Early XK120 Micram Adjusters -- Which is Which?

(Christopher Potempa) #1

In rebuilding the brakes, I took it for granted that the micram adjusters front and rear were the same. They’re not. And the parts catalog confirms it. Adjuster part numbers 21787 (front) and 23978 (rear). The attached picture shows how each is different. The one on the left is shorter, the one on the right longer. I know this is an obscure point, but can anyone tell me which is front and which is rear?


(Rob Reilly) #2

You should have 4 fronts and 2 rears. Does that help?
If not, then my next guess is that you would want more adjustment range on the fronts, assuming the front shoes wear out faster than the rears, so use the longer ones in front.

We are making the assumption that the parts catalogue is right, inerrant in the autograph, that when Lockheed decided to start sending them the latest improved micrams, somebody updated the SPC entry for rears and didn’t simply forget to update the entry for fronts?

(Christopher Potempa) #3

Me having four fronts and two rears? No, that doesn’t help if only because I’m thinking the brakes were one of the issues the car was laid up for decades earlier (that, and someone having replaced a fuse or two with a piece of round brass stock which led to a barbeque of wiring…). When I first got the car most of the front brake internals were simply…gone. Just not there. Go figure. Anyway, Rob, your logic regarding the frequency of brake adjustment makes sense so, in the absence of other references, I’ll go ahead and use the short ones for the rears. Thanks.

(Lee140FHC) #4

My thinking is the opposite…if one adjuster(rear) must take up wear on 2 shoes, then wouldn’t it need more adjustment range?

(Christopher Potempa) #5

Yes, that’s a good point too. Someone with a crusty, un-molested barn find would also be a help here as various books I have on the subject are sparse with information when it comes to the earlier (read: pre-'52) brake illustrations. Thanks.

(Rob Reilly) #6

I can see Lee’s thought. The rear cylinder moves as well when you adjust them. I don’t remember noticing any difference in mine.
If it warms up above freezing maybe I’ll have a look at them.
Searching I find early XK120 is the only Jag that used them, but they are also on MGTD TF and A, Morris Minor, TR2-4 and Wolseley and readily available.

(Christopher Potempa) #7

Okay, my rear brakes and wheels are back together again and I went with the shorter adjusters if for no other reason than they seem to have a smoother initial adjustment rise than the longer type. It’s good to know that other makes used them as when searching for '120 parts at Barratt and the like, I was coming up empty. I keep a restoration log so this is another of those fine points I’ll have to make a note of. Thanks.