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Front caliper mounts

Posted: 31 May 2012 08:17
by nickst4
Not what I was looking for as I replaced my serviced caliper: a stripped thread on the steady-pin! The Dart has a sliding caliper which is attached by a big 10mm screw through the fork mount, but the caliper is kept aligned by a smaller turned-down 8mm bolt below. It was this lower thread in the fork leg that had stripped, and there is so little material that a helicoil just wasn't on. The only discrete way of repairing it would be to weld the hole up and redrill/thread, but I devised a better in-situ method using a close-fitting stainless peg kept in place by a retaining-plate on the outside. I reamed the old hole out to a fraction bigger than the base of the old threads, and the new peg gets much more support than the short thread of the original provided, and without the wedging action that probably weakens the mount. Perhaps it look over-engineered, but it is safe and invisible behind the guard, where there is enough clearance for the hex-head bolt.

On to the next job!

Nick

Re: Front caliper mounts

Posted: 31 May 2012 20:58
by morini_tom
Nice repair!

I cracked the lug off on a set of dart forks a while back. I'd like to say it was my heroic braking at Cadwell but I suspect not! Forks are not the Darts strongest point...

In all seriousness though, that sliding calliper pin should have a thin plastic bush (actually more like a thin rolled tube, white plastic) which sits inside the slider on the calliper. I'd never seen one still in place on a Dart until I bought a NOS calliper which came with the pin and the bush, and then again when I bought a seal/slider overhaul kit. With the bush in place the calliper slides freely but there isn't radial play in it. I suspect without it the calliper is able to hammer the sliding pin and that's not going to help the weak pint on the fork lug.

If you don't have this bush I'd advise seeking one out- could be an impossible task but I'd happily photograph the one in mine if it helped, and I could probably find a brembo part number.

Re: Front caliper mounts

Posted: 01 Jun 2012 05:51
by nickst4
Thanks Tom!

My caliper has what I assumed is the original rubber sleeve cum bellows in that steady-pin hole, and the fit is snug so I've not investigated further. While being Brembo, it resembles closely the setup on my Honda NTV. Do you think your white plastic sleeve is in addition to the rubber tube or a substitute?

Nick

Re: Front caliper mounts

Posted: 01 Jun 2012 20:06
by morini_tom
The white plastic sleeve is in addition to the rubber bellows. If you look closely you will see a groove in the rubber on the side closest to the pads which retains the lip on the plastic sleeve (this is just visible in the bottom photo.

The plastic sleeve forms a tight sliding fit on the pin (actually I think the sleeve is probably teflon). Without it the pin isn't massively loose but you can certainly feel some sideways play.

I took some photos of my NOS caliper so you can see where the sleeve is and what it looks like.

When I overhauled a caliper a while back the overhaul kit had a new pin, bellows and plastic sleeve. This came from Motomecca. Afraid I can't find a part number but could dig deeper if motomecca don't know the number. This was probably 10 years ago now though.

Caliper, pin and sleeve just visible down the bore:
Image

Sleeve pushed out of the bore for better visibility:
Image

Pin pushed into the sleeve:
Image

Hope that#s some help,
Tom

Re: Front caliper mounts

Posted: 02 Jun 2012 06:50
by nickst4
The Man's a Star: I'm sure I said that already! :D

Yes, your guess was right, as there's no plastic bush inside the rubber sleeve on mine. Presumably, the corrosion everywhere had it sticking to the pin and it got damaged/lost when a PO took the caliper off. Now that I check it, I can see there is some slop in the fit of the pin in the installed rubber sleeve. I'd be grateful for some measurements, as I've got some PTFE or nylon that I could turn to fit. Presumably, it needs to be as thin-walled and long as possible, with that lip at the outer end. If you aren't able to measure the thickness, perhaps I could borrow your NOS one to see? I'm off to the US tomorrow for a couple of weeks and will check Motomecca for the Brembo overhaul kit when I get back. Or, indeed, ask TAW-Vehicle in the States, who are very good on Brembo parts at reasonable prices.

Thanks again, Tom!

Re: Front caliper mounts

Posted: 02 Jun 2012 07:42
by EVguru
I think the problem is caused by corrosion between the rubber sleeve and the caliper body. The pressue causes the caliper to sieze on the pin and it's either pushed out during re-fitiing (leaving the owner to puzzle over a strange bit of plastic) or left out deliberately.

I don't remember if it was on a Dart or a Freccia, but I've had that problem. The pistons, pins and seals are the same on an Excalibur RLX caliper (although the mounting centres are different) and that donated the required parts.

As I recall the sleeve is split so it can be installed.

Re: Front caliper mounts

Posted: 02 Jun 2012 09:12
by morini_tom
Paul's right, the sleeve has a slit.

Just checked my dart paperwork, the rubber and sleeve I bought from Motomecca was part number 122.4746.1 "Caliper mount pin rubbers SL2"

Just looked at their website and they list "122.4746.10 Caliper Mount Pin Rubbers Sl2.28 £7.92" which I assume are the same but a call to check model compatibility would be sensible as the part number is not identical to the one I bought.

Failing that then I'd happily measure mine but it's pretty thin walled and soft so wouldn't be a delight to turn (I would say that though, I'm crap on a lathe!!)

Tom

Re: Front caliper mounts

Posted: 03 Jun 2012 05:32
by nickst4
Thanks for the info, Tom and Paul. I'll chase that up in a fortnight, after a break in the US.

The last job I was doing yesterday was to brass-sleeve the flange of the speedo-drive hub so that it was actually concentric with the spindle hole, giving the (new) oil-seal in the NS of the front wheel a chance to keep grease in and muck out. How it got to be so eccentric I don't know, unless a PO pulled the whole thing apart and didn't get it back together properly. That big narrow oil-seal and the bearings were easily obtained from simplybearings.co.uk, with next-day delivery. Good firm!

Nick

Re: Front caliper mounts

Posted: 28 Jun 2012 07:24
by nickst4
Follow-up to the matter of the little sleeve in the caliper steady-pin hole: MotoMecca can no longer supply the repair kit we talked about. No surprise there then, so it's just as well that the rubber bellows are intact on mine. I did eventually manage to turn a sleeve out of nylon, 0.5mm wall thickness with a flange, and it slides sweetly on my new steady-pin. Even better, the caliper fitted to the forks without any strain, so the holes must be pretty well lined-up. Nice new EBC HH pads have gone in too, I glaze-busted the disc and remounted it with 8mm stainless bolts, it runs acceptibly-true and the hydraulics bled through nicely. I always finish front brake bleeding by tilting the bike so the reservoir is highest and tickle the lever, purging any air that is lodged in the top of the hose or banjo via the recuperation hole. Always works a treat, especially with integral reservoirs, and far better than fighting gravity by forcing bubbles down to the caliper. OK, so I could reverse-bleed them if I had the kit....

Nick