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Bottom Bracket Compatibility Question

Old 02-13-16, 08:58 AM
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kenshireen
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Bottom Bracket Compatibility Question

I have a 1988 Look KG96 which is all Shimano except the crankset which is Campy (Athena I think)
Anyhow.. I want to replace this with a 50/34 Shimano....

Will I need to replace my BB and what kind would you suggest.
My BB is 30 years old and has never been touched.

Thank you..
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Old 02-13-16, 09:08 AM
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Originally Posted by kenshireen View Post
I have a 1988 Look KG96 which is all Shimano except the crankset which is Campy (Athena I think)
Anyhow.. I want to replace this with a 50/34 Shimano....

Will I need to replace my BB and what kind would you suggest.
Yes, you will need to replace your BB.
My BB is 30 years old and has never been touched.

Thank you..
Depends on what Shimano crank you're getting, but if you're getting a modern Hollowtech II one you can't go wrong with this one:

Shimano Ultegra 6800 SM-BBR60 Bottom Bracket | Competitive Cyclist

Assume your bike has a standard English or Italian threaded bottom bracket.
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Old 02-13-16, 09:40 AM
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Pick a crankset first, then get a suitable/recommended BB for that crankset.
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Old 02-13-16, 09:58 AM
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repeat question? didn't you ask this before? Hollowtech cranks and BB CAN come in 1 box.

the 50-34 set is a Common build part.. want to keep square taper? the crank profile influences the BB spindle length.

lowest profile comes in to meet the short BB, Others with straight 90 degree to the spindle cranks need long ones..

Last edited by fietsbob; 02-13-16 at 11:16 AM.
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Old 02-13-16, 10:52 AM
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Originally Posted by fietsbob View Post
Hollowtech cranks and BB come in 1 box.
That's not universally true FYI.
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Old 02-13-16, 11:00 AM
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Hi kenshireen,

The advice you've received so far is good EXCEPT you must first know what kinds of bottom brackets will fit your bike frame. It would be easy to pick a crankset that uses an incompatible bottom bracket. Here are some points to consider:
  • One of the most common bottom brackets nowadays is BB30 (and similar variations like PF30). The bottom bracket shell in your 1988 frame is probably too small to accommodate the 30 mm spindle diameter of these modern bottom brackets. So stay away from any crankset that uses a 30 mm spindle like BB30 and PF30.
  • If you want to use a modern bottom bracket with a hollow spindle (they are lighter and stronger than the old square taper variety), then stick with a bottom bracket that uses a 24 mm spindle. But beware: There are several different 24 mm standards. This is where the previous advice is best: Pick a crankset that uses a bottom bracket with a 24 mm spindle and then select a bottom bracket made for it. For example, Shimano uses Hollowtech. Since you plan to use Shimano, this would be the logical choice. I use SRAM and they use GXP (actually GXP is 24 mm on the drive side and tapers to 22 mm on the non-drive side).
  • Many modern bottom brackets are "press fit" which means they are not threaded. Instead of screwing to your frame, they are pressed into the frame and held in place by friction. Since your bike was made in 1988, it probably has a threaded bottom bracket shell so the best solution would be to stick with a threaded bottom bracket.
  • When you pick a threaded bottom bracket, make sure it matches the threads on your frame's bottom bracket shell. The two common standards are English (ISO) with 1.37" x 24 tpi or Italian with 36 mm x 24 tpi. Your current bottom bracket should be labelled so you'll know which it is. Note: tpi = threads per inch.
  • If you are short and have a short leg length, you might want to pick a crankset with a shorter crank arm length. The shortest road crank arm that you'll usually find from the major manufacturers is 165 mm. My wife is only 5' 2" and has a 27" inseam (as measured for a bike fitting). She needs a 145-150 mm crank arm length. Otherwise, her knees rise to high at the top of the crank circle, which stresses both her knees and her hips. In order to get her a suitable crank size, we selected a crank for a BMX bike because many of them are available in short crank arm lengths. If you go this route, you'll have the best selection if you use a bottom bracket with a square taper spindle. Shimano's UN55 is excellent and is available with a wide variety of spindle lengths for both British and Italian threaded bottom bracket shells.
  • If you use a bottom bracket with a square taper spindle, then you must select the correct spindle length. If your current bottom bracket has a square tapered spindle, then get a new one with the same spindle length. The goal is to center the chainrings on your crank to the driveline (chainline) of the center of your cassette on your rear wheel. If the front of your driveline is not centered to the rear of your driveline, you can expect problems.
  • If you use a bottom bracket with a square taper spindle, be careful that its taper matches the taper of your crankset. There are two main standards JIS (Japanese) and ISO. Many bikes in the U.S. still use the JIS standard. If you mix the standards the wrong way (I forget which way is the bad way), you'll never be able to stop your crank from wiggling on the spindle.

Kind regards, RoadLight

Last edited by RoadLight; 02-13-16 at 03:15 PM.
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Old 02-13-16, 11:14 AM
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In his other thread he said he was getting an Ultegra crank, so I think a lot of wasted typing there.
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Old 02-13-16, 12:28 PM
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A couple minor corrections…

Originally Posted by RoadLight View Post
  • When you pick a threaded bottom bracket, make sure it matches the threads on your frame's bottom bracket shell. The two common standards are English (ISO) with 1.37" x 24 tpi or Italian with 35 x 1 mm (25.4 tpi). Your current bottom bracket should be labelled so you'll know which it is. Note: tpi = threads per inch.
35 x 1 is metric (aka "French") thread, not Italian, and pretty obscure these days. Italian is weird, mutant 36mm x 24tpi and still quite available, albeit usually (but not exclusively) on Italian frames. The 36mm ID of an Italian shell means an English or metric thread cup will fall into the shell without engaging any threads.

  • If you use a bottom bracket with a square taper spindle, be careful that its taper matches the taper of your crankset. There are two main standards JIT (Japanese) and ISO. Many bikes in the U.S. still use the JIT standard. If you mix the standards the wrong way (I forget which way is the bad way), you'll never be able to stop your crank from wiggling on the spindle.
"JIS" (Japanese Industrial Standard) is the other major spindle standard besides ISO. "JIT" is usually an acronym for "Just In Time" parts delivery to manufacturers to minimize warehousing costs.
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Old 02-13-16, 02:28 PM
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Originally Posted by fietsbob View Post
repeat question? didn't you ask this before? Hollowtech cranks and BB CAN come in 1 box.
no they don't, Shimano has never shipped road (HT2)BB's with cranks, they used to/still do this with MTB cranks, but it's becoming more common to have to purchase the BB separately for these as well now (they seem to have stopped this with the introduction of 11 speed MTB)
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Old 02-13-16, 03:19 PM
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@JohnDThompson: Thanks for catching my typos. I've corrected my post.
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Old 02-13-16, 03:29 PM
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Originally Posted by RoadLight View Post
Hi kenshireen,

The advice you've received so far is good EXCEPT you must first know what kinds of bottom brackets will fit your bike frame. It would be easy to pick a crankset that uses an incompatible bottom bracket. Here are some points to consider:
  • One of the most common bottom brackets nowadays is BB30 (and similar variations like PF30). The bottom bracket shell in your 1988 frame is probably too small to accommodate the 30 mm spindle diameter of these modern bottom brackets. So stay away from any crankset that uses a 30 mm spindle like BB30 and PF30.
  • If you want to use a modern bottom bracket with a hollow spindle (they are lighter and stronger than the old square taper variety), then stick with a bottom bracket that uses a 24 mm spindle. But beware: There are several different 24 mm standards. This is where the previous advice is best: Pick a crankset that uses a bottom bracket with a 24 mm spindle and then select a bottom bracket made for it. For example, Shimano uses Hollowtech. Since you plan to use Shimano, this would be the logical choice. I use SRAM and they use GXP (actually GXP is 24 mm on the drive side and tapers to 22 mm on the non-drive side).
  • Many modern bottom brackets are "press fit" which means they are not threaded. Instead of screwing to your frame, they are pressed into the frame and held in place by friction. Since your bike was made in 1988, it probably has a threaded bottom bracket shell so the best solution would be to stick with a threaded bottom bracket.
  • When you pick a threaded bottom bracket, make sure it matches the threads on your frame's bottom bracket shell. The two common standards are English (ISO) with 1.37" x 24 tpi or Italian with 36 mm x 24 tpi. Your current bottom bracket should be labelled so you'll know which it is. Note: tpi = threads per inch.
  • If you are short and have a short leg length, you might want to pick a crankset with a shorter crank arm length. The shortest road crank arm that you'll usually find from the major manufacturers is 165 mm. My wife is only 5' 2" and has a 27" inseam (as measured for a bike fitting). She needs a 145-150 mm crank arm length. Otherwise, her knees rise to high at the top of the crank circle, which stresses both her knees and her hips. In order to get her a suitable crank size, we selected a crank for a BMX bike because many of them are available in short crank arm lengths. If you go this route, you'll have the best selection if you use a bottom bracket with a square taper spindle. Shimano's UN55 is excellent and is available with a wide variety of spindle lengths for both British and Italian threaded bottom bracket shells.
  • If you use a bottom bracket with a square taper spindle, then you must select the correct spindle length. If your current bottom bracket has a square tapered spindle, then get a new one with the same spindle length. The goal is to center the chainrings on your crank to the driveline (chainline) of the center of your cassette on your rear wheel. If the front of your driveline is not centered to the rear of your driveline, you can expect problems.
  • If you use a bottom bracket with a square taper spindle, be careful that its taper matches the taper of your crankset. There are two main standards JIS (Japanese) and ISO. Many bikes in the U.S. still use the JIS standard. If you mix the standards the wrong way (I forget which way is the bad way), you'll never be able to stop your crank from wiggling on the spindle.

Kind regards, RoadLight
thank you so much for the detailed explanation...
As I mentioned earlier I have a 7 speed cassette with a 7/8 speed chain... want to switch to a 50/34.
I am looking at a FC 6750.... will a 24mm BB accomodate that crankset and will I have to add spacers to properly
line up my drive train?

thank you again
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Old 02-13-16, 05:08 PM
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Does your BB have cups and loose ball bearings? If so, you may be able to replace only the spindle at very low cost. Disassemble the BB and measure the spindle, including the diameter and the distance between the forged "cones." Look up BB spindles or "BB axles" at Niagara or Bikewagon and ensure that they carry spindles with the same dimensions. You'll have a choice of lengths, but hold off till you have a JIS-taper crank. Then buy the spindle length recommended by the crank manufacturer. I've done this a couple of times.

In the past, there were some odd variations in JIS, but a new spindle and new crank should match up fine.
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Old 02-14-16, 01:29 AM
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Originally Posted by kenshireen View Post
... I am looking at a FC 6750.... will a 24mm BB accomodate that crankset and will I have to add spacers to properly
line up my drive train? ...
Hi kenshireen,

The Shimano FC-6750 is designed to use a Hollowtech II bottom bracket. So the answer is "yes, this crankset will accommodate a 24 mm BB." In fact, as far as I can tell, this is your only choice for this crankset. So you will need to purchase a Hollowtech II bottom bracket from Shimano. Remember to make sure that it matches the thread type of your frame (English/ISO or Italian).

I looked up Shimano's Tech Service sheet for this crankset and have attached it to this post. According to it, the FC-6750 is designed to have a 43.5 mm driveline (chainline). Since this crank has two chainrings, the gap between them will be 43.5 mm from the centerline of your bike frame. What you need to do is measure the driveline at your 7-speed cassette to see if it matches. One way to do this is to get a large pair of calipers and measure the inside width of your frame's rear dropouts (where your rear wheel hub axle attaches to your frame). This measurement should be made with the wheel mounted. Next, measure the width from the driveside dropout to the center of your cassette. This would be the 4th cog in the middle of your 7-speed cassette. Use these two measurements to calculate your rear driveline as follows:

rear driveline = ( width between dropouts / 2 ) - driveside width to cassette center

This will tell you how far the middle of your cassette is from the centerline of your frame. Ideally, it should match the front driveline of your crankset. I'm not sure how much variance a 7-speed system can handle. Hopefully another bike mechanic with experience can help. And I have no experience with Hollowtech bottom brackets to know whether or not you can add spacers to increase the driveline distance, if needed. (My experience is primarily with Suntour and SRAM drivetrain components.) However, I doubt that spacers would be needed. Since your frame is from 1988, it might use a 126 mm rear dropout width instead of the 130 mm standard used today for road bikes with rim brakes. Plus, the width of your 7-speed cassette will be different from the 10-speed cassette width that the FC-6750 was designed for. So your rear driveline measurement will probably not be 43.5 mm and it may be only 31.9 mm (a common driveline for Shimano 7-speed cassettes). If it is significantly less than 43.5 mm (31.9 mm would definitely be "significant"), I'm not sure there is anything you can do to make the front driveline match with an FC-6750. In that case, it would be wiser to pick a crankset that better matches your driveline.

It is possible to correct the mismatch at the rear by having a bike shop spread the rear of your frame to accommodate a 130 mm width. Then purchase a modern wheel with a 130 mm wide hub, install a 10-speed Shimano cassette and compatible rear derailleur. Personally, I wouldn't recommend going this route unless you love your frame so much that you insist on sticking with it and you have access to the needed drivetrain parts. What I would do in your shoes, is buy a vintage crankset that matches the rest of your drivetrain.

One last comment: In your OP you wrote that your bottom bracket had not been touched in 30 years. That's awful!!! I'd hate to see the poor condition of the grease and bearings in your bottom bracket. Even if your bike was barely ever ridden and was safely stored for these 30 years, the grease in your bottom bracket will have dried and its bearings will be in desparate need of cleaning and new grease. If your bike has seen heavy use and/or been used in the rain, then I doubt the bearings are still good. Bicycles need regular maintenance!!!

Kind regards, RoadLight
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Old 02-14-16, 06:09 AM
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Can't spread his bike. Vintage lugged carbon.
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Old 02-14-16, 12:01 PM
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24mm pipe hollowtech 2 you have no options .. the external bearings screw into the frame & the tube
attached to the right arm goes through it it .. dont pre worry if it will be perfect or not .. without trying .
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Old 02-14-16, 12:01 PM
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Originally Posted by RoadLight View Post
Hi kenshireen,

The Shimano FC-6750 is designed to use a Hollowtech II bottom bracket. So the answer is "yes, this crankset will accommodate a 24 mm BB." In fact, as far as I can tell, this is your only choice for this crankset. So you will need to purchase a Hollowtech II bottom bracket from Shimano. Remember to make sure that it matches the thread type of your frame (English/ISO or Italian).

I looked up Shimano's Tech Service sheet for this crankset and have attached it to this post. According to it, the FC-6750 is designed to have a 43.5 mm driveline (chainline). Since this crank has two chainrings, the gap between them will be 43.5 mm from the centerline of your bike frame. What you need to do is measure the driveline at your 7-speed cassette to see if it matches. One way to do this is to get a large pair of calipers and measure the inside width of your frame's rear dropouts (where your rear wheel hub axle attaches to your frame). This measurement should be made with the wheel mounted. Next, measure the width from the driveside dropout to the center of your cassette. This would be the 4th cog in the middle of your 7-speed cassette. Use these two measurements to calculate your rear driveline as follows:

rear driveline = ( width between dropouts / 2 ) - driveside width to cassette center

This will tell you how far the middle of your cassette is from the centerline of your frame. Ideally, it should match the front driveline of your crankset. I'm not sure how much variance a 7-speed system can handle. Hopefully another bike mechanic with experience can help. And I have no experience with Hollowtech bottom brackets to know whether or not you can add spacers to increase the driveline distance, if needed. (My experience is primarily with Suntour and SRAM drivetrain components.) However, I doubt that spacers would be needed. Since your frame is from 1988, it might use a 126 mm rear dropout width instead of the 130 mm standard used today for road bikes with rim brakes. Plus, the width of your 7-speed cassette will be different from the 10-speed cassette width that the FC-6750 was designed for. So your rear driveline measurement will probably not be 43.5 mm and it may be only 31.9 mm (a common driveline for Shimano 7-speed cassettes). If it is significantly less than 43.5 mm (31.9 mm would definitely be "significant"), I'm not sure there is anything you can do to make the front driveline match with an FC-6750. In that case, it would be wiser to pick a crankset that better matches your driveline.

It is possible to correct the mismatch at the rear by having a bike shop spread the rear of your frame to accommodate a 130 mm width. Then purchase a modern wheel with a 130 mm wide hub, install a 10-speed Shimano cassette and compatible rear derailleur. Personally, I wouldn't recommend going this route unless you love your frame so much that you insist on sticking with it and you have access to the needed drivetrain parts. What I would do in your shoes, is buy a vintage crankset that matches the rest of your drivetrain.

One last comment: In your OP you wrote that your bottom bracket had not been touched in 30 years. That's awful!!! I'd hate to see the poor condition of the grease and bearings in your bottom bracket. Even if your bike was barely ever ridden and was safely stored for these 30 years, the grease in your bottom bracket will have dried and its bearings will be in desparate need of cleaning and new grease. If your bike has seen heavy use and/or been used in the rain, then I doubt the bearings are still good. Bicycles need regular maintenance!!!

Kind regards, RoadLight

Thank you Roadlight
I measured as you requested. 126mm rear dropout to rear dropout.... distance from the 4th cog to the drive side dropout is 20mm (maybe 21). According to your formula that nets out to around 42mm.
Should I be OK.... or will I need spacers. BTW, the reason I don't want to buy a vintage CS is because I am looking for a 34/50 . I thought BB are normally not opened for grease and cleaning... Thought they were sealed.
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Old 02-14-16, 12:07 PM
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Originally Posted by habilis View Post
Does your BB have cups and loose ball bearings? If so, you may be able to replace only the spindle at very low cost. Disassemble the BB and measure the spindle, including the diameter and the distance between the forged "cones." Look up BB spindles or "BB axles" at Niagara or Bikewagon and ensure that they carry spindles with the same dimensions. You'll have a choice of lengths, but hold off till you have a JIS-taper crank. Then buy the spindle length recommended by the crank manufacturer. I've done this a couple of times.

In the past, there were some odd variations in JIS, but a new spindle and new crank should match up fine.
Bottom brackets are reasonably inexpensive so I would buy one to match the 6750. I do not know if I have BB cups/loose bearings since I have not found the tool to remove my campy crank but my LBS will do this part for me. I assume that since I have campy that the BB will not accomodate shimano...or am I incorrect... Still learning.... thanks to all for your patience
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Old 02-14-16, 12:57 PM
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Originally Posted by kenshireen View Post
Bottom brackets are reasonably inexpensive so I would buy one to match the 6750. I do not know if I have BB cups/loose bearings since I have not found the tool to remove my campy crank but my LBS will do this part for me. I assume that since I have campy that the BB will not accomodate shimano...or am I incorrect... Still learning.... thanks to all for your patience
Campy crank will be ISO taper, so the existing BB spindle won't take a Shimano crank (JIS taper). However, a JIS spindle runs <$10, and I've never heard of problems with a well adjusted, well maintained cup & cone BB. Sealed BBs are fine, but you sometimes hear of them creaking, loosening, etc. Personal preference, I guess.

When you (or the LBS) remove the BB, clean and examine the bearing races in the cups. If they aren't scored or pitted, they should be reusable. I found stiff, tarry grease in a BB that hadn't been touched in 50 years, but the bearing races and even the ball bearings were fine when cleaned up. It depends on mileage and riding/storage conditions.

Last edited by habilis; 02-14-16 at 01:05 PM.
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Old 02-14-16, 05:09 PM
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Originally Posted by kenshireen View Post
... I measured as you requested[/B]. [B]126mm rear dropout to rear dropout.... distance from the 4th cog to the drive side dropout is 20mm (maybe 21). According to your formula that nets out to around 42mm.
Should I be OK.... or will I need spacers. BTW, the reason I don't want to buy a vintage CS is because I am looking for a 34/50 . I thought BB are normally not opened for grease and cleaning... Thought they were sealed.
Hello again kenshireen,

Actually, if the distance from the driveside dropout to the center cog of your cassette is 20 mm, then the rear driveline would be 43 mm which would be within 0.5 mm of the front driveline of the FC-6750. So you shouldn't have a problem and no spacers would be needed. Personally, I doubt you'd need spacers as long as the front and rear drivelines are within 2 mm or less. But here, my inexperience with a 7-speed cassette shows because, since it is less wide than a 10 or 11-speed cassette, a larger divergence may also be tolerable since the chain will have less distance to move at either end of the cassette.

What happens if your driveline is mismatched at each end? Answer: When your chain is on the cassette cog that is farthest away from a chainring, the plates of the chain will rub on the side of the teeth of both the cog and chainring. This will cause your drivetrain to be more noisy and it will cause the cassette, chainring and chain to wear quicker and need to be replaced more often. In the case of a vintage bike like yours where it may be difficult and/or expensive to find a replacement cassette, this would be bad. The shifting may also not be as smooth as it could be. But, again, it doesn't sound like this will be a problem in your case because it sounds like your driveline will be very straight with the FC-6750.

Regarding bottom bracket cleaning: It depends on the type of bottom bracket. Back in the 80's many bikes were still using unsealed bearings and you must remove the bottom bracket end caps (actually, they are bearing "cups") to service and/or replace the bearings. My family has a couple of vintage Fuji road bikes from 1985. Their bottom brackets didn't have any seals. I replaced both with a Suntour Microlite WTB Grease Guard bottom bracket that is sealed where the spindle passes through each cup. They use a special spindle that is hollowed at each end. The crank arm bolts are hollow, too. You press the nozzle of a grease gun to the hole in the crank arm bolt and squeeze fresh grease into the bottom bracket and the old grease is squeezed out past the rubber seal along the spindle. This was, as far as I know, one of the few bottom bracket designs available in the early-mid '80s that didn't require disassembly to service the bearings. They are still highly sought after for vintage bikes to this day.

Most modern bottom brackets use cartridge bearings that are sealed. So the standard procedure is to replace the cartridges whenever needed instead of servicing the bearings inside them. However, it is possible to service many of them if you care to---you pry off the seal, degrease the bearings and regrease them. But my preference is to install new bearings whenever a modern bottom bracket needs service. Why? Because high-quality steel bearings are so cheap that you may spend more money on degreasing them than it would cost to replace them. The exception would be ceramic bearings which are very expensive.

Regardless whether the bearings are old-style without seals from the 80's or modern sealed cartridges, the presence of seals does not provide you with a 30-year service interval. The seals are not perfect (especially if you want your crank to have a low spin resistance). It can take just one careless swipe with water pressure while cleaning your bike to blow dirt past a seal and into a bearing. And most of the grease used in bikes from the 80's will not last 30 years.

Kind regards, RoadLight
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