|The MGA With An Attitude
ADJUSTERS THAT SLIP - Drum Brakes, MGA - BT-106
This is a problem which I hope is rare, but as a safety issue it bears mentioning.
At 11:32 AM 5/31/05 -0400, Roger Bailey wrote:
>"I suffered another brake collapse yesterday. This time there was no heavy braking involved at all; the brake pedal simply went 2/3rds of the way to floor. I assume I am going to find that one or both snail cams have rotated again.
>I am having trouble with this notion, but it has been suggested that the brake drum diameter has been cut beyond spec's, or that the brake linings are worn out, or some combination of the two."
If the drums are excessively oversize, or the linings mostly gone, the cams could turn beyond the max high point and drop back to zero. But that happens while you are adjusting the things, not while you're driving. If they collapse while driving I suspect something wrong with the adjuster or the mask (or maybe both). I have not had this happen on my MGA in 345,000 miles of driving (with the same adjuster parts).
>"I have ordered two new mask boxes, since although it is possible to fettle the snail detents with a Dremel, one can't get inside the mask, and they are cheap enough anyway. .... Do you agree with the new mask / fettle snail cam approach (cams costing a fortune)?"
Yes, of course. Always see if you can fix the expensive parts to avoid buying new ones.
At 01:01 PM 6/9/05 -0400, Roger Bailey wrote:
>"I have found nothing glaringly or obviously wrong with the rear adjusters (as expected). However, there are some useful observations:
>With the original mask and snail cam on the bench (snail in notch 12 position), if one holds a finger lightly over the axle to apply some mild down pressure, it is possible, with the index finger of the other hand contacting the snail at its max diameter, to rotate the snail towards zero, and with hardly any effort, passing pretty easily over the detents. Exchanging the mask for the new one, I noticed that the effort required gets greater, unsurprisingly."
I think you're on the right track. This doesn't sound right to me. A moderate force against the detents should make it relatively hard to rotate the adjuster.
>" After grinding out the detents in the cam with my Dremel and tiny grind stone, this test now becomes very significant. If you apply any down force at all, you really can't rotate the snail over the detents any more."
Careful there. You don't want to get to the point where it cannot be adjusted with a screwdriver in the car. I don't suppose it could ever come to that, as the detent ridge in the mask has a ramp angle.
>".... I conclude that wear in the mask's nib, together with wear on the cam, can make the cam rotate much more easily. The effort to move the detented position with the drum assembled is significant, and about what one might expect; there is no sign that it would move on its own."
When I adjust these things it takes a real ham fist on a large screwdriver to crank it up tight. If I get it too tight against the drum it almost takes a wrench on the screwdriver to get it to back off a notch. When functioning properly they should be very resistant to moving out of position.
>"The DS drum measures 10-3/32" diameter."
That's too big I think you need to replace the drum. I believe standard practice allows turning the drum out to 0.060" oversize. As it gets larger the shoes need a longer run to wear in and seat properly when new, having to conform to the mismatched radius. Also you may have the problem of the adjuster slipping over the high end back to zero when the shoes are only half worn.
>"This drum's cam rotated back on its own, the PS side's drum is 10-1/32". Its cam did not rotate this time. Looking at its cam detents, I would say the DS cam had more aggressive detents, especially after my original light fettling. Now both have much more sticktion to the cam. I hope that this is an end to the foolishness!"
At 03:22 PM 6/9/05 -0400, Roger Bailey wrote:
>"Attached is a photo of the newer style mask. You can see that the original had a completely rectangular opening,
whereas the new one has an arched web. This does look the stronger of the two designs, bearing in mind that some folks have reported cracks in the base of the mask."
Yes, the arch design would be a stronger part. I have another observation. It looks like the dimple in the base of the old mask does not go all the way across. This may be intentional as a stiffener when the sides are nearly straight and it's not all in the same plane. However, this does make it appear like the dimple might bear primarily against the inside corners of the teeth on the adjuster. This may work well when new, but could lead to faster wear if it is only a small point of contact.
The detent ridge in the new mask appears to go all the way across, in that it is seen as a dimple in the side plate. This may provide a better bite on the teeth of the adjuster cam.
Considering the cost of new adjusters, I think it's jolly worth a try at touching up the teeth on the old ones with a thin grinding wheel. As the teeth wear they become blunt on the ends and slide more easily over the detent ridge in the mask. You would need to grind back the angled ramp on both sides of each tooth to make the tooth narrower to restore the sharper apex. Please let me know how this works, as we all want safer cars on the road.