bikemore 

So I have a Trek 720 with fork with a crown race of 26.6. I've read in numerous places that this can be fixed by milling. But it turns out that it will be a while before I can get to a bike shop that can do the milling. Has anyone sanded a crown race to get .2 or .15mm off? Is there too much risk of getting it out of round?

Eric S.

I've used a circular grinding attachment on a drill to open up a crown race a couple times. Kind of crude, but no ill effects.

Otis

Not sure why the crown race would be this size, but to cut to the chase. Have a pro cut the fork race if really needed. If you are going to try a backyard fix then try to file, sand, grind, or whatever, on the headset only.

bikemore 

definitely a mystery why it is at 26.6. Perhaps it was milled before from 27.

JohnDThompson 

Perhaps a replacement fork? Trek-built forks were all milled at the factory to ISO spec: 26.4mm

OP: a Trek 720 is a nice enough frame to justify doing the job properly. Have a bike shop mill the crown race seat with the proper tool.

bikemore 

It appears to be the original fork. I taken a hint from the previous HS used - I used a Stronglight HS with roller bearings. The crown race piece is plastic. .2 mm came off when I put it on. I think it will work for a while. I am considering finding a shop that will do it after I do a short bike tour.

JohnDThompson 

Does it have a Haden "Europa" fork crown?

bikemore 

Yup.

JohnDThompson 

Ok. It's likely original, then. Treat it nicely!

verktyg 

Question?

Is the headset crown race 26.6mm or the mounting area on the fork crown where the race fits 26.6mm?

Per @JonDThompson, the fork crown should be 26.4mm,

Sounds like you have a Stronglight B10 needle bearing headset. The cups, cones and crown race are made of Delrin plastic. The needle bearings run on hardened steel races.



Those headsets were available with 26.4mm, 26.5mm and 27.0mm bores in the Delrin plastic crown races.

A 26.6mm bore would be a "fall on fit" on a 26.4mm fork crown. It should be a slight interference fit.

0.2mm is a lot of slop in a crown race and after a long ride it will probably get larger! You will be able to sense this a clunk when you brake hard, come to a stop or hit a bump.

A suggestion... Take an aluminum beverage can, using scissors, cut out a shim to fit between the inside of the crown race and the fork crown. This will keep the crown race from getting further damaged.

If it were a metal crown race, I'd recommend Loctite bearing mount but it may not play well with olde plastic...



I always reface the fork crown and the top and bottom of the head tube anytime I have a headset apart. I have the tools to do it but many shops don't or don't know how to use them properly.

Trek built quality frames and yours was faced before it was painted so the head tube and fork are probably still OK (unlike many Holy,Holy scared European, especially high end Italian frames!). It wouldn't hurt to have the fork crown refaced just for yuks!

Your Stronglight B10 headset is ~30 years old and plastics get long in the tooth over time.

I'd recommend replacing the head set with either one of the new Stronglight A9 headsets with precision ball bearings in cartridges. They work very well and look the same as the old needle bearing A9s.



If you buy one of these, make sure that it's made of aluminum alloy. There are some black ones being sold that are black painted steel and have loose bearings not cartridges.



Another suggestion would be a Tange Levin CDS headset. They are very well made with through hardened precision ground steel races. They come in polished chrome, satin chrome and black finishes and have plastic dust seals. You can find them for under $30.00 USD.



BTW... the French have had an almost unholy, kinky thing about DuPont Delrin® plastic... Think Simplex derailleurs, MAFAC brake levers, Stronglight A10 headsets.... Maybe it has to do with the DuPont name being French or maybe their attraction to Jerry Lewis!

happy sailing...

verktyg

Chas.

verktyg 

The Stack Height for Stronglight B10 and A9 headsets are variously listed as 37mm, 38mm and all the way up to 43mm!

The ones that I've checked were around 37-38mm.

The Tange Levin CDS has a listed stack height of 33mm but I think that they are a little higher.

It's hard to go by published specs because they don't say if the height includes washers....

Use ball bearings in a cage to measure the A or B dimension. For assembly I use loose balls rather than cages in headsets and BBs.



Easy way to measure the stack height capacity of a frame:




verktyg

Chas.

bikemore 

Anyone have a clue what shops in Boston would do facing on short notice? Has anyone used Paramount in Somerville? I could try Belmont, but I don't know how good they are on turn around (ie, I am not big on being blown off).

Ya, I measure and quickly forgot the stack height, I think it is 40+.

I do have a box full of 26.4 HS. When they didn't go on like I am used to is when I got the caliper out to measure the diameter. The frame I have is pretty clean. Everyone swears that the race would be at 26.4. I really don't have a clue how it got the extra .2mm. I don't know about Trek, but it wasn't hard to find others who found unusual size fork races.

Chombi

Definitely keep the Delrin cups away from Locktite as some of them are known to attack plastics. I think the blue stuff is definitely one that does, as found out by many motorcyclist when they used it on fairing mounting bolts and have their fairing (ABS plastic) literally crumble on them overnight right next to the bolts they applied Locktite to.
Not sure if Locktite red or gap filler will do the same, or if Delrin specifically, is vulnerable to these compounds, but I would not take the risk if the headset was mine....

bikemore 

It is definitely the fork crown race that is 26.6. It seems like it is slightly out of round as the 26.4 HS race will fit one side better than the other. I believe the HS race will slide down on the back more than on the front.

verktyg 

There's some miscommunications in terminology here....

The headset "crown race" is the removable piece that fits on the top of the fork crown that the bearings ride on.

The somewhat industry standards for the inside diameters are 26.4mm, 26.5mm and 27mm.

The machined area of the fork crown where the crown race fits is called the "crown race seat".

The crown race seat area of fork crowns before the steerer is brazed in are at least 1mm to 2mm larger than after they seat has been cut down to the required size. The tools used to cut the seat to size usually insure that it is parallel and perpendicular to the steerer.



Sutherland's recommends that the crown race seat should be 0.025mm to 0.05mm (.001" to .002") larger that the inside diameter of the crown race to provide an interference press fit.

If someone unevenly forced a metal crown race onto the bearing seat it could have pushed up a slight ridge in one area of the seat causing it to be out of round.

Refacing the seat corrects for any out of roundness or flatness problems in the bearing seating area.

A Delrin plastic crown race will deform to fit an out of round boss/seat but could cause uneven bearing rotation.

Many of the fork crowns that I've refaced were not properly cut at the factory (especially on many top name Italian frames)...

A Trek frame was most likely properly machined before assembly.

verktyg

Chas.

CliffordK

I'm not sure what kind of fancy tools a bike shop might have. However, a good machine shop should be able to chuck the steer tube in a lathe. Let the fork tines spin... And cut down the bearing seat pretty quickly.

It shouldn't take more than a few minutes.

Do you know anybody with a lathe?

Just don't take too much off

As mentioned, it might be safer just to mill down the crown race from the headset, and not the seat.

miamijim

Hit it with a hand file. file a tick, rotate. file a tick, rotate, file a tick, rotate.

Road Fan

Another thing: I don't know what year the 720 is, but I recall 720s being equipped with A10s which did not have Delrin crown races. The B10, which came new on my 1984 610, did have a plastic crown race. There was another odd one called C10 which was all-plastic and which was rather cheaper than the B10.

If there is a B10 or B10 parts on the bike, there has been at least one overhaul in the past. I'd say a GOOD LBS should look at the frame/fork and make sure the new installation is sound.

If the OP can find a use a copy of Sutherland's, he should be able to find the correct amount of interference. Its not a lot of interference, but the crown race cannot be correctly installed with the hand.

rootboy 

I would see if Elton over at Harris could help you out. If he's still there, that guy knows his stuff. Knows vintage bikes and, I would think they would have a crown race seat cutter.

As for other suggestions mentioned here, I personally would be very careful if taking it to a machine shop. While those guys are used to precision work, I might not feel comfortable taking a fork to someone who didn't know bikes and had never done if before. Still, if you specified the exact size you needed for an interference fit, it might be OK. Still, be careful having a thin, hollow steerer tube chucked in a big lathe. 2 cents.

bikemore 

Thanks for the tip on Harris. They are opened tonight when I am actually able to visit the shop (really - time constraints are a big part of my problem).

As per your caution about using any machine shop, I will take that caution. It is only .1mm taken off so it isn't like there is a lot of time removing (I am guessing setup is more time than actual cutting).


I am far from an expert mechanic, but I have put headsets on at least a half a dozen bikes and never had much trouble. Bikes run fine.

When I removed the old stronglight crown race I notice a bit of it had been shaved off (didn't sink in why it had been shaved). And when I put the new (used) crown race on a small piece was shaved off then.

Sorry about the misuse of terms, I have a lifelong habit of imprecise use of language - hard to undo.

SJX426 

One of my goals is to keep things as original as is reasonable. That statement includes not changing dimensions unless there is damage. Note @JohnDThompson post #9 . I would take it seriously. @verktyg provides some great information, specifically about interference fit. .02mm max interference is the key. Your really don't want the crown race to rotate on the seat.

The seat and face on my '83 Colnago was milled prior to paint. I know this because the Super Record Crown Race was split when I purchased the bike used in 2009. After removing the CR, I discovered that the surface had overspray from the front fork painting process. Once cleaned up, the .02 interference was measured. I am convinced that the split CR was a result of the paint on the seat/surface. SR CR's have much less material and smaller dimensions compared to NR HS CR. They are known to split. Care to ensure specs are met is important in this case. This is not to say that all HS CR's are subject to failure but damage, such as galling on the CR seat surface, is a possibility resulting in a CR that is not correctly seated.

BTW, I used the split CR by rotating it in a position where the bearings apply the least loading overall. I didn't and don't feel the break when turning the fork. Try finding a SR CR only that is in acceptable condition for less than $100! I did find one for $150 but there was a frame set attached with a tie strap.

nlerner

Dave, a friend of mine had Broadway Bikes in Somerville take a fork crown from 27.0 to 26.4. He is a satisfied customer. But giving Harris/Elton a call before you head over is a good idea, too.

bikemore 

If I get the CR seat milled in time, I will be using a NR HS.

SJX426 

AHHH far more robust!

verktyg 

I reface the head tube and fork crown seat on every frame that I put together (that's how I know first hand about the poor quality facing on the sacred, holy top name Italian frames!).

A few years back, I bought a 1988 Peugeot Biarritz frame that came with a Shimano 600 alloy headset. When I removed the headset, I found a crack in the crown race and as you mentioned with your Colnago, it was probably caused by the paint overspray on the seat.



I refaced the seat and rotated the cracked crown race like you did but the slight interference fit caused the crown race to spread enough at the crack to be felt in use.

Rather than cutting the fork crown seat undersize which could cause problems if I ever replaced the headset, I used a fine grit mounted point in a Fordem tool to grind away a small amount of the inside of the race to eliminate the interference fit.

I added Loctite 660 bearing mount to make sure that the race stays put! The crack is so fine no one would ever notice it.

BTW, this has become my all time favorite riding and handling bike... my grail, my Goldilocks bike!

In my experience, better quality French frames required less material removal when refacing than most premium quality Italian frames that I've worked on!!!

verktyg

Chas.