it? You will have to take the old one off anyway. and all you need to do is countersink the engine front plate (next to the crank) and fit two countersink screws (easily obtainable from Mini specialists).

If you want a good feel of how to do the job then read the engine section in the Haynes manual, as well as the 'camshafts, cam timing and valve trains' chapter in David Vizard's book.

There is really not that much to it. You will have to remove the bumper, front panel and radiator, as well as the timing chain cover, crank pulley, crank and cam sprockets, timing chain & tensioner, oil thrower, cam securing plate, tappet covers, rocker cover and rocker assembly, pushrods and coax the cam followers out. Then remove the cam itself.

Assembly is the reverse of removal........ha ha.......NOT.

Use cam lube (!) on the bearing journals and be VERY careful not to catch the cam bearings when trying to slide the cam back in as it becomes quite heavy when you have got hold of only one end! - also turn it slightly to engage the oil pump into the end.

put the cam securing plate back on, woodruff key into the slot in the cam, sprockets & chain on (making sure it's lined up correctly), tensioner, oil thrower, timing chain cover and crank pulley, cam followers, pushrods, rocker assembly & rocker cover.

Put everything else back on, and I think that's it?? might have forgotten something but I can't recall........

Oh, and renew every seal, gasket and rubber in sight, especially the crank oil seal.

So, not that bad but not a 5 minute job, but if you are methodical most of it is obvious.

I would run the engine in again as if it was new, just to let everything bed in, and change the oil after 500 miles to get rid of the cam lube. But I suspect that that is a bit over the top, but it can't hurt!

p.s. watch out for cam suppliers calling peg drives slot drives, as I have heard of some referring to the end of the cam and some to the oil pump!
Just let them know what engine it is for and they will sort you out.

also depends on whether you can remove the fron panel without wrecking the hockey sticks etc...
Sometimes it can be quicker to pick the engine right out, then you also get nice access to the fiddly bits on a bench - your choice really.

I was thinking about changing Phyllis' timing chain for one of those nice, quiet duplex ones. I think I've changed my mind - vive la rattle!