Topic: The DOT3/DOT4 Gospels... Preach on brothers

[ev0lve]

I'm swapping to dual discs up front on my K5. I've slowly gotten all the bits in line to make it happen and one of those bits is a 1991 Katana master cylinder which clearly states to only use DOT4 fluid.

Reading up on DOT 3/4 it seems they're not chemically much different and mostly the rating is for boiling point. Poking around the accepted wisdom seems to be that you can run 4 in a 3 rated system but not 3 in a 4 rated system.

So, the questions are, why is that (if true) and, assuming a full flush, can you run either 3 or 4 without issue?

Thanks!

Edit: Some more reading finds that DOT 4 has "borate esters" to increase the boiling point and those may or may not corrode some components in unexpected ways as well as the viscosity is different which may or may not screw with the various seals. Urg.

So if I'm reading this right the Katana master cyclinder has been tested with 4 but since 3 doesn't contain the ingredients that the master cylinder is rated for I should be safe staying with 3? Double urg. Advice please...

[Chris Liston]

I'm pretty sure Dot 3 and 4 are compatible with each other.  Its DOT 5 that you need to fully flush.  The master cyl will be fine with either.

[Sporkfly]

The theory has always been that they can mix...

Here's a test... Take a clear bottle and put DOT 3 and DOT 4 in it. Watch for separation over the course of a few days.

Anyway, what I would do is pick DOT 3 or DOT 4, flush the system with it and go with it. I think DOT 3 would be fine - if you're running calipers that have had DOT 3 their entire life I'd go with that.

[Hush]

Can't get DOT3 here in Godzone anymore so we just gotta use DOT4.

[ev0lve]

Can't get DOT3 here in Godzone anymore so we just gotta use DOT4.

So that seems pretty definitive. Just pick one poison and stick to it. Since I have a jug of DOT3 I think I'll use it.