Front Brake Cylinders for BN1 -- odd sizes ?

[ianh]

Hi back again !!
I have an early BN1 which I think is supposed to have the larger 1 inch bore brake cylinders. On the later models they changed the cylider to a smaller 7/8 size.

Having checked on my car the drivers side has 2 smaller 7/8 inch cylinders and on the passneger side and have one larger 1 inch and 1 small 7/8 inch on the drivers side !

Does anyone know why they changed the size and whether I should get 3 new 1 inch cylinders to get back to original or simply replace the single 1 inch with a new 7/8 inch.

Presummably with the larger bore the brake pressure will be similar but maybe acting over a larger area of the base of the shoe and certainly requiring a greater swept voume of brake fluid.

I have had problems with the brakes jamming on when the brakes warms up but I suspect that may be due to insufficient slack between the brake pedal and master cylinder so the return hole is still partially blocked.

There seems to be slack in the pedal when cold but not when hot weird eh ?

Any help as always greater appreciated


Ian

 

[piman]

Hello Ian,

you should certainly have equal bore cylinders all round, the larger bore will give a greater force than the smaller. Having different bores side to side should give unbalanced braking.
If they went to smaller bores it sounds as though the front to rear balnce was,originally, too much to the front.

Alec

 

[zblu]

1" on the front on the BN1 7/8" on the BN2 front

 

[zblu]

The width of the drums was also increased at some stage, I think it was the change from 4 to 5 stud, again the change from BN1 to BN2 I think

 

[Dave Russell]

According to the "book", the BN1 prior to around Nov. 1954, had 1.75" wide drums & 1" wheel cylinders on all four corners.

In Nov. 1954 when the BN1 was changed to the Hypoid axle with 2.25" brake drums, the front wheel cylinders were changed from 1" to 7/8", "for improved brake balance". I don't think the balance problem had anything to do with the rear drum width increase because brake sensitivity is not much affected by drum width. Wider drums do however, decrease brake fade.

Beginning with the first BN2 in Aug. 1955, the front brake drums were changed to 2.25" wide. There were no further cylinder diameter changes.

I changed my BN2 front cylinders from 7/8" to 1" to give more front braking power in relation to the rear. Combined with the 1" rear cylinders, I find that balance is very good. At lockup, the front locks up slightly before the rear, which is usually considered to be optimum for stability & driver control.

As Alec said, no matter what the cylinder sizes selected, they should be the same on both sides of the front or the rear.
D

 

[ianh]

great many thanks
It would seem (although maybe I am reading too much into it) that the original front cylinders for the early BN1's were a little over powered if they were later downgraded to 7/8' and wider drums were put onto the back ?

I can't help wondering if that is why replacement 7/8' cylinder were (mainly!) put on the front on my healey although it is interesting Dave put 1' on the front of his BN2 to get the best balance (albeit now with different front dums too).

Presummably if wider drums are put on the back wheel this lowers the pressure per unit area of shoe but increases the contact area so limited nett effect on braking force as Dave suggests.

Still not quite sure which way to go. Dave from your comments it would seem 'the book' suggests better balance would be achieved with 7/8 cylinders on the BN1?

 

[zblu]

Knew you would have the right info Dave Russell!, altho I checked moss out and their info is a bit sparse!
Then as a rule of thumb 4 stud/1"/1.75"?
Depends if you want originality ianh
7/8" might just have been what was in the austin parts bin at the time!

 

[zblu]

Dave, if I'm reading my part numbers correctly they went back to these 1" on the 100/6's?

 

[Dave Russell]

zblu,
This would be reasonable. The Six has More weight further forward, so more bias toward the front brakes would be reasonable.

Ianh,
"Dave from your comments it would seem 'the book' suggests better balance would be achieved with 7/8 cylinders on the BN1?"

Possibly so, I really doubt if there would be a great difference either way. 1" cylinders vs 7/8" cylinders would give about 30% more shoe to drum pressure for the same pedal pressure. Unless you are racing, you won't be approaching wheel lockup very often. For that matter, with standard brake linings it's not easy to lockup at any time.
D

 

[ianh]

Thanks once again guys, it is so great to have such an indepth source of experience and reassurance especially in what seems to be a slightly rough (although I am sure short lived) maintenance period.
I will go with 1" as original.
Dave, where did you get your encyclopedic knowledge, it is truly outstanding ?
Ian

 

[Dave Russell]

[ QUOTE ]

I have had problems with the brakes jamming on when the brakes warms up but I suspect that may be due to insufficient slack between the brake pedal and master cylinder so the return hole is still partially blocked.
There seems to be slack in the pedal when cold but not when hot weird eh ?
Ian

[/ QUOTE ]
To address the second part of your question:
The BN1, BN2 master cylinder is a very strange arrangement. All of it's parts
must be exactly correct & fitted in the correct order & direction. The
Push rod free play is very critical. .030" to .040". If this is too
little the brakes will not fully release. At worst, the plunger ports
will not be uncovered when the MC is fully retracted, in spite of the
correct clearance, due to recuperating seal swelling or wrong design. Some
folks have slotted the plunger ports to give more retracted clearance
but I have not found this to be necessary.
I have attached Norman Nock's pic of the MC assembly for reference.
D