Shock mount size issue?

First experience of fitting shock bush kit and I’m bemused.

Bought a Rockshox Sid shock which came with a new metal DU bush (steel, phos bronze lined) already fitted. The Bore measures* 12.72/12.70 (I don’t think it’s round).

Not knowing it had a bush fitted, and prior to its delivery, I ordered a ProFin mount kit and I’m confused by the sizing. The ProFin pin measures 12.73** and unsurprisingly won’t fit the bush that has been pre-fitted to the shock.

The ProFin bush is plastic (probably oil filled nylon?) and the bore measures 12.95mm, so the pin is a very loose fit. Much too loose. Would the bush close down to, say. 12.74 when pressed into the shock?

I’d prefer to use the DU bush but obviously can’t as it stands. Is there some kind of standard these things are made to or are they designed to be not interchangeable?

Thanks.

*measured to the best of my ability with slightly tired Mitutoyo digital calliper

**measured with a very nice Mercer micrometer

You press out the metal du and insert the complete kit surely

Welcome to the world of inaccurate bushing sizes.

A key thing with polymer bushes, is that the sizes are only accurate when fitted in an accurate bore, so the size/play you’ve measured is about right for a non-fitted bush.

Profin will likely supply Igus G bushes (nothing oil filled, just generic hard wearing dark grey polymer), and having tested various Igus bushings, G tolerance is around 12.73/4. Igus bushes are officially meant to run on 1/2″/12.7mm bar, but not with zero play. Igus J (low friction/beige colour) is even worse, with fitted dimension coming in around 12.75-8mm.

Thanks for reply. I find it hard to accept that something that is supposed to be a precision fit is, er, not very precisely made. Surely, a 12.7mm dia pin, which is seemingly a standard size, should be 12.7mm+0/-.02, and should be a good slide fit in a 12.7mm bush**. It’s all a bit shit. In a previous life I’d have made my own out of phosphor bronze and it would work better than the expensive stuff I’ve been sold. Rant!

** or is it deliberately oversize so that it won’t fit anyone else’s mount kit. Dunno.

As mentioned remove the metal one insert plastic one.

I bought the tftuned bushing kit nice and easy, insert tool, gentle tap with hammer and it’s out

Never got on with metal bushings and always changed them for nylon ones that come in 2 halves. Easy to push in.

Had a shock wear prematurely due to metal bushings that didnt allow any lateral movement when the frame flexed a little. Softer nylon bushings just seemed safer.

It’s neither the bush or the wrong size. Profins bushes are made to 12.73 which is tight/too tight on a Rockshox shock, but will work better in a fox which tend to be slacker.

That’s my point, really. Why 12.73? Can’t we have an agreed size amongst manufacturers of shocks and bushes.

I’ll fit the ProFin plastic bush but I’m not optimistic the pin will be a good fit. I’ll report back.

Hi Tim,

The poly bushes do indeed compress when inserted into the eyelet. Metal ones do but they have a split to deal with the size change. There should be a sticker on the bag explaining.

12.73 is correct. 12.7 dead will be loose/too loose, however if you want one at this size feel free to get in touch.

Btw in addition the grey poly bush is from IGUS

Swap out the metal bush for the plastic one in the ProFin kit, as it should compress to fit. It’s odd that sizes aren’t consistent between brands.

Swap out the metal bush for the plastic one in the ProFin kit, as it should compress to fit. It’s odd that sizes aren’t consistent between brands.

Yes its a total crapshoot and you even find variation within brand.

Obviously there’s some allowance for the type of bush thats intended (some are more compressible than others) but you even find within brand that you can get some shocks where the hardware just falls through. Fox even sell 2 sizes of bushes to cater for slightly undersized (or was it oversized?) holes.

RS shocks are on the smaller end of the spectrum so more prone to being tight compared to say fox or formula, and as fox/formula then use a less compressible top hat style bush too, in combo it can mean sometimes the stock hardware just drops right through! Sometimes the solution is to change bush, sometimes it’s to change bush and hardware. Whats good about the stainless steel hardware is once you find the right size you can use that forever as unlike Alu/Brass hardware it won’t wear out and so its just a case of buying the same bushes each time *

* Ok so it will eventually, but it takes a really, really long time.

Okay, so I pressed the steel bush out and fitted the nylon bush, and yes it closed down to 12.74/12.73 (with my tired Mitutoyo callipers). The pin is a VERY tight fit, so tight that it won’t rotate in the bush, locked solid. When mounted the shock will be rotating on the 8mm did sleeved bolt that passes through the frame, so steel on steel, the poly bush doing nothing. Bonkers.

To say that Fox is a looser fit than Rockshox just reinforces the point I’m trying to make. From an engineering point it’s just not good enough.

When I had bushings replaced recently, I had the eyelets reamed as well to free things up.

Before I started making shock bush kits naturally I had to decide on fit and tolerance.

The problem is you have the shock manufacturer’s eyelet tolerance, the du bush tolerance and then trying to get the perfect fit with the hardware between the two.

I measured month’s worth of shocks in for servicing before settling on a size/tolerance, the tolerance is H7 btw so +0/-0.018mm.

With the measurements I took over most brands, Fox, X Fusion, Rockshox and DVO I predicted that by dropping 0.01mm on current size I would see between 4 and just over 20% come up slack depending upon the brand. This would be unacceptable hence current sizing.

As Ben says this happens a lot with oem hardware.

If you getting 12.73/74 on the ID I would expect that to be a little slack but as you say your mic may be tired.

It should be an interference fit and the frame should clamp the hardware in place. If you can spin ot freely by hand you will feel play when everything is bolted down.

I can’t see an order from anyone called Tim in the last month but even if you bought it somewhere else feel free to get in touch, I can make one with a smaller diameter if you like. That way when you settle on a size you are happy with I can make future orders to that if you let me know ahead of time.

Informative and very helpful response from Mr Bounce. Feels like he is being unfairly reamed (pun intended) for the wide tolerances of the shock manufacturers.

Bounce Cycles, thanks for the reply and it all makes sense, and I apologise for my rant, and I sympathise with regards to the manufacturers different sizing. To be fair, a digital calliper, tired or not, is not the right tool for measuring a bore, as I’m sure you know, so I cannot with confidence praise or condemn the machining. However, I’d be surprised if you made something outside of your own tolerances. What does surprise me, though, is the deviation between Rockshox and Fox. Surely a standard should have been established. Poor, in my opinion.

In a previous life I worked in the precision machining world, an army of CNC machine tools at my disposal. Regularly turned to less than 8 microns in stainless on our Star sliders. I’d have accurately measured the bore of the shock and turned something up.

For now, I’ll persevere with your products and see if it will bed itself in.

BTW, I bought from Merlin, always my first choice for mail order, they have been excellent over many years.

No apology necessary at all, I have confidence in what I do and I measure every one I send out so I have no need to be offended.

Your measurement on the OD of the hardware is the same as mine and that matches MC’s figure described above so sounds like 3 of us are getting it consistent.

I am in it for the long haul though so if someone isn’t happy, they have my support, no question and I will keep going until they are happy.

However and I hope this doesn’t sound dismissive, but bushings being “too tight” is a pretty rare occurance even with differing tolerances between manufacturers at the size I spec. I would be surprised if I have had half a dozen messages to this effect in 7 years and many thousands of units sold. Of those two that I can think of said “yeah I see what you mean can I go back to the original size?” etc when I sent them one around 0.02mm smaller.

Press fit poly bushes are actually really good at soaking up some of the eyelet tolerance issues which I think is probably one of the reasons a lot of the people selling hardware in the UK spec them.

Just clamp one end of the shock in the frame. It shouldn’t flop around freely, but with modest effort move unhindered, if it does you are golden. If it flops around it is too slack as even if there isn’t play straight away, there soon will be.

Yeah as you say verniers won’t do the trick and would explain you getting .73/.74. I had it in my head you were using a micrometer. You should be getting more like .71/.72.

Sure it will be fine but holla if you want to try a smaller size, as I say just clamp one end and feel how it goes.

As for bike manufacturers all committing to standard, well, I have some thoughts on this………

As best I can tell with a decent mic, the pin measures 12.735mm

No I mean 12.71/72 on the bore sorry. 12.73 on the OD.