AC Aceca Front Disc Brakes: Calipers, Discs and What Fits

The calipers: Girling, and shared more widely than you'd expect

The original front calipers are Girling, not Dunlop. The earliest cars used the Girling Type 'B'; some competition cars had the alloy Type 'BR'; and over the years many cars have run the 16-series calipers, the 16B (solid) and the 16P (split).

The surprise is how widely the caliper is shared. According to the Girling catalogue, as documented by AC Owners' Club members, the Ace/Aceca front caliper carries the same part number as the 1959 to 61 Greyhound and the 1961/62 TR4: 64032520/1. The later 1961 to 63 Greyhound used the split caliper (64032602/3), and a later TR4 caliper even found its way onto some Cobras.

Mounting-hole spacing is where the accounts part company. One long-standing owner, writing from direct experience of the parts, held that the AC caliper carriers are spaced differently from the aluminium TR3 carrier and that AC and TR3 units are therefore not interchangeable. A second account, worked up from a Girling master catalogue, states that the mounting hole spacing is the same for the AC and the TR and that the calipers do fit. What happened next is worth recording, because it is rare. The member who made the first claim received the catalogue account privately, and then posted it on the forum himself, against his own earlier post, with the remark that it matters that everything shared is accurate. He published the evidence against himself. The archive records both accounts, but notes that one rests on a printed Girling master catalogue with part numbers, the other on handling the components, and that the member who handled them was the one who put the catalogue account into the public record. Measure your own carriers before you commit to anything.

The discs: specific to AC, and not interchangeable

The disc is the part that is genuinely AC-specific. The AC front disc is larger in diameter than the TR3's, where the TR3 disc is 11 inches, AC owners measure theirs at around 11.7 inches, so treat the exact figure as something to check by measuring your own. The Girling disc part number recorded up to 1961 is GB49613, and the discs were specially made for the Ace, Aceca and Greyhound.

So even though the caliper is shared, the AC disc is not interchangeable with a TR3 disc. Don't assume a Triumph disc will fit, it won't.

Fitting a caliper to the larger AC disc

Because the AC disc is larger in diameter than the discs the shared calipers were otherwise used with, something has to give when a caliper is fitted. What that something is depends on which account you follow, and, more usefully, on which caliper you are fitting.

The catalogue account says the bottom groove of the caliper should be relieved, opened out, to clear the larger AC disc, and warns that if the AC disc is reduced in diameter the pads will stand proud of the disc edge, overhanging where there is no disc to grip. That warning is worth taking seriously, and it comes from the catalogue-based account rather than from the archive.

A second owner, however, recollected that converting to Girling 16/3 calipers required the AC discs to be machined down slightly in diameter. The two accounts may not actually be in conflict, because they may be describing different calipers. That is the practical point: what you have to do to fit a caliper appears to depend on which caliper you are fitting. Establish exactly which caliper is going on the car, and what that caliper needs, before anyone touches a lathe.

Original, or a later conversion?

If your Aceca left Thames Ditton with disc brakes, it has the AC Girling setup, the larger AC discs and the shared-pattern Girling caliper. If the front brakes are a later conversion or upgrade, you may find Triumph-based or other parts instead. That matters when ordering: measure the disc, and look for part numbers or casting marks on the caliper, before assuming which you have.

Where to find parts

Specialists can supply and identify these parts. BG Developments manufactures a full range of Girling historic calipers to original specification and lists AC among the cars it covers. Triumph and Austin-Healey specialists, who handle a great deal of Girling equipment, may be able to identify a caliper type from its dimensions. As always, confirm the exact part against your own car before ordering.

What is still not established

• The exact dates at which front disc brakes became an option and then standard fitment. Dates are widely quoted but the archive has not traced them to a primary source, so none is given here.
• Whether Girling made these calipers solely for the Ace and Aceca. The question was asked and never answered. The catalogue evidence argues against it, since the TR4 shares the part number.
• Whether AC relieved the caliper groove themselves at Thames Ditton. The correspondent working from the master catalogue believed so, but said plainly that he had no proof.
• Whether an owner can identify their calipers from markings at all. The owner who began the discussion found no part numbers on his.

In short

The front brakes on a disc-braked Ace or Aceca are Girling. The Girling master catalogue gives the caliper the same part number as the 1961/62 TR4 and the early Greyhound (64032520/1); the disc is AC-specific and larger than the TR3's, so it is not interchangeable. Whether the carriers really interchange, and whether fitting a caliper calls for relieving the caliper groove or reducing the disc, are questions on which experienced owners disagree, and the answer may depend on which caliper you are fitting. Measure your own parts, establish which caliper you have, and take advice from a brake specialist before machining anything.

Sources. This article draws on the AC Owners' Club forum thread AC Aceca front disc brakes, started 25 March 2026 and running to 14 April 2026, in which members compared their own experience of the parts against a Girling master catalogue and did not agree on every point, and on specialist suppliers including BG Developments. No individual member is named here.

Brakes are safety-critical and this is a general reference, not a substitute for the workshop manual or a brake specialist. Part numbers are a starting point, confirm them against your own calipers and discs, and with a specialist, before ordering or fitting.