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MG MGA - Brake Drum Diameter

I have just been checking the car over for its annual MOT. When I came to the rear brake drums, the adjuster went straight through the clicks and back to beginning. Suspecting the linings were worn I carried out an inspection only to find masses of wear left. That left worn drums as the likely problem and where I have become confused.

The book lists my 1500 drums as 10 inch, so I was surprised to find the inside diameter 9.938 inches (25.243 cm). The drums were as fitted on the car when I took it over as a wreck 12 years ago. If the mileage on the speedo was correct at the time, these drums could well now have done 105,000 miles. However, the car may have been raced during its early life, so they could have been replaced.

Logic tells me that these are worn out later 1600 drums rather than 1500 drums. During the rebuild I religned the 'as supplied' brake shoes, so I did not come up with any conflict of wrong brake shoes for the drum. Any thoughts?

Steve
Steve Gyles

I have just been checking through the 1500 and 1600 parts lists. Both quote the same part number for the drum even though workshop manuals list the 1500 drum as 10" diameter and the 1600 drums (front discs) as 9.6" diameter. However, the parts books list different numbers for the brake shoes.

Does this mean that I can save on new brake drums by skimming to the 1500 diameter of 10 inches and fitting a set of 1500 shoes?

Or am I getting it totally wrong somewhere? Bit confused.

Steve
Steve Gyles

Steve, When you measured the ID of the drum did you go into the worn part because if the lining width is a bit less than the drum depth there will be an unworn rim on the drum of the original ID. It might even be less than the original due to rust build up which might account for the less than 10" you measured. So your drum may still be overworn in the wearing area. I believe the maximum amount recommended ground off the drum is 60 thou (120 on the diameter).
Mike
m.j. moore

Mike

No, I measured from the shiny bit using an internal caliper.

I have just been speaking with Bob West. He is convinced that there was only ever one size of brake drum and brake shoe. Can anyone prove/disprove? All evidence welcome. My MG Workshop manual does not appear to mention different sizes, but the Haynes manual does.

Steve
Steve Gyles

Steve,
Are you sure the adjuster was correctly fitted/located? Were both sides the same?
Mike
Mike Ellsmore

Mike

Yes, correctly fitted. One less click on rear right. It was the one extra click on rear left that flipped it over the cam and back to the beginning.

Steve
Steve Gyles

I am now of the opinion that Bob West is correct and there was only ever the 10" drum.

Have a look at the attached image. I have posted 2 specifications from the official MGA Workshop Manual. You will then see that the Haynes MGA Manual appears to have interperated the 1600 brake shoe material length (9.63 inches) as the drum diameter.

Steve

Steve Gyles

Steve,

I also have my MOT booked and have been adjusting the brakes and have run out of travel on the adjusters. There is plenty of meat left on the linings but I cannot adjust them far enough to lock the brakes. I haven't measured the drums but am now wondering if they have sometime in the past been skimmed a little too enthusiastically.

I am certain you are correct and there is only the one drum size. Looks like Mr Haynes has got his transcription wrong.

Malcolm
Malcolm Asquith

Barney has a page on his site that talks about ill fitting brake shoes. http://www.mgaguru.com/mgtech/brakes/bt102.htm
He says there is a variation it fit between shoes from different manufacturers and "Some shoes are such a loose fit that the adjuster is mostly extended when the shoes are new". If the drums measure correctly in several locations, this is a possibility.
Jeff Schultz

Steve - If I am not mistaken he MGA has twin leading shoes on the front wheels and use the same adjuster front and rear like on the TD and TF. If so, it just takes very little wear on the rear brakes before run out of adjuster. This had be scratching my head on our TD before I finally realized that because the rear adjuster is operating on two shoes, it requires twice the adjustment to tank up the same amount of wear on the front brakes where each shoe has its own adjuster. The solution is to tack weld a piece of 1/8 inch strap iron on the bottom of the adjusters on the rear brakes. I ordered some modified adjusters from a place here in the states to use on our TD (I kept the unmodified adjusters for the day when I need to get new shoes). Cheers - Dave
David DuBois

Dave

That sounds a great idea. I have a number of adjusters lying round from when I converted my front wheels to discs. Looks like a task for this weekend. Car is in for its annual health check (MOT - Ministry Of Transport Test) today, fingers crossed.

Steve
Steve Gyles

Car passed its MOT without a hitch. Licensed to thrill for another year. No problem with plastic sports windscreens this part of the empire - Australia take note.

Nevertheless, taking a leaf out of Dominic's book of advice, I am not taking chances with the brakes, replacing all round this weekend.

Steve
Steve Gyles

This thread was discussed between 05/05/2009 and 07/05/2009

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