Dunstall caliper seals

madass140 · Oct 25, 2017

The twin disc slider/calipers. I'm playing around with some of these parts and that the length of thread on the outer caliper plug covers the o'ring groove in the caliper when screwed home flush with the outer face of the caliper, its as if the thread on the plug is to long. I dont have the seal that fits in the caliper groove , if thats what the groove is for. maybe I'm missing something , anyone have experience with this "great" bit of engineering?

ando · Oct 26, 2017

Don

The caliper plug does not screw in that far. When the 0 ring is fitted to the plug it bottoms on a lip and holds the plug about 1mm proud of the outer face of the caliper. I have been using viton 0 rings for the piston and plug for years with no problems. The 0 rings are thin, about 1mm from memory and they are both a different size

ando

madass140 · Oct 26, 2017

Removed broken link

thanks ando for reply. my plug screws in until the thread hits the bottom side of the groove in the caliper. its sitting .5mm above the caliper face when screwed in, fitting an o'ring in the caliper wont work as the thread on the plug will destroy it. The caliper groove has a diameter of 1.450" and the piston 1.250" so if the thread was shortened so as not to damage the thread then an o'ring of more than .100" (2.5mm) section would ne needed. I think that the caliper plug should have its own groove and o'ring so it seals inside the caliper piston bore which is 1.375". I dont know if my pistons are genuine, but I think having the o'ring on the plug is the best idea.

ando · Oct 26, 2017

Don

It has been a couple of years since I had mine out. The piston looks correct. The O ring for the plug is a slightly larger dia than the 0 ring for the piston but the same thickness. The 0 ring for the plug fits over the reduced end of the plug and slides up against the beginning of the thread. From memory the outside dia of the 0 ring when fitted is about the root dia of the thread so the 0 ring goes in without damage.

I have a spare set of 0 rings here that I am currently trying to find and measure, so far without success. I will look again tomorrow when I get home from work and post the size.

ando

madass140 · Oct 26, 2017

yes, in hindsight and doing some measuring , my idea of having the o'ring on the plug is not such a good one as the caliper bore and thread in the caliper is the same diameter so to have some crush on the o'ring it would have to be forced past the thread. More measuring and shortening the thread on the plug and fitting a 37mm O.D x 3mm o'ring in the caliper groove would give the o'ring .47mm crush either side

madass140 · Oct 26, 2017

I might make some new pistons , the inner one with a threaded raised section inside for easy removal

madass140 · Oct 26, 2017

I tried the plug with the old caliper o'ring which is the same size as the o'ring for the plug I suggested above 37 O.D x 3mm
and it works fine except the thread is to long and needs to be shortened about 3mm

Mike B · Oct 26, 2017

madass140 said:
The twin disc slider/calipers. I'm playing around with some of these parts and that the length of thread on the outer caliper plug covers the o'ring groove in the caliper when screwed home flush with the outer face of the caliper, its as if the thread on the plug is to long. I dont have the seal that fits in the caliper groove , if thats what the groove is for. maybe I'm missing something , anyone have experience with this "great" bit of engineering?

It has been a few years since I played with my Dunstall forks, but I do remember the part# of the seals (they were all the same by the way). Their were a few versions of the forks, don't if they are all the same. At any rate, National seal # 123 is what fits my units.

Mike B

madass140 · Oct 26, 2017

yes, I believe they are the same size also, the size I calculated is only slightly different, I'll see how they go when they arrive.
I made a new pair of plugs with shortened thread so the thread doesnt chew out the seal. seems spot on now with a seal fitted.
Broken links removed

ando · Oct 27, 2017

Don

I found an 0 ring and it is .066”, so it would be a 1/16” thick which is a lot less than what is fitted to your piston. So it would appear that as Mike suggested, that different seals may have been used. Good to see that you have yours sorted.

ando

madass140 · Oct 27, 2017

I spoke to Ron Fraturelli and he said yes there were 2 different thread lengths on the plugs , so it seems I was correct in making
new ones accordingly.

pierodn · Jan 8, 2018

Hi.
Please, i must replace the pistons seals.
Do you know who can supply them or can i use an aftermarket types?.
Thanks.
Piero

Dunstall caliper seals

Similar threads