V Brake fitting problem

[Si90]

Hi all.

As I said in my introduction, I've returned to cycling after a 10 year gap, dusted off my 94 Orange Clockwork and embarked on restoring it to it's former glory, and taking the opportunity to upgrade a few parts at the same time. I am putting on XT M739 components and also want to take the opportunity to add suspension forks to replace the F7's I had put on when I bought it.

This is where I have hit my first problem. I have a beautiful set of Pace RC36Evo forks with a 1" threaded steerer that I have had serviced by Tim at Fork English (superb service by the way) and have a set of XT M739 V brakes. My problem is the brake won't slide all the way onto the boss and stops about 2.5mm from the end. This means that the small pin only just seats in the hole and there is rather a large gap where the fixing bolt goes. Can anyone tell me from the pictures below if the bosses are not original Pace item or they are but I need to replace them with some more suitable ones, in which case does anybody know where I can get the right ones. The rim/step on the bosses that are fitted is deeper than the recess in the clamp even though they are screwed all the way in and the depth of the flat area for the spanner seams to be too deep also. I know that these brakes will go on these forks as I have seen it done on this forum.

Thanks to anyone who can help me out.

Si90

 

[Chute55uk]

I would say the shoulder of the boss where you would put the spanner to screw the boss in is too big, get some calipers and measure it and the hole in the vbrake, if it is too big then get some smaller bosses of a file and make them smaller.
on a separate note this thread is the gold standard of how you ask a technical question for all you newbies out there.

 

[legrandefromage]

undo the brake bosses and show us a pic of the thread - I'll compare and see if I have and spare that will fit, yours look as if they dont sit deep enough into the Pace part.

 

[Chute55uk]

looking closer it seems that either the ramped part of the shoulder (closer arrow) is interfering or the shoulder is too deep. (further away arrow)

 

[legrandefromage]

the boss is sitting proud when it should be flush.

 

[Chute55uk]

Got you

 

[Si90]

Thanks you guys for the quick remarks.

undo the brake bosses and show us a pic of the thread - I'll compare and see if I have and spare that will fit, yours look as if they dont sit deep enough into the Pace part.

I will take out the boss again later this evening and post a picture with the key measurements.

looking closer it seems that either the ramped part of the shoulder (closer arrow) is interfering or the shoulder is too deep. (further away arrow)

Your right, although I think it is a combination of everything being that little bit too big. The first shoulder going into the clamp recess is too deep, the ramped part is too big and the flat shoulder for the spanner is also too deep. Again I will confirm with measurements later.

I have looked at getting some new ones but nobody seams to give any key measurements only the thread size, which I know is M10 but not sure of the thread pitch. I don't want to just keep buying bosses until I get some that fit. Can't help think though that there should be some out there somewhere that fit without resorting to a file but if it comes to that then that will be the path I will have to take. I have noticed in my travels that M8 bosses don't have the first shoulder that sits in the recess of the clamp but all the M10 ones do. May have to have a trip to LBS but they don't hold many spares.

 

[Si90]

So after taking a few measurements I now know what I am looking for. The picture shows the bosses I currently have.



From the measurements its clear I have more than 1 problem.

1. the shoulder that goes into the Pace clamp is 3.05mm but the recess in the clamp only measures 1.7mm - thats why it sits proud and not flush.
2 The shoulder used to tighten the boss up measures 5.45mm but the recess in the V brake is only 4.3mm.
3 All this adds up to a 2.5mm gap between the brake and the the boss clamp as in my earlier picture.

The long shot of all this is that I need a brake boss that measures in the region of 4.3 from the top of the tightening shoulder to the second shoulder and then that shoulder to be 1.7mm to recess into the clamp. This would also mean that the plain shaft part would also have to be longer than its current 16.58mm. The M739 measures in the region of 25mm thick so I need the plane shaft to be about 20mm this would give a correct distance of 24.3mm from the end of the boss to flush with the clamp.

If anyone has a set of bosses they think will do the job I would much appreciate it if they could tell me where they are from - if they know so I can use that as a starting point to try and get some.

Sorry this seems to have become rather long winded for what seams like a simple component, god knows what will happen with the rest of the bike when I get round to it :roll: :roll:


Thanks in advance

 

[We_are_Stevo]

Is it a 'shoulder' turned on the boss or a very tight washer?

I ask because I had some like that and didn't realise until I dropped one on the floor and dislodged it!

 

[Rampage]

I have some of these somewhere, I will try and measure one later.