Could the banjo bolt be restricting brake pressure?

[andrewp]

This may become a topic of risk/benefit engineering discussions.

On the front calipers, the 10 mm diameter banjo bolt possesses tiny internal drillings. The bolt's longitudinal drilling is 2.5 mm and the single lateral drilling is even smaller at 1.9 mm. Most of the brake systems I have worked on have double lateral drillings on the banjo bolt. Since the single drilling is so small . . . is it not possible this restriction limits the rate of fluid flow to the caliper piston? Moreover (as these cars are over 30 years old), rubber dust -I assume from master cylinder seal wear- accumulates in the banjo bolt drillings. Mine were about 25% blocked by this fine black dust.

What I plan on doing is machining the central bore to 3.5 mm diameter and drilling through the lateral hole (to make two holes) of 2.2 mm diameter. This will at least partially relieve the flow restriction at the banjo bolt. The bolt is 10 mm in diameter so I do not suspect this will weaken it to point of snapping. But that is the risk of modding this piece.

I just want to note that diesel Ford truck owners are doing similar modifications to their fuel rail banjo bolt. After enlarging the bore and increasing the lateral drilling number, some owners have reported a 0.4 second improvement on a 1/4 mile drag run through this mod alone.

What do you think of this idea?

Andrew in Edmonton

[dapinky]

I understand what you are saying about increasing the fluid flow capacity by (I assume) cross drilling at 90 degrees to the current hole.

I have never seen a banjo bolt with 2 drillings (4 holes in the circumference), and haven't ever measured to see if they all have the same size drillings anyway......

...... my concern is that if this were necessary, then someone would produce such a bolt, and they would be widely used - and I've never heard of it (but I may not have been listening!). From that, my concern is that if it weakened the material sufficiently to cause potential catastrophic failure. As with all things, the maximum pressure in the joint will be at exactly the moment that you require maximum braking - and is the time when losing that pressure will have the most effect.

Without doing a material stress test, I wouldn't be doing such things to my standard bolts...... but...... if I thought it necessary, I may buy a pair of titanium bolts and try the extra drilling on those.

Maybe a better approach would be to simply increase the 1.9mm to 2.5mm (or whatever number is suitable). 1.9mm hole = 2.84sq/mm area whilst 2.5mm drilling = 4.91sq/mm..... so just a little less than a pair of cross-drilled holes, BUT without so much weakening of the structure, and with less fluid dynamic losses due to the wall areas.

TBH, I can't see the necessity, but if you feel that the current bore is insufficient due to debris contamination, then maybe a change of fluid every 2 years would prevent the worry.

[andrewp]

The banjo bolt has one single hole in it. It is NOT through the bolt itself (so NOT two holes).

I was suggesting to drill through the bolt (to make two holes) when I enlarge that lateral hole.

[Jamie N]

Most modern front brakes that I've ever worked on have a central and lateral hole between 3 and 3.5 mm, can't imagine taking the Elan ones up to those sizes will do any harm,
and quite possibly will make an improvement.

The real improvement comes from ditching them for something far more efficient.

[Tuga2112]

have you tried to look up if there are banjo bolts made with bigger holes ?
if the holes would be an issue, the manufactorers would be ordering them made with bigger holes.

its worth checking out multiple listings of banjo bolts with the correct thread and ask the sellers for the hole diameters.

do the PMN upgrades have different banjo bolts ? if they dont. I'd stay away from it.

[dapinky]

The banjo bolt has one single hole in it. It is NOT through the bolt itself (so NOT two holes).

I was suggesting to drill through the bolt (to make two holes) when I enlarge that lateral hole.

Ah, right, I misinterpreted your wording and read it as one hole, all the way through the bolt, not as one hole from the centre bore to the outside....

In that case, I can see no great problem in continuing it all the way through, as that is how most banjo bolts already are.