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PostPosted: Wed Feb 19, 2020 10:21 pm 
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My docs say not to mix the trials seal on a c2 but agree that the c2 closed will be ok. 20 microns is not much in production items for brake calipers. Unless the measuring device used, was very high end, most digital calipers only have a resolution of 10 microns.


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PostPosted: Thu Feb 20, 2020 1:02 am 
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Joined: Mon Feb 10, 2020 7:05 pm
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dyna-ti wrote:
Howdo :D

I've some seals, but as to what exactly they are is anyone's guess, and tbh they are really close in inside diameter.
I would say one is thicker slightly, but I know these to be mini pro trial caliper seals, and checking the documents it lists it as a different parts number for the seal. inside diameter appears to be 25mm, which is the same for C2 and mini trials. I cant see that the seat on those two can be so radically different, and for that matter cant see how the O2 is that much different if its the same sized piston.

0.06 or 0.07 of a mill arent critical sizes here. This isnt microbiology those differences are so tiny that im sure -/+ in manufacture would easily eat them up. Its rubber, not ceramic alloy.

Without looking at all the components, and the caliper sides and piston, and im not in any way denying you're at fault, or even wrong, I just cant see how it would make that great a difference that tiny.
If thats a help :? sorry cant really offer any more assistance :)


Hi Dyna-Ti - thanks for your reply.

Whilst I agree that 0.06 or 0.07 of a mm (I assume you didn't mean mill as in american 1/1000 inch) is not very much at all and falls inside tollerances, surely the piston should be a fairly tight fit and stretch the rubber a little to ensure it seals? In that case, even a gnats d..K too big and it would leak - especially under pressure?

I'll have to strip my back brake caliper again, which is a CLOSED one, and messure the pistons - but I know they are a little bigger than the OPEN system ones, hence the 25mm id seals being nice snug fit, and no leaks.


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PostPosted: Thu Feb 20, 2020 1:07 am 
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mdvineng wrote:
My docs say not to mix the trials seal on a c2 but agree that the c2 closed will be ok. 20 microns is not much in production items for brake calipers. Unless the measuring device used, was very high end, most digital calipers only have a resolution of 10 microns.


So I presume that is saying that the 'trials' and C2 Closed can both use a 25mm id seal. But leaves the C2 Open still unresolved?


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 Post subject: Re:
PostPosted: Thu Feb 20, 2020 5:45 am 
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Sadly yes as the data for those was sort of drawn on a fag packet and never correlated to the rest of the brake group components, from what I have found. Dyna ti is the expert on these things. When the seal is fitted into the shell groove the id becomes smaller than when sat on the table.
The trials seal should never be used in a c2 as the dimensions are not compatible.
So for now you could try the c2 seal and see what happens ,then try the motorcycle route and hope it isn't too tight and not retract the pads.

see https://www.hopetech.com/_repository/1/ ... -chart.pdf

ideally we need the groove dimensions to determine the best route. or you could just source a closed C2 caliper and have a current spares supply :)

Dave s


Last edited by mdvineng on Thu Feb 20, 2020 8:42 am, edited 1 time in total.

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PostPosted: Thu Feb 20, 2020 8:39 am 
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rwm1962 wrote:
Pretty sure mine were round. I'll look them up online later as not stripping down again! May be able to tell from pic.

Wouldn't square section be more like to leak in a moving application like a piston?


never use round section in brake calipers. the seal area contact is; a) smaller b) the flex of the square section is used in part to retract the piston


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 Post subject: Re: Re:
PostPosted: Thu Feb 20, 2020 11:20 am 
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Joined: Mon Feb 10, 2020 7:05 pm
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mdvineng wrote:
Sadly yes as the data for those was sort of drawn on a fag packet and never correlated to the rest of the brake group components, from what I have found. Dyna ti is the expert on these things. When the seal is fitted into the shell groove the id becomes smaller than when sat on the table.
The trials seal should never be used in a c2 as the dimensions are not compatible.
So for now you could try the c2 seal and see what happens ,then try the motorcycle route and hope it isn't too tight and not retract the pads.

see https://www.hopetech.com/_repository/1/ ... -chart.pdf

ideally we need the groove dimensions to determine the best route. or you could just source a closed C2 caliper and have a current spares supply :)

Dave s


Yes looking at the chart, the seals shown for trial and c2 are slightly different widths (0.4mm). So if that is too much to be compatible, that strengthens my feeling that the 3mm width I'm looking for is never going to work with a 2mm or 2.4mm width seal even if the 25mm id reduces when pushed into the groove.#

The grooves on my c2 Open system are 3mm.
Also the old seals are a very snug fit (stretching the seal slightly) on the 24.95mm pistons, whereas the new c2 Closed seals are pretty slack fit.

I also agree with your statement about square section being the way the pads retract from the disc when pressure is released. That the way nearly all car braking systems work. I guess the closed system may be different as it has return springs behind the pistons - the open system does not.


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 Post subject: Re:
PostPosted: Thu Feb 20, 2020 11:33 am 
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personally I would replace the caliper to the closed C2, if you want to stay period. Alternatively if you want to stay open C2, you could have the grooves machined to take the motorcycle 3.14 seals, as I tend to think they will be too resistive as is.


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PostPosted: Thu Feb 20, 2020 12:03 pm 
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Joined: Mon Feb 10, 2020 7:05 pm
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I'd also have to replace the lever master cylinder as the closed and open are different.
I hadn't thought about machining.... that may be worth looking into... might be expensive though?
I wonder if the 0.14mm could be removed from width of the seal with wen and dry on a flat surface? I had a car engine oil pump which you could get back into tollerance by 'grinding' with paste on a flat glass sheet.


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PostPosted: Thu Feb 20, 2020 4:23 pm 
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pburger wrote:
I'd also have to replace the lever master cylinder as the closed and open are different.
I hadn't thought about machining.... that may be worth looking into... might be expensive though?
I wonder if the 0.14mm could be removed from width of the seal with wen and dry on a flat surface? I had a car engine oil pump which you could get back into tollerance by 'grinding' with paste on a flat glass sheet.


why change the m/cyl both calipers are the same piston config.

I would only do that if you can freeze the ring and keep it flat at the same time, as when it is flexible you will end up with a rounded surface and poor sealing performance. make something 3.14 thick and see how it fits in the groove.


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PostPosted: Thu Feb 20, 2020 4:41 pm 
Dirt Disciple

Joined: Mon Feb 10, 2020 7:05 pm
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mdvineng wrote:
pburger wrote:
I'd also have to replace the lever master cylinder as the closed and open are different.
I hadn't thought about machining.... that may be worth looking into... might be expensive though?
I wonder if the 0.14mm could be removed from width of the seal with wen and dry on a flat surface? I had a car engine oil pump which you could get back into tollerance by 'grinding' with paste on a flat glass sheet.


why change the m/cyl both calipers are the same piston config.

I would only do that if you can freeze the ring and keep it flat at the same time, as when it is flexible you will end up with a rounded surface and poor sealing performance. make something 3.14 thick and see how it fits in the groove.


The open system does not have the adjuster assembly that the closed system has.

Get your point about keeping it flat... might be difficult.


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