Brake drums - acceptable scoring?

[Oldmogman]

Is there such thing as acceptable scoring on brake drums?

I was going to get my rear drums skimmed but on closer inspection I wonder if it's necessary. I don't want to reduce life of the drum or, worse still, risk damage if the tool/drum judders on the lathe!

One drum has two small grooves each measuring approx 1.5mm wide by about 5 thousands of an inch deep (measured with a straight edge and feeler gauges), the other is perfect.

I've attached a pic, for what it's worth, but have to say that the grooves look worse in the pic because they're highlighted by surface rust (I've just cleaned them up after retrieving them from my shed!)[frame][/frame]

[philthehill]

I would suggest that the drum shown is acceptable if and when used with new brake linings.
The new brake linings will soon bed in to the drums irregularities.

[beero]

I doubt it would affect the braking enough to either notice or to fail an MOT, especially on the rear.

[oliver90owner]

I doubt it would affect the braking enough to either notice or to fail an MOT

Almost doubtless, but any reduction in brake performance is not something to advocate. In actual fact, once bedded in (provided the components always run true and concentric) the contact area will be increased, so braking would be increased (think here oil filters are folded to increase the effective fitering area, as an analogy). The problem might arise if the components actually contact on the 'peaks' rather than the peaks and hollows, hence it is preferable to operate with flat curved surfaces (in the two planes).

Brake drums are a wearing, service item. They are less visible to an MOT inspector than a scored disc brake and brakes are passed as long as they meet a minimum standard. Are you satisfied with brakes just above the minimum standard? I would suggest the best standard should be aimed for.

In conclusion, the drums should be skimmed, if necessary, and replaced when outside specification. Linings will last longer with drums in good condition. I would never say 'they are OK unless checking 'in the flesh'. PTH is probably right but there may already be other issues not visible over the internet.

[Big Jim]

I would not be worried at all by that (but I am not signing anything ).

[Oldmogman]

Thanks guys - you were right!

Showed the drum to a mechanic friend who said the scoring was so slight it wasn't worth worrying about, especially since it was on the rear.