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Builder's Corner Restoration and preservation. Keep them running in perfect shape. What size is your bottom bracket?

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Old November 15th, 2010, 12:34 AM   #1
turd-not
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Default crack help

First post, first Fat

In junior high I would cut out Fat Chance magazine adds and hang them on my wall, bought a new 1987 Trek 830 and the dream of the bike disappeared and transformed to just riding what I had.

Finally got one!
Just looking for some advice on a crack that I blindly overlooked when I picked it up. Previous owner was running a narrow post. Any harm it just throwing a 29.4mm in and drill stopping the crack? I
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Old November 15th, 2010, 9:09 AM   #2
fat-tony
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There are a lot of folks here on Fatcogs more knowledge than me, but I would say that it is never a good idea to ride a frame with a crack. Period. Plus, it is going to be your primary ride, you live in Utah (there is a place there named Moab, ever heard of it?), and you are going to be thrashing the frame the way ift was meant to be ridden.

Maybe get in line with Rody or another frame builder than does repairs and let them hook you up.
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Old November 15th, 2010, 9:23 AM   #3
Doug Carter
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Are you referring to the horizontal slot at the bottom of the seat clamp area?


That doesn't look like a crack to me
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Old November 15th, 2010, 11:56 AM   #4
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[QUOTE=Doug Carter;13824]Are you referring to the horizontal slot at the bottom of the seat clamp area?


That doesn't look like a crack to me
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Old November 15th, 2010, 12:30 PM   #5
I-ROBOT
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Definitely a crack!

We were getting several frames back with that same issue. It would happen due to the way the bottom of the seat post slot was drilled. The hole would be drilled first and then the slot was cut. Since the drill would produce microscopic ridges in the hole, the stress from rider's bodies landing hard on the seat after jumps and whatnot would start a crack propagating. We then started adding the brazed on washer to reinforce the hole but obviously some still fail. Others failed due to people using the wrong size seat post and tightening the seat clamp way too much to get it to hold.

The best way to repair the crack is as follows: Remove the seat post (already done - I have a keen sense of the obvious don't I?), thoroughly clean the inside of the seat tube - make sure all of the grease is removed at least one inch either side of the crack, strip the paint away from the crack - I would recommend at least a quarter-inch below the crack and above, get a 1/16" drill bit and stop-drill the very end of the crack, carefully heat the existing washer around the hole until the silver solder melts and remove it, carefully remove any remaining silver solder from the seat tube, find a piece of copper tube that will fit snugly in the ID of the seat tube at the crack and clamp the copper tube to the seat tube, have the crack TIG welded with ER80S-D2 filler rod - DO NOT USE 4130 WELD ROD, remove the copper backing tube and make sure the repair weld has penetrated all the way through. If any weld bead is protruding into the seat tube, it will have to be ground or reamed so the seat post will fit and slide up and down, find a large stainless steel fender washer and round it so it fits snugly where the old washer was and carefully braze it to the seat tube, re-cut the slot and sand all of the edges with fine sand paper, prime and paint the affected area.

That's all there is to it! A job better left to a pro if you want it done right so it will last.

Good luck
Scott
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Old November 15th, 2010, 12:44 PM   #6
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They cracked through the brazed-on washer like that?


Wow.
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Old November 16th, 2010, 7:41 AM   #7
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Quote:
Originally Posted by Doug Carter View Post
They cracked through the brazed-on washer like that?


Wow.
Yep happened on my '92 as well.
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Old November 21st, 2010, 11:52 PM   #8
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Thanks so much Scott,

Does the Mandrel need to be copper? Thinking about an old seat post to save a step.
Anyway I have a welder at work and he spends the whole day welding 2
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Old November 22nd, 2010, 11:32 AM   #9
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An aluminum seat post would serve the same purpose. Its just to keep the burn-through to a minimum and reduce the chance that the top of the seat tube will distort. You can weld the crack with an ER70S-2 or ER70S-6 wire if you don't have ER80S-D2 wire available. I would prefer the ER80 filler but the lower strength carbon steel wire will be OK.

I would think a small reamer would do the trick or some fine sandpaper wrapped around a a small rod or rat-tail file. Again, anything you can do to eliminate any areas that can cause failure will help.

Make sure the cracked washer is completely removed and get as much of the silver brazing out as you can. It will have a slightly yellowish tinge to it compared to the silvery surface of the chrome-moly tube. If you have a magnifying glass, use it to examine the prepared area. Excess silver alloy in the weld will cause it to crack due to the low-melt point zinc that's also contained in the brazing alloy. We used to use a 56% silver brazing rod - formally known as BAg-7 and I would recommend that you use that or something similar to braze on a new reinforcing washer.

You can use a stainless steel washer or form a some piece of chrome-moly or even plain carbon steel to make a reinforcement. Do not use a plated washer or any plated material unless you completely remove the plating which will interfere with the brazing bond to the seat tube.

Best of luck and let me know how it turns out
Scott
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